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below the double ionization potential

This content has been downloaded from IOPscience. Please scroll down to see the full text. 1997 J. Phys. B: At. Mol. Opt. Phys. 30 2177 (http://iopscience.iop.org/0953-4075/30/9/018) View the table of contents for this issue, or go to the journal homepage for more

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J. Phys. B: At. Mol. Opt. Phys. 30 (1997) 2177–2186. Printed in the UK

PII: S0953-4075(97)78092-1

Double photoionization of H2 S below the double ionization potential J H D Eland†, P Lablanquie‡, M Lavollée‡, M Simon ‡§, R I Hallk, M Hochlafk and F Penentk † Physical and Theoretical Chemistry Laboratory, Oxford University, Oxford OX1 3QZ, UK ‡ LURE, Université Paris-Sud, 91405 Orsay, France § SPAM/CEA/CEN Saclay Bt522, 91191 Gif sur Yvette, France k LDMA, CNRS and Université P et M Curie, 4 place Jussieu T12-B75, 75252 Paris, France Received 18 September 1996, in final form 14 February 1997 Abstract. Three separate experiments demonstrate that an indirect process of double ionization can populate part of the potential energy surface of the H2 S2+ ion at energies below the lowest bound vibrational level. The process seems to involve autoionization at a curve crossing of a singly ionized (or possibly neutral) state with a repulsive part of the adiabatic ground state doubly charged ion surface.

1. Introduction Double ionization, the ejection of two electrons by a single photon, is strictly forbidden as a one-step process in the single-particle frozen orbital approximation. The fact that the phenomenon is observed with significant intensity for all atoms and molecules is due in part to electron correlation, but in larger measure to the occurrence of two-step or multi-step ionization processes such as the Auger effect. Many detailed experiments have demonstrated the importance of indirect pathways even for the rare gas atoms ionized at energies near threshold [1]; in many cases electron correlation is implicated in formation of the intermediate states. The elegant Wannier law, which describes the effect of electron correlation in direct double ionization near threshold and has been the starting point of innumerable researches [2], is applicable only in specially selected cases. In the normal Auger effect, whether initial electron ejection is from an inner shell or from the outermost shell, both steps of the indirect mechanism involve only electron motions. In some molecular systems it has been found, however, that nuclear motion may intervene between two electronic transitions. Such phenomena were first observed in HBr [3] then in other hydrides including H2 S [4, 5], and recently in O2 [5a]. In all these cases the first process is excitation or ejection of a core electron; the molecule then begins to dissociate before an electron leaves in an autoionization step. It has recently been suggested specifically [6] that in core-excited H2 S neutral superexcited states dissociate before one of the fragments ionizes. It has also been shown that dissociation intervenes before valence-shell autoionization of O2 [7] and in double ionization of O2 [8] and CO [9, 10]. These examples are all evidence of the general problem of failure of the separation of nuclear and electronic motions, which is illustrated for example under conditions as various as Penning ionization [10a], dissociative electronic attachment [10b], collisions with negative ions or protons [10c] and neutral dissociation of c 1997 IOP Publishing Ltd 0953-4075/97/092177+10$19.50

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superexcited states [10d]. In this paper we present evidence that a two-step double ionization with intervening nuclear motion occurs in the valence shell of H2 S and has the effect of allowing double ionization below the double-ionization potential as normally defined. Double ionization of molecules always produces at least three charged particles (m++ + 2e) in the final state (and often more). Coincidence methods are indispensable for a full characterization of such reactions and accordingly, the experiments reported here are all of this type. Two electrons of known (zero) energy are detected in coincidence in the threshold photoelectrons coincidence (TPEsCO) technique; their abundance as a function of the energy of the ionizing light is a spectrum of energy transfer in double photoionization. This is a spectrum of nascent doubly charged ions produced, here H2 S2+ , in which the intensities of different states and levels are governed by the special requirement to produce two zero-energy electrons. The yield of stable (or metastable) H2 S2+ ions as a function of the light energy is determined using photoionization mass spectrometry, and gives a separate measure of the double-ionization potential. Most of the incipient H2 S2+ ions (or their precursors) actually dissociate to yield product ion pairs H+ + HS+ , H+ + S+ or + H+ 2 +S . The mechanism of formation of these pairs and their abundance as functions of the light energy are investigated by the coincidence technique photoelectron-photoion–photoion coincidence (PEPIPICO). Previous work on the double ionization of H2 S has used the Auger technique [11, 12], double-charge exchange [12] and an earlier form of photoion–photoion coincidence PIPICO [13]. From high-resolution Auger spectroscopy it is known that the ground state of H2 S2+ is bound and supports at least four vibrational levels with a spacing of about 250 meV. The double-ionization potential is at 31.7 eV (adiabatic value [12]). The inner valence photoelectron spectrum excited by x-rays has been studied by Adam et al [14], and shows some interesting structures in the energy region covered in the present work. 2. Experimental methods In the TPEsCO experiment, ionization occurs where an effusive gas beam crosses a beam of wavelength-selected synchrotron radiation from a positron storage ring. Electrons are extracted using the penetrating field technique of Cvejanovic and Read [15], which provides high sensitivity and selectivity for electrons of zero kinetic energy. Pairs of electrons are registered as separate pulses at a three-element microchannel plate (MCP) detector, and their yield is recorded as the light wavelength is scanned. Details of the technique have been given elsewhere [16a, b]. The experiments reported here were done at beamline SA31 of SuperACO at LURE, whose characteristics have been published [17]. The basic apparatus common to the mass spectrometry and PEPIPICO experiments is a time-of-flight (TOF) mass spectrometer in whose source region gas molecules in an effusive jet are ionized where they meet a light beam. Some of the experiments reported here were done with monochromatized synchrotron radiation, others using individual selected lines from gas discharges in helium or neon. Mass spectra, recorded as TOF distributions with time zero provided by the detection of a photoelectron, are recorded in conjunction with all the experiments. PEPIPICO spectra are two-dimensional TOF mass spectra accumulated by the detection of pairs of coincident ions, also timed from a photoelectron detection. These techniques have been described before [18, 19]. H2 S was a commercial product of adequate purity, used as received. For work on the hydrogen isotopomers HDS and D2 S, samples were synthesized by the reaction of D2 O or H2 O/D2 O mixtures with Al2 S3 . The spectrometer inlet system had to be conditioned for several days with D2 O before spectra of pure D2 S could be taken.

Indirect double ionization of H2 S

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Figure 1. TPesCO spectrum (lower part) and TPES spectrum (upper part) of H2 S over the energy range 28–40 eV. The photon resolution was around 30 meV. Vertical lines on the upper figure indicate the position of Rydberg series of singly ionized states converging towards the ground state of the doubly charged ion (see text).

3. Results The TPEsCO spectrum of H2 S is shown in figure 1 together with the simultaneously measured threshold (single) photoelectron spectrum (TPES) over the energy range 28– 37 eV. They were measured with our first experimental set-up [16a] which is somewhat less effective in the rejection of energetic electrons than our new set-up [16b]; however, a new measurement with this new set-up gave exactly the same results; the lower statistics we reached fully confirmed the features presented in figure 1 and justify that we do not present here the two sets of measurements. In the single-electron spectrum, intensity dies away slowly in this region as the energy increases (note the expanded intensity scale), and there are some structures below the onset of the two-electron signal. In the two-electron signal there is a strong continuous structure starting at about 28.4 eV, with two sharp peaks at 31.74 and 31.98 eV visible upon it. From the Auger and charge exchange results, the sharp peaks can be identified at once as the ground and first excited vibrational level of triatomic H2 S2+ . The magnitude of the interval between the two peaks, together with the usual selection rule (totally symmetric modes only) allows it to be identified as one quantum of the symmetric stretching vibration ν1 . The adiabatic double-ionization potential of H2 S is thus determined as 31.74 ± 0.02 eV.

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Figure 2. Partial PEPIPICO spectra of H2 S at different photon energies. Successive contours represent changes in intensity by a factor of two within each spectrum. The shapes of the peaks unambiguously identify the H+ /HS+ peaks at all wavelengths as two-body charge separation reactions.

In the single-electron TPES spectrum there is one very prominent pair of peaks at 29.10 and 29.36 eV with the same spacing as the vibrational levels of the doubly charged ions. Several other similar pairs of peaks can be seen, the most prominent group having first members at 28.0, 29.1, 29.95 and 30.44 eV. It seems natural to interpret these as members of a Rydberg series of singly ionized states converging towards the ground state of the doubly charged ion. The effective principal quantum numbers are 3.8, 4.54, 5.53 and 6.47. The apparent quantum defects are satisfactorily constant for the higher members and suggest a series with low l in the active electron (s or p series). It is notable that none of the peaks seen in the single-electron TPES appear in the two-electron double-ionization channel. Above the energy of the sharp lines the double-ionization spectrum shows a strong, structureless peak. This peak may represent double ionization to purely repulsive higher states of H2 S2+ , of which there are several in this energy range, or population of an unbound part of the ground state. Below the sharp lines there is also significant intensity in double ionization; this can only be dissociative double ionization. The thermodynamic threshold for formation of the fragments H+ + HS+ is 27.9 eV, below the onset of threshold electron pair formation at 28.4 eV. To confirm the reality of the low-energy dissociative double ionization and to investigate its nature we turn to the other coincidence techniques. In figure 2 partial PEPIPICO spectra of H2 S are shown at three photon energies, two above the ionization potential and the other below it; the lowest photon energy, which is that of a line in the emission spectrum of neon, is marked in figure 1. A peak, characteristic of the formation of the H+ + HS+ ion pair, is clearly visible at the lowest photon energy, proving that ionization into the product channel does indeed take place below the adiabatic double-ionization potential. The intensity is very low at the lowest photon energy, however; relative intensities at two wavelengths and for the three hydrogen isotopomers are given in table 1. Intensities in the different double-ionization channels are expressed relative to the total intensity of single ionization forming S+ , HS+ and H2 S+ (or isotopomers), which are thought to vary only slowly with energy in this energy range; the data have not been corrected for bias in the electron detector, which favours low-electron energies. The relative intensity of the ion-pair channel at different photon energies was also measured by the PEPIPICO technique, using synchrotron radiation as an excitation source; the results, corrected for an estimation of electron and ion detection efficiencies, are shown graphically in figure 3. The relative intensity of the stable H2 S2+ product is rather difficult to measure

Indirect double ionization of H2 S Table 1. Abundances (percentage of ionization potential. H2 S

P

2181 (H2 S+ + HS+ + S+ ) above and below the double-

D2 S

HDS

Wavelength

H+ /HS+

D+ /DS+

H+ /DS+

D+ /HS+

HDS2+

˚ 304 A (40.8 eV) ˚ 407 A (30.5 eV)

22.1 ± 1

27 ± 2

38 ± 4

13.8 ± 2

9.6 ± 1

Ratio

0.40(4)

0.55(2)

0.80(9)

0.24(6)

—

0.018

0.020

0.021

0.017

—

Table 2. Ion pair abundances as a percentage relative to ˚ Wavelength (A)

H+ /HS+

H+ /S+

+ H+ 2 /S

256 304 326 356 407

26.4 22.1 15.4 5.1 0.4

28.5 8.9 2.4 — —

2.0 1.3 0.7 0.15 —

P

(H2 S+ + HS+ + S+ ).

because the mass spectral peak coincides with a peak for the OH+ ion from the inevitable water background or impurity. Only in the HDS isotopomer is the doubly charged parent ion peak (m/z 17.5) measurable without interference, but it is reasonable to assume that the intensity of the OH+ fragment from water does not vary rapidly in this energy range. Measurements on the mass 17 ion from H2 S at different wavelengths with constant OH+ intensity subtracted gave the relative yield of the stable doubly charged H2 S2+ parent ion shown in figure 4; the curve extrapolates to an onset near 31.7 eV in exact agreement with the position of the resolved TPEsCO peaks. The yield curves for parent doubly charged ion and for fragment ion-pair formation are put on a common intensity scale by using the estimated value of the ion detection efficiency, which was about 30% in all experiments; however, error bars given by this procedure can be as large as 30%. In contrast to the H2 S2+ ion, the HS2+ fragment, which is known to exist as a stable ion [20, 21], should be easily visible at m/z 16.5 in the mass spectra. In fact this ion is seen only at high photon energy, when a sulphur 2p inner-shell electron can be excited or ionized [22]. The PEPIPICO peak shape for H+ + S+ suggests that at lower excitation energies, a very short-lived HS2+ intermediate may be involved, but this is not confirmed. The relative yield curves (figures 3 and 4) and the data for HDS (table 1) show that at photon energies well above threshold, stable doubly charged ions are only roughly onefifth as abundant as cation pairs. This means that most of the states populated by double photoionization are repulsive or fully predissociated. If barrier tunnelling were involved in the dissociations either above or below threshold, different isotopomers should show different relative yields of stable and dissociative ions. No such differences are apparent in the data of table 1. The relative yields of sharp peaks and continuum in the TPEsCO spectrum cannot be compared with the ion yields because the production of zero-energy electron pairs is only one small part of the total double ionization. A third quantity obtainable from PEPIPICO and PIPICO spectra is the kinetic energy

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Figure 3. Yield of the ion pairs H+ + HS+ as a function of photon energy, expressed as a percentage of the yield of singly charged H2 S+ ions. Error bars represent statistical errors deduced from the mass spectroscopy measurements only, and do not include the 30% uncertainties associated with the estimate of the ion detection efficiency (see text). Open squares represent the threshold zone magnified by a factor of 15: the low signal around 26 eV is attributed to residual second-order light, the data suggest a threshold for H+ + HS+ production at 28.5 ± 1 eV.

Figure 4. Intensity of doubly charged H2 S2+ ions as a percentage of the H2 S+ ion signal at different photon energies. A constant OH+ background has been subtracted from the raw observed signal at m/z 17. Error bars represent statistical errors deduced from the mass spectroscopy measurements.

release in ion-pair formation, which is related to the peak shapes, particularly the widths. Because of the low mass of hydrogen, which produces a small width, the energy release cannot be measured very accurately for H+ + HS+ pairs and the kinetic energy distribution


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Table 3. Kinetic energy releases (eV). ˚ Wavelength (A)

H+ /HS+

256 304 326 356 407

5.4(4) 5.2(4) 3.6(5) 2.9(4) 1.6(3)

D+ /DS+

H+ /DS+

D+ /HS+

5.3(3)

5.3(4)

5.2(5)

2.1(3) 1.6(3)

1.5(4)

+ H+ 2 /S

+ D+ 2 /S

5.2(5) 4.8(5)

(KERD) cannot be determined at all. For D+ + DS+ the kinematic factors are more favourable; the average energy releases are more precise, and some indication of the KERD can be given. Results are collected in table 3. There is no detectable difference in kinetic ˚ where the KER is apparently energy release (KER) between the isotopomers except at 356 A, slightly higher in H2 S than in D2 S. At the lowest photon energy where we have good data, ˚ below the double ionization potential of H2 S, the average energy release at 30.5 eV (407 A), is 1.6 eV with a spread of about 0.5 eV. This is about half the maximum available energy (photon-energy threshold) and is considerably less than the energy release at higher photon ˚ and above the principal energy release energies. At all photon energies of 40.8 eV (304 A) + is constant at about 5.2 eV, with a rather narrow distribution. In the formation of H+ 2 +S and its isotopomers, which is measurable only at the higher photon energies, the kinetic energy release is 5.2 ± 0.5 eV. 4. Discussion The form of the TPEsCO spectrum of H2 S (figure 1) is rather surprising; not only does it show intensity below the double-ionization potential but it also fails, except in locating the dication ground state, to resemble either the Auger spectrum or the double-charge transfer spectrum of H2 S. The peculiar and largely unknown but presumably indirect mechanism by which zero-energy electron pairs are formed must be held responsible. To discuss the dissociative double-ionization processes, one needs the correlation diagram; an attempt to construct this is shown in figure 5. The ground state of H2 S2+ , 1 A1 , which has a double hole in the outermost 2b1 orbital, does not correlate adiabatically to the ground-state products (H+ (1 S) + HS+ (3 6 − ) which form only a triplet surface [21]. Instead it correlates adiabatically to HS+ (1 1), which is about 1.2 eV higher [12]; similarly, + adiabatic correlations in the C2v direction to H+ 2 + S or to atomic products also go to excited states in each case. The first excited state of the H2 S2+ ion, which is 3 B1 according to theory and interpretation of the Auger spectrum, is the one which can correlate to groundstate products adiabatically. Contrasting diabatic correlations, based on orbital correlation, probably connect the ground state of H2 S2+ to the first limit for H + HS2+ , rather than to any limit for H+ + HS+ , because this is the only way the products can have a configuration which retains a double vacancy in the out-of-plane sulphur p orbital. The potential barrier to deprotonation of H2 S2+ that allows vibrational structure and stable doubly charged ions to exist has been calculated to be about 2 eV [21] and was attributed to an avoided crossing between the attractive diabatic surface and the repulsive surface from the cation pair. The dissociative processes that occur at high photon energy seem, according to the unchanging kinetic-energy releases, to be dominated by single pathways which we can try to interpret using the correlation diagram. If the dissociation to H+ + HS+ goes through

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Figure 5. Correlation diagram for H2 S2+ . Positions of the asymptotes have been calculated from thermodynamic data and ionization potentials. Adiabatic correlations are shown as full curves, while dotted and broken curves represent suggested diabatic correlations. It is not clear whether a separate correlation to atomic products as well as to H2 + S species is permissible in the C2v direction.

the dication ground state and follows the adiabatic path, the central energy at which the FC zone intersects the H2 S2+ surface can be estimated from the kinetic energy release as 27.9 + 5.2 + 1.2 = 34.3 eV. This energy matches the main maximum in the TPEsCO curve and suggests that the vibrational peaks and the intensity maximum may represent transitions to the same surface. The apparent vibrational FCFs do not support this idea, however. Cesar et al [12] have reported limited molecular orbital calculations on H2 S2+ which indicate that the bond angle is almost unchanged by comparison with neutral H2 S, ˚ longer. An unchanged bond angle explains why there but the H–S bonds are about 0.1 A is no apparent excitation of ν2 in the TPEsCO spectrum or in the Auger spectrum. To explain the formation of threshold electron pairs and charge-separated dissociation products at energies below the lowest bound state of H2 S2+ we must postulate the formation of an intermediate state of the neutral or singly ionized molecule that can be populated over a range of energies and can transfer its population to the doubly ionized products. The absence of isotope effects suggests that a ‘single’ curve crossing is involved. The rather narrow KERD at 30.5 eV implies either a substantial non-zero electron energy, which would leave the TPEsCO unexplained, or strong excitation in the products. If HS+ is produced in the 1 1, state the observation of both ionic products and threshold electrons at this photon energy is naturally explained. In support of this idea, the yield of threshold electron pairs starts at 0.5 eV above the H+ + HS+ (3 6 − ) onset, but its first main rise corresponds to the threshold for H+ + HS+ (1 1) at 29.1 eV. The low KER means that the final electron ejection must take place at a long range, with ˚ as the nuclei may already have some velocity an internuclear distance greater than 9 A, when they get to the position where the full charge separation is developed. Either just one or both electrons may be ejected at this point, but in either case the intermediate must


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persist for some time. It takes 100 fs–0.5 ps even for a hydrogen atom to get to a distance ˚ We can consider the following two separate hypotheses. of 9 A. (1) Prompt ejection of one electron, deferred ejection of the second. (2) Ejection of both electrons at long range after nuclear separation. For hypothesis (1), a repulsive intermediate state of H2 S+ is required, whose potential energy surface crosses the ground-state H2 S2+ surface at long range in the H–SH bond coordinate so that a zero-energy electron can be ejected. The intermediate ion state could be a Rydberg or doubly excited state with a dication core, which would begin to dissociate into two cations; the heavier cation then captures an electron on its way out yielding H+ + HS∗ . Later, at a distant curve crossing, the excited fragment must autoionize, often by ejecting a zero-energy electron. Since not all curve crossings necessarily lead to autoionization it would be interesting to look for confirmation of this model in the form of superexcited fragments, e.g. in emission studies of H2 S. Unfortunately the emission studies carried out hitherto [23] do not extend to short enough excitation wavelengths. From comparison with the TPES, it seems that the Rydberg states converging to the bound levels of H2 S2+ are not involved, but an unbound Rydberg state may be. There is a weak bump in the TPES at the same energy as the main peak in the TPEsCO spectrum, but there is a stronger broad peak (or shoulder) at the same energy (30 eV) as the shoulder on the TPEsCO curve below the bound levels. There is an intense peak at this same energy in the x-ray excited inner valence photoelectron spectrum [14]; both spectra point to a strong candidate as the singly ionized intermediate state. For hypothesis (2) a neutral intermediate state is needed. It must be populated over a range of energies and then undergo two successive or simultaneous autoionizations giving threshold electrons. If both crossings are missed, superexcited neutral products should be left and the quantum yield of ionization should be less than one (when the fraction of double ionization has been allowed for). This mechanism is not ruled out, but there is less supporting evidence for it than for mechanism (1). 5. Conclusions Although many other possibilities remain open, the most natural explanation for the dissociative double ionization of H2 S below the normal double-ionization threshold seems to be as follows. Over a range of energies near 30 eV a highly excited and mainly repulsive state of H2 S+ is formed, and is seen in both the threshold photoelectron and normal photoelectron spectra. The ions in this state dissociate by HS∗ –H+ bond extension, ˚ HS∗ autoionizes to HS+ (1 1) releasing a but at a particular distance near or above 9 A, low-energy electron. Acknowledgments We thank the staff of LURE for their help and assistance. JHDE particularly thanks the Université P et M Curie for hospitality when this paper was being prepared. References [1] Schmidt V 1992 Rep. Prog. Phys. 55 1483 Lablanquie P 1995 J. Elecron Spectrosc. Relat. Phenom. 76 63 [2] Mazeau J, Andric L, Jean A, Lablanquie P, Selles P and Huetz A 1996 Atomic and Molecular Photoionization ed A Yagishita and T Sasaki (Tokyo: Universal Academy) p 31

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