A photofragment imaging study using resonant ... - CiteSeerX

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Photodissociation of superexcited states of hydrogen iodide: A photofragment imaging study using resonant multiphoton excitation at 13.39 and 15.59 eV Hans-Peter Loock, Bernard L.G. Bakker, and David H. Parker

Abstract: Jet-cooled HI has been excited using a resonant three-photon excitation scheme to energies corresponding to 13.39 and 15.59 eV. Analysis of velocity mapping images of the iodine atom fragments allowed the identification of the HI excited states at these energies as − − the (4 61/2 ) 6p superexcited state and the repulsive 4 61/2 state of HI+ , respectively. Following excitation at 13.39 eV, we observe formation of iodine atomic fragments through the H(2 S) + I[(3 PJ ) 6p] (J = 0, 1, 2) fragment channels, as well as through the H(2 S) + I[(1 D2 ) 6p] channel. This observation is explained by extensive nonadiabatic interactions between the − (4 61/2 ) 6p state with the repulsive (4 51/2 ) 6p state and the weakly bound (A 2 6 + ) 6p state. In support for this proposed dissociation mechanism excitation of the corresponding ionic 4 − 61/2 state at 15.59 eV also results in formation of comparable quantities of I+ in its 1 D2 , 3 P0,1 , and 3 P2 levels indicating again extensive nonadiabatic interactions with other repulsive − curves. A similar mechanism based on the local interaction of the 4 61/2 state with the A 2 6 + 4 and the 51/2 state is proposed. PACS Nos.: 82.50F, 32.80R Résumé : Du HI refroidi dans un jet a été excité par résonance à trois photons à des énergies correspondant à 13,9 et 15,59 eV. L’analyse des images de projection de vitesse du fragment iode permet d’identifier les états excités du HI à ces énergies comme étant l’état superexcité − − (4 61/2 ) 6p et l’état répulsif 4 61/2 du HI+ respectivement. Suivant l’excitation à 13,39 eV, nous observons la formation de fragments d’ions atomiques via les canaux H(2 S) + I [(3 PJ ) 6p] (J = 0, 1, 2). Ceci s’explique par le fort couplage non adiabatique entre le (4 51/2 ) 6p et l’état répulsif (4 51/2 ) 6p ainsi que l’état faiblement lié (A 2 6 + ) 6p. En accord avec le − mécanisme de dissociation proposé, l’excitation de l’état ionique correspondant 4 61/2 à 15,59 eV résulte aussi dans la formation de quantités comparables de I+ dans ses niveaux 1 D2 , 3 P0,1 et 3 P2 , ce qui indique encore des interactions non adiabatiques avec d’autres courbes répulsives. Nous proposons un mécanisme similaire basé sur l’interaction locale de l’état 4 − 61/2 avec les états A 2 6 + et 4 51/2 . [Traduit par la Rédaction]

Received June 3, 2000. Accepted December 5, 2000. Published on the NRC Research Press Web site on May 9, 2001. H.-P. Loock.1 Department of Chemistry, Gordon Hall, Queen’s University, Kingston, ON K7L 3N6, Canada. B.L.G. Bakker and D.H. Parker. Department of Molecular and Laser Physics, University of Nijmegen, Nijmegen, P.O. Box 9010, NL-6500 GL, The Netherlands. 1

Corresponding author (e-mail: [email protected]).

Can. J. Phys. 79: 211–227 (2001)

DOI: 10.1139/cjp-79-2/3-211

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1. Introduction A superexcited state is a neutral electronic state of a molecule lying at higher energies than the molecular ionization potential. A number of competing processes such as direct dissociation, autoionization, predissociation, or other processes driven by excited-state interactions can follow excitation to superexcited states. For many simple molecular systems, these processes take place at high energies and the experimental effort to produce superexcited states frequently involves generation of pulsed vacuum ultraviolet or synchrotron radiation. The fate of the superexcited state is commonly determined by monitoring, for example, the photoelectron emission or photofragment yield. Experimental results may then be compared with predictions for the interactions of these excited states and a complete picture of the excited-state interactions may be obtained. Tunable laser multiphoton excitation is an alternative and probably more convenient method to produce superexcited states, but has been limited by the comparably low efficiency of nonresonant multiphoton excitation schemes. Furthermore, it is difficult to distinguish the interesting processes occurring to superexcited states from processes occurring on other than the desired multiphoton level. Photofragment imaging using the velocity-mapping technique is suited to overcome these problems for some molecules as is demonstrated in this report. With this technique, the superexcited state is excited by absorbing several photons from a narrow linewidth, pulsed laser. The consequent emission of charged particles (electrons and positive ions) is monitored using a time-of-flight mass spectrometer with a position sensitive detector. Neutral but highly excited fragments are detected by ionization with one additional photon, whereas the detection scheme is insensitive for the parent molecule and photofragments, if they are in their electronic ground state. The position sensitivity of the detector allows us to determine the total kinetic energy release of the dissociation process, and thereby not only to determine the quantum state of the detected fragment but also, by energy and momentum conservation, the quantum state of the correlated dissociation product. It is, therefore, straightforward to distinguish processes occurring via competing excitation processes or dissociation pathways. Furthermore, it is possible to probe directly the dynamics of purely repulsive states for which a photoelectron signal may not easily be obtained. With this work, the photofragment-imaging scheme has been tested for previously unobserved superexcited states of hydrogen iodide. HI was selected because there already exists a large body of information on the excited states of HI and HI+ to support our observations. Since nonresonant multiphoton excitation may not be able to produce superexcited states, an excitation scheme is proposed that makes use of resonance-enhanced excitation via the diffuse A-band at the one-photon level and a well-characterized excited state at the two-photon level. The third photon will then populate the superexcited state. In this study, two similar resonant three-photon excitation schemes have been used to prepare a superexcited state at 13.39 eV, and, as will be shown, its corresponding ionic state limit at 15.59 eV, and the dissociation dynamics of these states has been followed by detecting the iodine atomic fragment. In the experiment at 13.39 eV, HI is excited to a repulsive superexcited state approximately 3 eV above the adiabatic ionization potential but below the I+ + H dissociation limit. In this region between about 10.5 and 13.5 eV Cormack et al. [1] found a large number of vibrational bands in the threshold photoelectron spectra (TPES). HI and HI+ have very similar geometry since a nonbonding electron is removed, thus from Franck–Condon considerations only the lowest vibrational levels of HI+ are expected to be formed. The observation of TPES bands in the non-Franck–Condon region indicates that strongly absorbing superexcited states were able to enhance the oscillator strength of the X 2 53/2 and 25 + 1/2 (v = 4–17) states. Cormack et al. [1] proposed that interactions with the n = 6–8 members of the Rydberg series converging to the A 2 6 + state of HI+ were responsible for the enhanced absorption. As will be shown in this report other superexcited states will also have to be considered. This paper is organized as follows. After a brief description of the experiment, the photofragment images will be analyzed to confirm that the dissociation process with 277.67 nm light is due to a ©2001 NRC Canada

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resonance-enhanced three-photon excitation process to a superexcited state. It will be shown that the neutral iodine fragments are formed in three different electronically excited states, whereas the hydrogen fragments are formed in their ground state. Analysis of the image recorded with 238.66 nm light indicates the excitation of an electronically excited state of HI+ that again dissociates to yield excited iodine atoms. In the following section, the findings are discussed in terms of excitation with 277.67 nm light to a repulsive superexcited state, which dissociates nonadiabatically to yield fragments via three different dissociation channels. Applying the same mechanism to the ionic state dissociation with 238.66 nm light verifies this hypothesis.

2. Experimental The experimental setup has been described before [2] and therefore only details relevant to this particular experiment are given below. A pulsed molecular beam was produced by expanding a 2% mixture of HI (Matheson) in helium into a high vacuum and skimming the supersonic expansion by a 1 mm skimmer 30 mm downstream of the nozzle. The early part of the expansion was used to avoid contributions of HI dimers and clusters to the signal. The molecular beam was intercepted by the focused output of a Nd:YAG laser/dye laser system, which was frequency doubled to give a wavelength of around 277.67 and 238.66 nm, in both cases with pulse energy of less than 0.5 mJ/pulse. Ions created in this region were accelerated in an electrostatic lens assembly coupled to a time-of-flight mass spectrometer and detected using a gated position-sensitive microchannel plate detector. The length of the drift region is adjustable and was kept at 370 mm when detecting iodine atoms. The ion signal was amplified and projected onto a phosphor screen, from where the resulting image was read with a CCD camera. The raw image was analyzed by performing an inverse Abel transformation and the ions’ velocities and spatial anisotropy were determined from the transformed image. It is necessary to calibrate the size of the acquired image to a known ion velocity. For this experiment we used the well-known HI A-band dissociation to obtain calibration factors. For the calibration experiment HI was dissociated with light at 277.867 and 277.398 nm. The iodine atomic photofragments were detected via their I (2 P3/2 ) and I∗ (2 P1/2 ) transitions via the 6p 4 Do3/2 and 8p 2Po1/2 levels, respectively. The resulting images were then used to check for the presence of clusters and for the purity of the linear polarization of the laser beam. The velocity of the I (2 P3/2 ) atomic fragments can be calculated from the excitation energy and the known HI dissociation energy of 24 632 cm−1 [3] as being 129.3 m/s, and the velocity of the I∗ (2 P1/2 ) fragment as 74.9 m/s. Since the diameter of the ion image scales linearly with the radial velocity of the fragment atoms, these values could be used to determine the velocities of iodine atomic fragments arising from other processes. The error in the fragments’ kinetic energies is given mainly by the resolution of the ion image, i.e., the spatial resolution of the ion detector and the CCD camera. For the images of the iodine fragments this error translates into a 2000 cm−1 resolution in total kinetic-energy release. This uncertainty is unusually high because the iodine atom receives only 1/128 of the total kinetic energy and a small error in the velocity measurement of the iodine fragment corresponds to a large error in the total kinetic energy release. Atomic iodine ions were detected in this experiment by gating the gain of the channel plates at the I+ arrival time. This gating is not 100% effective and at mass 127 a small background originating from HI+ is observed. Since the parent HI molecule is given negligible radial velocity by its excitation and ionization processes, this signal can readily be distinguished from the I+ signal at mass 127.

3. Results To populate the HI superexcited states it is necessary to set the excitation laser at an energy that coincides with a resonance at the one and two-photon level.Young [4] has demonstrated that for moderate nanosecond laser pulse energies the (2 + 1) photon ionization can effectively compete with the A-band dissociation and has recorded and assigned spectra for the single-photon forbidden 1 = 2 transitions. ©2001 NRC Canada

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Fig. 1. Partial REMPI spectrum of the I 1 1(2) band of HI. The assignment of the spectrum closely followed the assignment of Pratt and Ginter [5].

Pratt and Ginter [5] published an extended survey of (2 + 1) resonance-enhanced multiphoton spectra and among others recorded two strong 1 = 2 bands. The band at around 70 228 cm−1 was readily identified as F1 1(2) – X1 6(0+ ) (0–0), whereas the band at around 71 990 cm−1 was assigned to I1 1(2) – X1 6(0+ ) (0–0) based on largely on its intensity. We re-recorded the spectrum of the latter transition (Fig. 1) and, for the photofragment imaging experiment, kept the laser energy fixed on the strong feature at around 72 027.0 cm−1 , which was assigned to the blended S(0) and R(2) line by Pratt and Ginter. These workers determined the rotational constant of this I 1 1(2) state to be approximately 6 cm−1 and therefore close to the value of the ionic ground states. The second photofragment image was recorded by keeping the excitation laser fixed near 238.66 nm ≈ 83 800 cm−1 , which is in the ionization continuum above the X 2 53/2 state ionization potential at 83 745.84 cm−1 [6], but below the resonance of the n = 5 Rydberg state series converging to the X 2 51/2 state at 83 880 cm−1 . Raw images and their inverse Abel transforms are displayed in Figs. 2a and 2c and Figs. 2b and 2d using 277.67 nm and 238.66 nm light, respectively. Both images and their inverse Abel transforms show the contribution of three different dissociation processes. The kinetic-energy release for the I+ ions for these three processes following excitation at 13.39 and 15.59 eV is comparable, despite the obvious difference in excitation energy. The radial velocity distribution (Fig. 3) and the spatial anisotropy for each dissociation channel is obtained in a manner described previously [2] and the fitting parameters are given in Table 1. The images could possibly be interpreted as the result of a one-photon dissociation process via the A-band, followed by nonresonant three-photon ionization of the iodine atomic fragment. Another possibility is a two- or three-photon dissociation process, which, should it result in excited iodine atoms, is followed by single-photon ionization of the iodine atomic fragment. These three excitation schemes are described in the following paragraphs. Clearly, one-photon dissociation of HI via theA-band cannot account for the observed photofragment channels in either one of the images. While for each image there are two iodine velocities that match ©2001 NRC Canada

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Fig. 2. (a) Raw photofragment image of the iodine fragment following three-photon excitation at 277.67 nm with (b) its inverse Abel transform. (c) Raw photofragment image of the iodine fragment following three-photon excitation at 238.66 nm with (d) its inverse Abel transform.

dissociation to H (2 S) + I (2 P3/2 ) and H (2 S) + I∗ (2 P1/2 ), there are three observations that render this dissociation scheme unlikely: (i) a third dissociation channel remains unaccounted for; (ii) the spatial anisotropy parameter for the H (2 S) + I∗ (2 P1/2 ) channel is positive, which is contrary to our own experiments on the A-band dissociation dynamics2 and all previous studies on the dissociation dynamics of the 3 5(0+ ) state [7,8]; and 2 H.-P. Loock, B. Bakker, and D. Parker. Manuscript in preparation.

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Fig. 3. Iodine fragment velocity distribution following excitation at (a) 277.67 nm and (b) 238.66 nm. The profiles have been obtained from Fig. 1c and Fig. 1d by radial summing of the image intensity. The velocity scale has been calibrated using the well-known energetics of the HI A-band dissociation as described in the text. Broken lines are the result of a least-squares fit to four Gaussian functions.

(iii) the intensity of the iodine atomic signal is much stronger than expected from a nonresonant three-photon ionization process. The photofragment images also cannot be rationalized based on a two-photon excitation process. For the excitation to the I1 1(2) – X1 6(0+ ) (0–0), R(2), S(0) with 277.67 nm light one could propose predissociation of these states to yield excited- or ground-state iodine atoms, which would then be ionized by another photon of the same pulse. Consideration of the energetics shows that this process is impossible: the internal energy of the fragments corresponds to 28 700–43 800 cm−1 (Table 1), which could only have been achieved by electronic excitation of the atoms. The lowest electronically excited states of hydrogen are at around 82 258 cm−1 and of iodine at around 54 633 cm−1 and (pre-)dissociation into excited atomic fragments can safely be ruled out. Alternatively, the HI parent molecule could have ©2001 NRC Canada

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Table 1. Summary of the results of Gaussian least-square fitting routines to the velocity distribution. The iodine atoms velocity was obtained directly from the fit to the images after proper calibration and was used to calculate the overall kinetic energy release. The expected iodine atom internal energies (in columns 3 to 5) following n-photon excitation were calculated using nEexc − Ekin − D0 with Eexc = 36 014 and 41 899 cm−1 and D0 = 24 632 cm−1 [3]. E(I )int (I) V (m/s)

Ekin

n=1 (cm−1 )

n=2 (cm−1 )

n=3 (cm−1 )

Intensity

β

277.67 nm

73.15 129.42 165.99

3624 11345 18663

7758 37 –7281

43772 36051 28733

79786 72065 64747

11% 45% 44%

1.1 1.7 1.9

238.66 nm

79.47 120.91 156.68

4278 9903 16621

12989 7365 641

54889 49264 42540

96788 91164 84440

27% 32% 39%

0.9 1.7 1.9

been electronically or vibrationally excited before laser excitation, which could account for the excess kinetic energy in the fragments. Again, given the number of vibrational quanta needed and that the lowest bound, excited states of HI — corresponding to the b 3 5 and C 1 5 states [9] — are around 55 500 cm−1 , this mechanism is considered highly unlikely. A similar reasoning can be applied to rule out two-photon dissociation initiated by excitation with 238.66 nm light. Two-photon absorption leads to excitation into the ionization continuum about 53 cm−1 above the first (v + = 0) ionization limit of HI at 83 745.84 cm−1 . Mank et al. have described the photoionization efficiency (PIE) spectrum in this region [6], and the threshold photoelectron spectrum was recorded by Cormack et al. [1]. Both spectra show a structure around 10.39 eV (∼83 800 cm−1 ), i.e., the excitation energy at the two-photon level using 238.66 nm radiation. The structure is readily attributed to the ionization to the lowest HI (v + = 0) level and is broadened by residual field effects in the case of the PIE spectrum. In our experiment, a fragmentation process that occurred following this two-photon excitation, however, cannot explain the photofragment image. For two-photon dissociation, the fragments’ kinetic energy corresponds to an internal energy of 42 500 to 54 900 cm−1 , which cannot be accounted for by either electronic excitation of the atomic fragments or electronic or vibrational excitation of the parent HI molecule. As expected, we do, however, observe a strong HI+ signal as indicated by the presence of ions with near zero kinetic energy at mass 128. Due to the imperfect mass gating of the detector, this strong ion signal also contributes weakly to the signal at mass 127 (Fig. 2b). After having ruled out single- and two-photon dissociation of HI as being responsible for the fragments’ kinetic energy, one can try to understand the photofragment images as being caused by threephoton excitation. As will be argued below, the processes that follow this excitation are quite different in nature for the two different excitation energies. At 277.67 nm, we observe dissociation of a superexcited state into two neutral but excited atomic fragments, whereas at 238.66 nm we observe nonadiabatic dissociation on a repulsive potential energy curve of the HI+ ion. In the following paragraphs these two processes will, therefore, be discussed separately. 3.1. Three-photon excitation at 13.39 eV In this case the excitation can be described as a (1 + 1 + 1) resonant excitation process via the repulsive states of the A-band and the I 1 12 R(2), S(0) states to a state above the potential minima of the 2 53/2 and 2 51/2 ground states of HI+ but below their H(2 S) + I+ (3 P) dissociation limit: IP(HI) + D0 (HI+ ) = 83 745.8 cm−1 + 25 181.5 cm−1 = 108 927.3 cm−1 [1,10]. The three-photon excitation ©2001 NRC Canada

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Table 2. Iodine atom internal energy following excitation at 277.67 nm observed and expected according to the three mechanisms described in the text. The energy ranges given reflect all J -states of the iodine atom.

Observed (3 P2 ) np; n = 6,7,8 (C) 6p; C = (3 P2 ), (3 P1,0 ), (1 D2 ) (3 P1,0 ) 6l; l = 0, 1, 2

Outer ring

Middle ring

Inner ring

64 747 64 904.31–67 062.09 64 904.31–67 062.09 60 896.23–63 186.75

72 065 74 965.62–75 621.41 71 501.47–73 387.15 71 501.47–73 387.15

79 785 78 732.22–792 24.70 78 415.25–80 125.45 82 028.55–82 792.79

energy (108 042 cm−1 = 13.39 eV) is also below the adiabatic ionization threshold for the A 2 6 + state (13.958 eV) and just below the threshold for the ion-pair channels H− + I+ (3 P0 )

13.527 eV

H− + I+ (3 P2 )

13.569 eV

−

+ 3

H + I ( P1 )

13.606 eV

Ionic states in this region have been studied using photoelectron spectroscopy [1,6,9,11] and — as will be later discussed — different mechanisms have been proposed to explain the unusually strong signal arising from highly excited vibrational states of the 2 53/2 and 2 51/2 states. Fragmentation of ionic states or possible superexcited states has only indirectly been observed, however [9]. Clearly, for energetic reasons the fragments must be neutral atoms and energy conservation dictates that their kinetic energy is dependent on their electronic state. The observed kinetic-energy releases correspond to about 64 750, 72 050, and 79 800 cm−1 (±2000 cm−1 ) in internal energy of the fragments, which in turn point to only a few possible fragment channels. In particular, the kinetic energy release is too high to correlate with excited H-atoms (lowest excited state ∼82 258 cm−1 ). Also, it is obvious that iodine can not be formed in its electronic ground state, since it would then take a nonresonant three-photon ionization process to ionize the atom — a process that is much too inefficient to account for the observed ion signal. However, given that the ionization potential of iodine is 84 295.4 cm−1 , ionization of electronically excited iodine atoms with an additional photon of the same laser pulse will be very efficient. Electronic excitation of iodine would fall in the n = 6–8 region of the iodine (3 PJ ) ns, (3 PJ ) np, and (1 D2 ) np Rydberg states. Using iodine level diagrams [10,12] as a guide, one can identify numerous atomic levels with the respective (3 PJ ) and (1 D2 ) cores, which match the respective internal energy. These levels are listed in Table 2. As can be seen from Table 2, three different dissociation processes will have to be considered. The first one involves formation of iodine atoms in their (3 P2 ) np states with n = 6, 7, and 8. A second possible process involves excitation of HI into a n = 6 Rydberg state and subsequent dissociation, yielding iodine fragment states which correspond to (C) 6p with the state of the ion core C = (3 P2 ), (3 P1,0 ), (1 D2 ). Finally, a third process may yield iodine atomic fragments in their (3 P1,0 ) 6l states, where the atomic orbital angular momentum l equals 0, 1, and 2. Other iodine fragment states can be excluded since their energies are too high. As discussed before, the error in the fragments internal energy is around 2000 cm−1 and, therefore, all three processes will have to be considered. As will be shown in the discussion of the experiment it is most likely the second process, i.e., excitation to a “6p” superexcited state of HI with subsequent dissociation of the ion core, that is responsible for the formation of the electronically excited iodine atomic fragments. Another possible source of I+ atoms following excitation of HI at 277.67 nm is photodissociation following excitation of HI+ X2 53/2 (v + = 0–17) and X 2 51/2 (v + = 0–12), i.e., photodissociation on the four-photon level. Three-photon excitation to an autoionizing superexcited state could yield a wide range of vibrationally excited HI+ . These stable HI+ states may absorb a fourth photon and dissociate ©2001 NRC Canada

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2

+ 1

H( S) + I ( D1 ) H(2 S) + I+ (3 P1 ) H(2 S) + I+ (3 P0 ) H(2 S) + I+ (3 P2 )

Observed

Series (with 2 P core)

Obs–Calc

96 788 91 164 91 164 84 440

98 071 91 430 90 791 84 340

–1283 –266 373 100

into I+ atoms. While there is some indication of HI+ formation from the leak-through signal in the center of the I+ image (Fig. 2a), this is small in comparison to the HI+ signal at 238.66 nm. Since each HI+ vibrational state would yield a unique set of dissociation rings, a large total number of rings would be expected. Also from energetic considerations, ionic dissociation does not yield a satisfactory explanation for the simple signals seen for 277.67 nm dissociation. Further experimental observations include the high spatial anisotropy, which indicates that the dissociation process is fast compared with the rotational period of the dissociating molecule. The spatial anisotropy is positive and the spatial anisotropy parameter has values of β = 1.1 (inner ring), 1.6 (middle ring), and 1.9 (outer ring) for the three channels, indicating that the parallel two-photon excitation to the I 1 1(2) state is followed by a third parallel excitation process to the (pre-)dissociated state. The trend in the β parameters may be coincidental or an experimental artifact (spatial anisotropy parameters, β, are less accurate for lower fragment kinetic energies) but may also indicate that the dissociation to the higher atomic Rydberg states is slower, and rotational effects decrease the spatial anisotropy [13]. 3.2. Three-photon excitation at 15.59 eV In this case, the three-photon excitation is resonant via the repulsive states of the A-band but likely nonresonant on the two-photon level, which corresponds to the ionic continuum just above the adiabatic IP of HI [4]. Excitation at 125 700 cm−1 (15.59 eV) is high above the 2 53/2 and 2 51/2 ground states of HI+ , the H(2 S) + I+ (3 P) dissociation limit, and the thresholds for the ion-pair channels H− + I+ (3 PJ ), J = 0, 1, 2 [1]. The excitation is also about 1 eV above the reported A 2 6 + state of HI+ , which shows diffuse, but vibrationally resolved structure in the photoelectron spectrum [1]. The fragment ion signal (Fig. 2c) may be interpreted in two ways. To account for the large amount of internal energy, one may propose excitation of the H-atom into some low-lying Rydberg states. Indeed, there is a reasonable agreement between the observed kinetic energy release and the H(np2 P) + I(2 P3/2 ); n = 2–4 channel [12], and better agreement if we propose spin–orbit excitation of the iodine atom. This mechanism requires, however, formation of I atoms in their electronic ground states, which would in turn require ionization via a nonresonant three-photon process. If this very inefficient process occurred at all, formation of iodine atoms would be dominated by the dissociation of the repulsive 3 5(0+ ), 3 5(0+ ), and 3 5(0+ ) states of the A-band with large contributions of perpendicular excitation on the one-photon level. Since the spatial anisotropy of all three channels is positive, this process can safely be ruled out. Alternatively, dissociation via the ionic curves to formation of ground and excited iodine atomic ions may be proposed. The internal energy for four H + I+ channels is listed in Table 3 and there is excellent agreement between the observed and calculated internal energy. The ion signal is expected to be strong, since there are only three excitation steps required to produce the ions. Unfortunately, the kinetic-energy resolution of the apparatus is not sufficient to distinguish between the formation of I+ (3 P1 ) and I+ (3 P0 ), so that in the following paragraphs both of these channels are discussed together. The branching ratio ©2001 NRC Canada

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Fig. 4. Schematic display of the states involved in the dissociation dynamics. The 6p Rydberg states (continuousline curves) arise from the respective diabatic and adiabatic curves (broken-line) and were placed into the diagram by shifting the excited HI+ states down by 2.35 eV.

between the three fragmentation channels can be obtained by simple radial integration of the averaged ion signal. We obtain a ratio of 33:27:40 for the I+ (1 D1 ):I+ (3 P1,0 ) and I+ (3 P2 ) channels, respectively. Furthermore, high spatial anisotropy is observed, which indicates that the dissociation time is short compared with the rotational period of the dissociating molecule. The spatial anisotropy is positive and has values of β = 0.9 for I+ (1 D1 ), β = 1.7 for I+ (3 P1 , 0), and β = 1.9 for I+ (3 P2 ), indicating that the transition is largely parallel. Again, as described above, the trend in the β parameters may be coincidental or an experimental artifact, but may also indicate that the dissociation to the higher excited atomic states is slower, and rotational effects decrease the spatial anisotropy. It should be noted that the proposed mechanism of three-photon excitation to a superexcited HI state is energetically similar to photodissociation of HI+ (v + = 0) to the same three [I+ (1 D1 ):I+ (3 P1,0 ) and I+ (3 P2 ) + H(2 S)] channels. Two-photon excitation produces large amounts of HI+ (v + = 0) along with a photoelectron with 53 cm−1 kinetic energy. The kinetic-energy release for I+ atoms from HI+ will be only (53/128) cm−1 or about 0.5 m/s less than the direct three-photon dissociation of HI, which is not distinguishable with the present velocity resolution. Consideration of the Franck–Condon overlap of the HI+ (v + = 0) wave function with the direct repulsive curves or the repulsive wall of the A-state curves (Fig. 4) suggests that HI+ dissociation to the lowest (3 P2 ) limit is likely going to contribute to the signal. For the following discussion it is not relevant whether the ionization occurs after the absorption of the second or third photon.

4. Discussion There exists an extensive body of information for the ionic states of HI. Most studies focused on the characterization of the X 2 53/2 and 2 51/2 states and their interactions. Two photoelectron spectra have recently been recorded, spanning the region 10–31 eV, i.e., the region between IP(HI) and IP(HI+ ) [1,14]. The more recent threshold photoelectron spectrum recorded by Cormack et al. [1] proves especially useful for the analysis of our data. The resolution of their synchrotron source is ©2001 NRC Canada

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about 10 meV and thus sufficient to resolve vibrational structure. These workers found sharp and strong contributions from vibrational bands of the two spin–orbit components of the X 2 5i states up to the H + I+ dissociation limit. They also observed diffuse but vibrationally resolved bands from the A 2 6 + state at around 13.958 eV and a threshold for dissociative autoionization at 13.45 eV. This means that the process (HI)∗ → H + I+ +e− takes place as soon as the dissociation limit, D0 , for the 2 5i states is exceeded. While there is a weak signal at around 13.39 eV arising from excitation and autoionization of the X 2 53/2 (v + = 17 at 13.401 eV) and (or) 2 51/2 (v + = 12 at 13.423 eV), there is no apparent photoelectron signal at 15.59 eV. Configuration interaction is significant in HI [15] and one may question the usefulness of the united atom orbital designations of the type (σ 2 π 3 ) nlλ, which we are using throughout this report. Although this notation does not describe the configurations well, it appears to be more descriptive when discussing photodissociation processes than other notations that may be more accurate [5]. In the following paragraphs the dissociation processes at 13.39 eV and 15.59 eV are discussed separately. 4.1. Excitation at 13.39 eV Given that we observe dissociation into electronically excited iodine atoms at 13.39 eV, it is proposed that the exited states of the HI parent molecule will reflect the electronic excitations of the fragments. It becomes therefore necessary to locate “low-n” Rydberg states belonging to series that converge to the first excited ionic states of HI. Previous evidence of these states is only indirect and will in this section be reviewed. The comparably high intensity of the X 2 53/2 and 2 51/2 vibrational bands in the TPES by Cormack et al. [1] and in the earlier photoionization mass spectrum of Eland and Berkowitz [16] and the photoelectron spectra of Böwering et al. [17] and Zietkiewics et al. [9] cannot be explained by the Franck–Condon overlap with the ground-state wave function. Cormack et al. proposed interactions of these vibrational states of the ion with more strongly absorbing Rydberg states. From energy considerations these workers attributed the intensity of the vibrational bands to mixing with the (A 2 6 + ) nsσ 1 6 + Rydberg states with n = 6, 7, and 8. These Rydberg states have their onset at around 10.6, 12.5, and 13.1 eV, respectively, as was estimated from the position of the A 2 6 + state with a measured series limit of 13.958 eV [1] and an estimated quantum defect of 4.00. Similarly, the position of the respective (A 2 6 + ) npπ 1 5 Rydberg states (n = 6, 7, 8) can be calculated as 11.5, 12.8, and 13.3 eV from an approximate quantum defect of 3.63 [10] and the same series limit. Finally, the (A 2 6 + ) ndπ 1 5 Rydberg states (n = 6, 7, 8) are expected to be around 13.3, 13.5, and 13.6 eV, respectively. Note that the ndπ (n = 7 and 8) energies are higher than the X 2 53/2 (v + = 17 at 13.401 eV) and 2 51/2 (v + = 12 at 13.423 eV) states and cannot, therefore, be responsible for their intensity. The A 2 6 + state, which correlates diabatically to the I+ (1 D2 ) + H(2 S) limit is known to be strongly predissociated by the 4 6 − state, leading to perturbations at the first vibrational level of the A state [1,14]. Furthermore, spin–orbit interaction with the purely repulsive 4 51/2 state yields an avoided crossing at large internuclear distance and an adiabatic correlation limit at 14.307 eV to the I+ (3 P0 ) + H(2 S) channel. Cormack et al. estimated the strength of this interaction as 0.464 eV. The potential energy curves for these states are schematically displayed in Fig. 4. As can be seen from Table 2, three different dissociation processes need to be considered. The first one, which we favor, involves excitation of a n = 6 Rydberg state and subsequent dissociation yielding iodine fragment states which correspond to (C) 6p with the state of the ion core C = (3 P2 ), (3 P1,0 ), (1 D2 ). This process would involve either multiple excitation to different 6p Rydberg states in the Franck–Condon region or extensive electronic interactions between the potential energy curves, while the 6p Rydberg electron would act as a spectator. Should only a single 6p Rydberg state be excited we would, from the potential-energy curve diagram, expect this to be the purely repulsive − ) 6p state — the corresponding ionic state of which correlates diabatically to the I+ (3 P2 ) + H(2 S) (4 61/2 ©2001 NRC Canada

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limit at 13.507 eV. This ionic state is responsible for the predissociation of the A 2 6 + state, and one can anticipate that interaction between these states is strong enough to cause an avoided crossing. Consequently, formation of fragments via the I+ (3 P0 ) + H(2 S) and I+ (3 P1 ) + H(2 S) pathways can occur through adiabatic dissociation along this state and formation of I+ (1 D2 ) + H(2 S) may be envisioned if the dissociation is nonadiabatic in the interaction region of the A 2 6 + state with the 4 51/2 state. We − propose that the dynamics following excitation to the superexcited (4 61/2 ) 6p state are identical to the dynamics of the ionic state and that the 6p electron serves only as a spectator in the dissociation process. − at around 15.5 eV to yield iodine Should this hypothesis be correct we expect excitation to the 4 61/2 3 3 1 fragment ions in their P2 , P1,0 , D2 states. This experiment is discussed below. − ) 6p state in Fig. 4 has been estimated from the known energy levels The position of the (4 61/2 of the iodine atom [10]. The energy difference between the iodine atoms (3 P0 )[1] 1/2, 6p and (3 P0 ) [1] 3/2, 6p levels and their 3 P0 ionization limit is about 18 850 cm−1 . This is roughly the same as the average difference between the iodine (3 P1 )[0, 1, and 2], 6p levels to their (3 P1 ) ionization limit at − ) 6p curve can be obtained by shifting 18 725 cm−1 . In Fig. 4 it was therefore assumed that the (4 61/2 − potential by 18 000 cm−1 or 2.25 eV. This is based on the crude assumption the corresponding 4 61/2 that the 6p electron does not interact with the excited ion core and the superexcited potential curves can be obtained by simply subtracting the electron binding energy of the 6p electron from the ionic potential curves. This difference of 2.25 eV corresponds approximately to the difference of the excitation energies 15.59–13.39 eV used in these experiments. It should also be noted that based on the electron configuration the excitation of a 6p superexcited state is not unlikely. The intermediate I 1 1(2) state can be described as having (σ 2 π 3 ) npπ configuration − from which ion core excitation to the (4 61/2 ) 6p state with configuration (σ 1 π 4 ) 6p is possible. The measured principal quantum number of the I 1 1(2) state was determined to be n∗ = 2.53 [5], which combined with an estimated quantum defect of ∼3.5 for the iodine p-series [10], leads to a configuration that may be described as a (σ 2 π 3 ) 6p configuration. Absorption of the third photon would in this picture correspond to an ion core excitation, as indicated in the scheme below. X1 6 + → {1 5(1),3 5(0+ ),3 5(1)} → I1 1(2) 5(σ 2 π 4 ) →

5(σ 2 π 3 σ ∗ )

→

(4 6 − )6p

→ 5(σ 2 π 3 )6p → 5(σ 1 π 4 )6p

In the next section the photodissociation dynamics initiated by three-photon excitation with 238.66 nm light will be described. As will be shown, the observed mechanism confirms our hypothesis of excitation − ) 6p state followed by extensive nonadiabatic interactions with the (A 2 6 + ) 6p of the repulsive (4 61/2 and (4 51/2 ) 6p states. A second possible process is in accord with the observed kinetic-energy distribution at 13.39 eV. Here iodine atomic fragments would be produced in their (3 P1,0 ) 6l states, where the atomic angular momentum l = 0, 1, and 2. These fragments may arise through excitation of a single n = 6 Rydberg state, either by simultaneous excitation of all three angular momentum components or by subsequent mixing of angular momenta. This state will then dissociate to yield fragments exclusively in their 3 P1,0 states. The only Rydberg states that fulfill these requirements are those based on the A 2 6 + ion core. They would dissociate adiabatically through the avoided crossing with the (4 51/2 ) 6l state to give H(2 S0 ) + (3 P0 ) 6l atomic states. We consider this process as not being very likely, since selection rules should favor the excitation of one or at most two angular momentum states. Selection rules prohibit excitation from the I 1 1(2) state to the [(A 2 6 + ) nsσ ] 1 6 + and [(A 2 6 + ) npσ ] 1 6 + states so that the 1 5(1) and 1 1(2) Rydberg states based on the A 2 6 + core appear as the primary candidates for the final state. These states, however, are not able to directly form atomic fragments in 6s states. From Fig. 2a it is apparent that the channel with the highest kinetic-energy release, i.e., the “H(2 S0 ) + (3 P0 ) 6s” channel in this model, is very strong. These considerations lead us to believe that this model is incorrect. ©2001 NRC Canada

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Finally, a third possible process involves formation of iodine atoms in their (3 P2 ) np states with n = 6, 7, and 8. In this process of simultaneous excitation of approximately parallel Rydberg potentialenergy curves the electronic state of the ion core is predetermined and the difference in kinetic energy arises from the excitation of the Rydberg electron. The only superexcited states that have 7p and 8p states in the vicinity of 13.39 eV are the [A 2 6 + ] nl states. This explanation is in accord with the explanation given by Cormack et al. for the intensity of the high vibrational bands of the X 2 53/2 and 25 1/2 states. The potential-energy curves of the Rydberg states, which converge to the A-state limit, are likely to have a similar shape as the A 2 6 + state itself. They interact with the corresponding Rydberg − and 4 51/2 states, but apparently not too strongly with their limiting ionic states, states of the 4 61/2 since we observed that the Rydberg electron remains attached to the iodine fragment upon dissociation. Cormack et al. proposed that at 13.39 eV the [(A 2 6 + ) nsσ ] 1 6 + (n = 6, 7, and 8) states also interact with the X 2 53/2 (v + = 17 at 13.401 eV) and (or) 2 51/2 (v + = 12 at 13.423 eV) states, leading to spin–orbit autoionization in the former case and vibrational autoionization in the latter [6]. The dissociation limit, as implied from the fragment kinetic energy, is the lowest I [(3 P2 ) np] + limit, with the (3 P2 ) np series converging to 13.507 eV. Dissociation to this limit is not possible by excitation of the (A 2 6 + ) np state alone, since the A-state correlates diabatically to the I+ (1 D2 ) + H(2 S) limit. The avoided crossing with the (4 51/2 ) np state also cannot be responsible, since the 4 51/2 ionic state diabatically correlates to the I+ (3 P0 ) + H(2 S) limit. Therefore, this dissociation mechanism − ) np state, of which the implies that the (A 2 6 + ) np state is very effectively predissociated by the (4 61/2 − 4 + 3 corresponding ionic 61/2 state indeed correlates to the lowest, the I ( P0 ) + H(2 S) dissociation limit. H(2 S)

Although there is no direct evidence against this mechanism, we believe that it does not describe the experiment well, based on three experimental observations. First and foremost, the agreement of the observed fragment kinetic energies with the energies expected from this mechanism is rather poor even when taking into account the limited kinetic-energy resolution of the apparatus. Secondly, the mechanism is not likely, based on the expected Franck–Condon overlap, which is from this mechanism supposedly similar for n = 6, 7, and 8 but is really expected to be much smaller for the lower n values. While this is in agreement with the lower intensity of the innermost ring in Fig. 2a, it will not explain the approximately equal intensities of the other two rings. Finally, the dissociation process is expected to be slow if it is caused by predissociation of the (A 2 6 + ) np Rydberg states. Dissociation processes that take place on rotational time scales are characterized by a reduced angular anisotropy of the photofragments and our findings of an angular anisotropy parameter β near its limiting value of β = 2 for a parallel transition is in disagreement with a slow, predissociative photodissociation process. The velocity-map imaging technique is well suited for recording dispersed photoelectron spectra as has been demonstrated before [18]. One may, therefore, expect that monitoring the photoelectrons released by the iodine atomic fragments will provide additional information on the energetics of the dissociation process. Unfortunately, our analysis is not limited by the resolution of the iodine kinetic energies but by the range of J -states that the iodine atoms may assume following dissociation. Since these ranges overlap for the three proposed dissociation mechanisms (see Table 2) one cannot decide on the mechanism by increasing the kinetic-energy resolution. In any case, it is doubtful that the kineticenergy resolution in the photoelectron spectra is going to be much larger compared with the iodine photofragment images, since the autoionization channels leading to HI+ in its X 2 53/2 (v + = 17) and (or) 2 51/2 (v + = 12) states, release a strong photoelectron signal that will obscure the much smaller photoelectron signal from the ionization of the iodine photofragments. A better way to verify the mechanism is to conduct photodissociation experiments at 15.59 eV to probe the dissociation dynamics of the corresponding ionic state. If the proposed mechanism was true, the dissociation of the corresponding ionic state will yield iodine photofragments in the same (3 P2 ), (3 P1,0 ), (1 D2 ) levels. ©2001 NRC Canada

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4.2. Excitation at 15.59 eV There is no indication of resonant ionization around 15.59 eV in the TPES spectrum [1], which indicates that either the state we are exciting is not coupled to an ion state or that the ionization bandwidth is so broad as to be being undistinguishable from the TPES background. However, there are eight band systems identified in the TPES spectrum above 16 eV. They all have a poor signal-to-noise ratio in the TPES [1] and also in the HeII and HeI spectra [14] and are commonly labeled A-H. With exception of the barely resolved band labeled A at around 19 eV, the dissociation limits have been identified as the H+ + I∗ limits using proton emission spectroscopy. By analogy with the problem discussed above it is tempting to propose excitation to Rydberg states converging to this band, however, the high intensity of the fragment ion signal indicates that the excited state is purely ionic. We observe formation of comparable quantities of H(2 S) + I+ (1 D1 ) and H(2 S) + I+ (3 PJ ) with J = 2, 1, 0, indicating that either multiple states are excited or extensive long-range interactions cause nonadiabatic dynamic effects. From the high spatial anisotropy parameter it can be concluded that the dissociation is fast and direct. It is instructive to consult the potential-energy curves proposed by Cormack et al. [1]. In the region − state. This state spin–orbit of 15.2–15.8 eV these workers predicted the absorption of the repulsive 4 61/2 + interacts with the bound A2 61/2 state at energies near the bottom of its well, causing predissociation of + − . The 4 61/2 state is presently poorly characterized, but predicted the lowest vibrational levels of the 2 61/2 2 + 3 to correlate diabatically to the lowest H( S) + I ( P2 ) dissociation limit [1]. If the interactions with the + state are sufficiently strong, a new set of adiabatic curves is formed, the upper one of which A2 61/2 correlates to H(2 S) + I+ (1 D1 ) and assumes the σ 1 π 4 configuration at long internuclear distance. This state, however, also may interact with the a4 51/2 state, a repulsive state with σ 1 π 3 σ ∗ [1] configuration that diabatically correlates to the closely spaced H(2 S) + I+ (3 PJ ) limits with J = 1, 0. Consequently, − state — can through extensive nonadiabatic excitation of a single excited state — the repulsive 4 61/2 interactions produce fragments via four fragment channels. The strength of the interactions has to be intermediate for effective competition of the channels to occur. Through fitting of potential-energy− and 4 51/2 states to their spectroscopic data Cormack et al. curve parameters for the diabatic 4 61/2 determined the spin–orbit coupling matrix element as 5300 cm−1 , which leads to an interaction strength of 3740 cm−1 in the crossing region at rc = 3.345 Å. Given that the branching ratio between the fragment channels obtained from our experiment is 33:27:40 for the I+ (1 D1 ):I+ (3 P1,0 ):I+ (3 P2 ) channels, it is − + state with the 4 51/2 and the 2 61/2 state are of similar apparent the interactions between the 4 61/2 magnitude. A rough estimate for the interaction strength at these two avoided crossings can be obtained from our experiment using the Landau–Zener equation [18]. Within the Landau–Zener theory and using the calculated curves of Cormack et al. to estimate the differences in slope (∼1 eV Å), we calculated the − state and the interaction strength to be 1500 cm−1 for the first interaction between the diabatic 4 61/2 2 6 + , and 2000 cm−1 for the higher lying avoided-crossing with the 4 5 state. These values provide 1/2 1/2 not much more than an order of magnitude estimate for the interaction term, because of the known shortcomings of the Landau-Zener theory [19] and the rather approximate properties of the potential energy curves. However, the results are qualitatively in agreement with the independently determined − and 4 51/2 states and therefore interaction term by Cormack et al. of 3740 cm−1 between the 4 61/2 support the dissociation mechanism. More importantly, this dissociation mechanism provides strong support for the proposed dissociation − ) 6p Rydberg state is mechanism following excitation at 13.39 eV. Here, we suggested that the (4 61/2 3 primarily excited and then dissociates to its diabatic limit to form I [( P2 ) 6p] + H(2 S) in a nonadiabatic dissociation. The avoided crossing with the (A 2 6 + ) 6p state causes a new set of adiabatic curves to be formed and opens the I[(1 D2 ) 6p] + H(2 S) channel as the diabatic limit of the (A 2 6 + ) 6p state. Finally, a second avoided crossing with the repulsive (4 51/2 ) 6p state allows for formation of the I [(3 P0 ) ©2001 NRC Canada

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6p] + H(1 S) fragments. This mechanism is in complete analogy to the one for the respective ionic states proposed above. The other two mechanisms discussed in the paragraphs above involved excitation of either a (HI+ ) np or (HI+ )6l superexcited state and would involve formation of I(3 P2 ) np; n = 6, 7, 8 and I(3 P1,0 ) 6l; l = 0, 1, 2. Excitation of the corresponding ionic states is then expected to yield iodine atoms via only a single channel, i.e., of I+ (3 P2 ) or I+ (3 P1,0 ), respectively. While this argument cannot be used to completely rule out the contributions from these channels it is apparent that they play only a minor role compared with the excitation of the (HI+ ) 6p superexcited states.

5. Summary and conclusions − Using a three-photon resonant excitation scheme at 13.39 eV dissociation of the (4 61/2 ) 6p superex3 2 cited Rydberg state into two neutral fragments, I[( PJ ) 6p] + H( S) with J = 0, 1, and 2, as well as into I[(1 D2 ) 6p] + H(2 S) has been observed. To account for the formation of the iodine fragment into its excited I [(3 P0,1 ) 6p] and I(1 D2 ) 6p states it was necessary to propose nonadiabatic interactions with the (4 51/2 ) 6p state and the A(2 6 + ) 6p state. This model of a photodissociation mechanism was tested by three-photon excitation of the corresponding ionic states at 15.59 eV and detection of the iodine ionic fragments in their respective excited electronic states. It was found that the dissociation mechanism is indeed very similar to the mechanism proposed for the superexcited state, producing iodine atomic fragments in their I+ (1 D1 ), I+ (3 P1,0 ), and I+ (3 P2 ) states. Our measurement of the photofragment branching ratio supported the dissociation mechanism and is in qualitative agreement with a more accurate determination of the spin orbit interaction term by Cormack et al. [1]. Perhaps the most intriguing result from this study is the direct observation of the formation of neutral atomic photofragments following excitation at about 3 eV above the adiabatic ionization potential and subsequent direct photodissociation. These also prevail over excitation and dissociation of HI+ (v + ), which is formed in large amounts. Despite the fact that there exist many paths for autoionization of the parent molecule, a detectable fraction of the superexcited molecules dissociates nonadiabatically into neutral atomic fragments. Clearly this is a minor process compared to autoionization, but since the experiment is very selective for the formation of neutral Rydberg atoms it is nevertheless easy to probe. Predissociation of superexcited states has been observed previously. Morin and Nenner [20] have used synchrotron radiation and photoelectron spectroscopy, for the first time, to probe the direct dissociation of core-excited HBr into neutral but highly excited fragments, which subsequently autoionized. In a number of synchrotron studies Wills and co-workers [21,22] reported similar processes. In their dispersed photoelectron study Wills et al. reported a detailed analysis of the repulsive superexcited states of HCl excited by synchrotron radiation [21]. Their innovative way of mapping the photoelectron spectra allows for a straightforward distinction between electron signals arising from the ionic states, the autoionizing superexcited molecular states, and the autoionizing atomic fragments. The latter process was further divided between processes that involve the fast dissociation of a repulsive core-excited Rydberg state of HBr and the predissociation of a bound core-excited Rydberg state by another neutral state. The results from these synchrotron studies nicely confirm our analysis of the photofragment images. In a number of photoelectron studies the measured linewidth was used to determine predissociation lifetimes [23–26]. For example, in a pulsed-field ionization – zero kinetic-energy photoelectron spectroscopy study of the A 2 6 + state Mank et al. [26] observed that the predissociation of v + = 2 and 3 levels can be followed by excitation to high-n Rydberg states converging to the A 2 6 + state. During the predissociation of the ion core the Rydberg electron follows the Br+ fragment thereby yielding a high-n atomic fragment, which is then probed by pulsed-field ionization. The authors argue that the small interaction of the Rydberg electron with the ion-core dynamics can be understood considering the much higher classical velocity of the high-n Rydberg electron compared to the Br+ recoil velocity. A similar argument can be made in the experiment described above. The terminal velocity of the recoiling iodine atom is between 73 and 165 m/s, which is very slow with respect to the orbital motion of the electron.

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It is still somewhat surprising that, even in our case, the model of a noninteracting low-lying n = 6 Rydberg electron acting as a spectator to the dissociating ion core seems to describe the dissociation dynamics well. In coherent control experiments [27,28], Zhu et al. demonstrated that the difference in molecular phase between autoionization and predissociation of a superexcited low-n Rydberg state can be exploited to control the respective branching ratio into autoionized HI parent molecules and I atomic fragments. In contrast to our study, the repulsive superexcited state was excited at 10.47 eV, i.e., just above the minimum of the X 2 53/2 ground-state curve of HI+ . For energetic reasons it was assumed that the I-atoms were formed mainly in their I [6s 2 P3/2 ] and [6s 4 P5/2 ] states, although formation of iodine atoms in their two lowest states 5p5 (2 P3/2 ) and 5p5 (2 P1/2 ) could not be excluded. The authors identified the superexcited state responsible for the predissociation tentatively as a 3 5 (0+ ) state correlating diabatically to the I [6s 2P 2 3/2 ] + H( S) limit. Despite the uncertainty in the assignment of the superexcited state this experiment confirms our observation that photodissociation of low-n superexcited states, even if the process is highly nonadiabatic, can frequently compete with autoionization processes.

Acknowledgements HPL thanks Zygmunt Jakubek and John Stone for useful comments on the manuscript, Ralph Shiell for stimulating discussions, and is very grateful for the hospitality of the Nijmegen group. The authors thank the referees for useful comments on the manuscript. Financial support from the EU-TMR network program IMAGINE ERB 4061 (PL 97-0264) is acknowledged.

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