Photoexcitation of He Hollow-Ion Resonances ... - APS Link Manager

VOLUME 93, N UMBER 19

PHYSICA L R EVIEW LET T ERS

week ending 5 NOVEMBER 2004

Photoexcitation of He Hollow-Ion Resonances: Observation of the 2s2p2 4 P State R. C. Bilodeau,1,2 J. D. Bozek,2 A. Aguilar,2,3 G. D. Ackerman,2 G. Turri,1,2 and N. Berrah1 1

2

Western Michigan University, Physics Department, Kalamazoo, Michigan 49008-5151, USA Lawrence Berkeley National Laboratory, Advanced Light Source, Berkeley, California 94720, USA 3 University of Nevada, Department of Physics, Reno, Nevada 89557-0058, USA (Received 26 April 2004; published 4 November 2004)

Highly correlated states are studied in He , a fundamental 3-electron system and prototypical negative ion. The 2s2p2 4 P state is observed for the first time. This state is detected in a resonant simultaneous double-Auger decay of unprecedented strength. In addition, the first measurements of photodetachment cross sections, positions, widths, and shapes of triply excited resonances in He are reported. These measurements provide a sensitive test for several sophisticated ab initio calculations, and indicate differences in the position and shape of some structures. DOI: 10.1103/PhysRevLett.93.193001

Electron correlation is a crucial element in the theoretical descriptions of negative ions, hollow atom and ion states, and the Auger electronic decay pathways of core excited states. Detailed studies in computationally tangible 3-electron prototype systems are of particular interest [1,2]. Hollow-ion states of He , which decay by two sequential Auger processes or by simultaneous emission of two electrons in a double-Auger (DA) process, are therefore an ideal proving ground for the latest high-level theoretical models of electron correlation. Close interaction between these theoretical models and basic experiments can elucidate processes of importance in many fields. The interpretation of cosmic spectra relies heavily on atomic and ionic data which must be obtained from experiments or state-of-the-art calculations. In addition, inner-shell ionization and hollow-atom formation are attractive for x-ray lasing efforts [3]. Triply excited (hollow-ion) states of He were first observed in the attenuation of electron beams scattered on He [4], and later also in the formation of He [5]. Triply excited states of Li have also been studied in double ionization by electron scattering experiments on Li [6]. In such studies, triply excited states appear as a small perturbation on the typically 2 to 3 orders of magnitude larger direct-ionization continuum. The selectivity of photoabsorption studies provides a more specific probe of these processes, with improved energy resolution over electron scattering experiments. However, dipole selection rules limit investigations to odd-parity 2 P0 states from the 1s2 2s 2 S ground state of 3-electron systems [1,2]. Photoexcitation of even-parity triply excited states was recently studied with sequential 2-photon excitation in Li [7], but was still limited to doublet final states. He forms the exotic, highly correlated 1s2s2p 4 P0 ground state (bound below He 1s2s by 77:5166 meV [8]), providing a unique opportunity for the study of the quartet manifold in 3-electron systems. The nuclear Coulomb attraction is efficiently screened in negative ions and correlation effects are greatly enhanced, resulting in spectra that are significantly differ193001-1

0031-9007=04=93(19)=193001(4)$22.50

PACS numbers: 32.80.Hd, 32.80.Gc

ent from those of atoms or positive ions. Photoexcitation to hollow states of the ion involves a highly correlated ground state, proving a further challenge to theoretical studies. Being a fundamental 3-electron system as well as a prototypical negative ion, He core photoexcitation has been the subject of extensive theoretical study [9–13]. A recent experimental investigation in He [14] indicated that some of the observed structure was inconsistent with predictions, and stimulated renewed theoretical interest [12,13]. Included in these new ab initio calculations were detailed investigations of hollow-ion resonances ascribing some structure, previously believed to be nonresonant, to triply excited states. The positions, widths, and cross sections of triply excited resonances present sensitive parameters for evaluating the calculations. The energy resolution and calibration precision of the previous experimental study (70 and 200 meV, respectively) is not sufficient to differentiate between the models, and absolute photoionization cross section measurements remain unavailable. In addition, while the lowest triply excited quartet state in He (the 2s2p2 4 P state) [5,15] was predicted 25 years ago [16], until now it has eluded observation, largely due to the fact that formation of quartet states is forbidden in electron scattering [9]. Similarly, this state has not been observed in photoexcitation of any 3-electron system [7,17]. This Letter reports the first observation of the 2s2p2 4 P state. Since this state lies below the 2s2 threshold [at 38.08(4) eV [18] ], sequential Auger decay is not possible and the observed decay to He must proceed via a DA decay, a highly correlated process involving all three electrons of the ion. This is the only known observation of DA decay in a photoexcited negative ion. In addition, measurements of absolute cross sections, widths, and line shapes of core-photoexcited He are reported for the first time. In particular, the 2p3s3p 4 D and 2p3s3p 4 P states are studied (approximate state assignments taken from [13]). Finally, a fourth previously unresolved feature lying above the 2p3s3p 4 P resonance is observed to be Lorentzian in shape, contrary to predictions [9–13],  2004 The American Physical Society

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which describe this feature as nonresonant structure arising primarily from the 4 D continuum. The experiments were performed using the Ion-Photon Beamline [19] at the Advanced Light Source (ALS). A National Electrostatics Corporation rf Rb-vapor chargeexchange ion source [20] is used to produce a 9.96 keV He beam in the 1s2s2p 4 P0 ground state. A 60 nA beam of He is merged with a counter-propagating photon beam from ALS BL 10.0.1. Auger decay following photodetachment or excitation of He in the merged region leads to 2-electron loss. The resulting He ions (the signal) are deflected by the demerging magnetic field and counted using a multichannel plate –based detector. The interaction length itself is defined by energy-tagging He ions produced in a 29.4 cm long chamber (biased at 0:506 kV) in the merged-beam section (see [19]). The photon beam is chopped at 6 Hz to subtract the 8 kHz background rate produced from collisional stripping with residual vacuum gas (5 1010 Torr). Finally, we note that the distribution of J levels of the metastable He 1s2s2p 4 P0 ions in the interaction region departs from statistical weighting due to differing autodetachment rates of the J 12 , 32 , and 52 levels (16, 12, and 350 s lifetimes, respectively) [21] during the transport time (6:5 s) of the ions. This does not affect the spectrum, within the LS coupling approximation assumed in the calculations [22]. The photon energy was calibrated against seven accurately known ( 1 to 5 meV) absorption lines of Ne and Ar [23] between 26.5 and 48 eV (all uncertainties are quoted to one SD [standard deviation]). The uncertainty in the calibrated lab-frame photon energy is estimated to be 3 to 7 meV. The fast He ions produce an ion-frame Doppler shift of 88.9 to 103.6 meV for photon energies of 37.6 to 43.8 eV and a beam energy of 10.46(11) keV [7104 km=s]. The total Doppler correction uncertainty is estimated to be 0.6 meV. To reduce the chances of distortion from slight changes of the ion-photon beam overlap and other experimental

parameters over time, the spectra were obtained from repeated scans of the monochromator over the energy regions of interest. In addition, all scans are scaled to the continuously monitored photon flux and He current. No significant variation was observed between individual scans. Figure 1 presents measurements over photon energies near the 2s2p2 4 P state with the monochromator slits set for a nominal spectral bandwidth of 6 meV. The profile shape is essentially Lorentzian, consistent with the predicted large negative shape parameter (174:64 [10] and 171:35 [12]) for total photoexcitation. The resonance was fit to a Voigt profile, allowing a linear background (best fit consistent with 0) and a variable bandwidth [best fit 5:58 meV]. A line center and width of, respectively, 37:6687 eV and 9:720 meV were obtained. Calculations compare favorably with this measurement, although R-matrix calculations [11,13] tend to underestimate the binding energy (see Table I). The resonance was also measured at 35 meV bandwidth. The same line center was obtained within a statistical uncertainty of 1 meV, which verifies that the monochromator energy has no dependence on bandwidth. Finally, the direct 2-electron photodetachment background was estimated to be 0.1(3) Mb. Absolute cross sections were obtained at 4 photon energies (see Table II), using a method similar to previous experiments with this apparatus [19]. Given the signal rate R, photon flux , and the target-ion current I, velocity v, and charge q, the cross section can be obtained with qvR=IF; where the form factor F is a measure of the quality of the overlap between the ion and photon beams. F is determined by integrating the quadratic interpolation of 2-D form factors (measured at three positions) over the interaction length. The interpolation was estimated to be accurate to 1:5%. The main instrumental uncertainties in the cross section measurements arise from the photon flux determination ( 7:3%), the signal (He ) collection and detection

TABLE I. Measured positions and widths of triply excited states compared with recent calculations. is the difference between calculated and measured line centers. Feature 2s2p2 4 P

FIG. 1. Measurement of double-Auger decay from the 2s2p2 4 P state. The curve is the best-fit profile. The solid circle is the measured absolute cross section.

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Position [eV] Meas. Theory 37.668(7) 37.685 37.669 37.703 37.669

0.017 0.001 0.035 0.001

Width [meV] Meas. Theory Ref. 9.7(20)

10.8 9.66 9.74 9.85

[13] [12] [11] [10]

2p3s3p 4 D 42.972(4) 42.980 0.008 42.985 0.013

29(5)

35.0 34.3

[13] [12]

2p3s3p 4 P 43.322(4) 43.332 0.010 43.330 0.008 43.370 0.048

12(3)

14.0 13.8 14.4

[13] [12] [11]

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TABLE II. Measured absolute cross sections, reported to one SD. h is the bandwidth used in the measurement. Photon energy [eV]

h [meV]

37.668(7) 37.668(7) 42.969(4) 43.160(4) 43.536(4)

5.5(8) 35(5) 35(5) 35(5) 35(5)

Cross Section [Mb] Measured Corrected 579 8 244 3 275 3 3:66 5 5:38 7

6712 10 417 6 3:66 5 5:59 8

efficiency (9:2%=8:7%), the primary ion (He ) collection and detection efficiency (2:1%=2:9%), and the form factor measurement and integration (9:6%= 5:8%). Errors are added in quadrature, for a total instrumental error of 15:3%=13:1%. An additional uncertainty of 1:5 to 5% is obtained from signal counting statistics. To verify that all random and long-term drift effects have been accounted for, two sets of 12 measurements were collected at 37.668 eV (5.5 meV bandwidth). The sets each took 1 h to collect and were separated by 24 h. The 24 individual measurements were statistically scattered, and no trends were discernible within or between the two sets. A further uncertainty in the cross section measurements arises from the finite bandwidth. Measurements taken at energies where the spectrum is flat over a region larger than the bandwidth are representative of the true cross section. However, on or near resonances, an average of the actual cross section is measured, weighted by the spectral profile (reported under the ‘‘Measured’’ column in Table II). For measurements taken at the peak of a resonance, the effect of a Gaussian spectral profile was modeled to determine the natural-width cross section; these are listed in the Table II as ‘‘Corrected’’ with an additional error stemming from the uncertainty in the bandwidth. As a test, measurements at two bandwidths were performed at the 2s2p2 4 P line center. The corrected cross sections obtained are in agreement: 66(4) and 73(9) Mb for 5.5 and 35 meV bandwidths, respectively (excluding systematic errors), giving a weighted average of 6712 10 Mb (including systematic errors). For the 2s2p2 4 P state, only the DA decay cross section is observed. With DA decay generally expected to be very weak (cross sections 1 Mb [2,24,25]), the measured 67 Mb cross section is unprecedented. Assuming 600 Mb total peak cross section for this state (e.g., 581 [12] and 620 Mb [10]), DA accounts for 11% of the decay. This is significantly larger than other light systems [24], such as Li 2s2 2p2 P0 for which Simons and Kelly [25] and Wehlitz et al. [2] calculate ratios of 6.7 and 3.3%, respectively. Figure 2 shows the energy region where the triply excited 2p3s3p 4 P and 2p3s3p 4 D resonances are expected, well above the 2s2 threshold. A 37 meV nominal 193001-3


spectral bandwidth was used here, with an actual bandwidth of 35(5) meV determined from fits to the observed He resonances, including the narrow 2s2p2 4 P resonance. The CSCI calculation by Sanz-Vicario et al. [12] is also shown for comparison. (Note that these resonances decay almost exclusively via sequential Auger decay to He [12 –14], making this comparison appropriate.) Good agreement is observed over most of the spectrum, with the notable exception of the 2p3s3p 4 D resonance amplitude. Measurements of the higher-energy region with improved statistics show that while the amplitude and position of the feature agree with predictions, the asymmetric shape predicted by the calculations is not observed. The best-fit Voigt profile to these data, with the Gaussian component fixed to 35 meV and a constant background, yields a center and width of 43.537(4) and 0.100(12) eV, respectively. While a Lorentzian line shape is suggestive of a resonance, calculations [12,13] conclude that this feature is nonresonant structure originating primarily from the 4 D continuum. Higher resolution scans of (a) the 2p3s3p 4 D and (b) the 2p3s3p 4 P resonances (20 and 10 meV nominal bandwidths, respectively) are shown in Fig. 3. The observed symmetric (a) or asymmetric (b) profiles are expected from computed line-shape parameters [10,12]. Leastsquare fits using Voigt profiles (a) or Fano profiles [26] convoluted with Gaussian spectral profiles (b) were performed. Consistent results were obtained for fits with the bandwidth fixed to the nominal value, or allowed to vary [with best-fit values of 18(6) and 10(3) meV for (a) and (b), respectively]. For the 2p3s3p 4 P resonance, a Fano

FIG. 2. Double photodetachment observed with 35(5) meV bandwidth near the triply excited (a) 2p3s3p 4 D and (b) 2p3s3p 4 P resonances. The broken curves are calculations by Sanz-Vicario et al. [12], convoluted with the experimental bandwidth. Inset: Higher-statistics scan (magnified by a factor of 6) showing a third feature (c); the solid curve is the best-fit profile. The data were scaled to three measured absolute cross sections (filled circles). Error bars are one SD.

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We would like to thank B. S. Rude for timely assistance during the experiment and C.W. Walter, N. D. Gibson, J. L. Sanz-Vicario, O. Zatsarinny, T.W. Gorczyca, A. Mu¨ller, M. Pindzola, and K. T. Chung for valuable conversations and correspondence. This work was supported by DoE, Office of Science, BES, Chemical, Geoscience and Biological Divisions. A. L. S. is funded by DoE, Scientific User Facilities Division.

FIG. 3. Scans of the (a) 2p3s3p 4 D and (b) 2p3s3p 4 P states at bandwidths of 20 and 10 meV, respectively. Thin curves following the data are best fits to the features (see text for details). Theory by Sanz-Vicario et al. [12] is shown (broken curve) and convoluted with the bandwidth (thick curve). The dotted line indicates the noninterfering continuum level.

line-shape parameter of 6:420 is obtained, close to the Sanz-Vicario et al. CSCI value of 10:41 [12], but significantly different than the B-spline multiconfiguration Hartree-Fock (MCHF) result of 14:52 by Xi and Fischer [10]. While the predicted widths of these lines are consistent with the measurements (see Table I), calculations underestimate the binding energy of these states by 2 SDs. Some systematic effects in both the experiment and calculations can be eliminated by considering the 2s3s3p 4 P–4 D term splitting. The measured 350.6(23) meV splitting agrees with the R-matrix value of 352 meV by Zatsarinny et al. [13], but is larger than the CSCI value of 345 meV [12]. In summary, we have measured the He 2s2p2 4 P state for the first time, and observed the unexpected large DA cross section of 6712 10 Mb. The absolute cross section measurement for DA decay (as yet unavailable for any other 3-electron system) offers a tangible target for future theory. We hope these measurements will help initiate theoretical investigations of DA decay in negative ions and further studies of 3-electron systems in general. The exceptionally large excitation strength of this state also makes it a promising target for studies of correlated electron emission from DA decay in 3-electron systems. In addition, the line widths and shapes of the 2s2p2 4 P, 2p3s3p 4 D, and 2p3s3p 4 P resonances were found to compare well with theory, although the states appear to be more strongly bound. Finally, a feature just above the 2p3s3p 4 P resonance was well resolved for the first time, and displayed a significantly different shape than predicted. We hope these measurements will stimulate further calculations of the 2p3s3p resonance region. 193001-4

[1] L. M. Kiernan, E. T. Kennedy, J. P. Mosnier, J. T. Costello, and B. F. Sonntag, Phys. Rev. Lett. 72, 2359 (1994); Y. Azuma et al., Phys. Rev. Lett. 74, 3768 (1995). [2] R. Wehlitz, M. T. Huang, K. A. Berrington, S. Nakazaki, and Y. Azuma, Phys. Rev. A 60, 17(R) (1999). [3] M. A. Duguay and M. Rentzepis, Appl. Phys. Lett. 10, 350 (1967); K. Moribayashi, A. Sasaki, and T. Tajima, Phys. Rev. A 58, 2007 (1998), and references therein. [4] C. E. Kuyatt, J. A. Simpson, and S. R. Mielczarek, Phys. Rev. 138, A385 (1965); U. Fano and J.W. Cooper, Phys. Rev. 138, A400 (1965). [5] J. J. Que´me´ner, C. Paquet, and P. Marmet, Phys. Rev. A 4, 494 (1971). [6] A. Mu¨ller et al., Phys. Rev. Lett. 63, 758 (1989). [7] D. Cubaynes et al., Phys. Rev. Lett. 77, 2194 (1996). [8] P. Kristensen, U.V. Pedersen, V.V. Petrunin, T. Andersen, and K. T. Chung, Phys. Rev. A 55, 978 (1997). [9] D. S. Kim, H. L. Zhou, and S. T. Manson, J. Phys. B 30, L1 (1997); Phys. Rev. A 55, 414 (1997). [10] J. Xi and C. F. Fischer, Phys. Rev. A 59, 307 (1999). [11] H. L. Zhou, S. T. Manson, L. Vo Ky, A. Hibbert, and N. Feautrier, Phys. Rev. A 64, 012714 (2001). [12] J. L. Sanz-Vicario and E. Lindroth, Phys. Rev. A 65, 060703(R) (2002); N. Brandefelt and E. Lindroth, Phys. Rev. A 65, 032503 (2002); J. L. Sanz-Vicario, E. Lindroth, and N. Brandefelt, Phys. Rev. A 66, 052713 (2002). [13] O. Zatsarinny, T.W. Gorczyca, and C. Froese Fischer, J. Phys. B 35, 4161 (2002). [14] N. Berrah et al., Phys. Rev. Lett. 88, 093001 (2002). [15] K. T. Chung, Phys. Rev. A 51, 844 (1995). [16] K. T. Chung, Phys. Rev. A 20, 724 (1979). [17] The Li 2s2p2 4 P state was inferred from Li gas-target collisions: K. T. Chung, Phys. Rev. A 25, 1596 (1982); M. Rødbro, R. Bruch, and P. Bisgaard, J. Phys. B 12, 2413 (1979). [18] P. J. Hicks and J. Comer, J. Phys. B 8, 1866 (1975). [19] A. M. Covington et al., Phys. Rev. A 66, 062710 (2002). [20] J. H. Billen, IEEE Trans. Nucl. Sci. 28, 1535 (1981). [21] T. Andersen et al., Phys. Rev. A 47, 890 (1993). [22] O. Zatsarinny and T.W. Gorczyca (private communication); J. L. Sanz-Vicario (private communication). [23] K. Schulz, M. Domke, R. Pu¨ttner, A. Gutie´rrez, and G. Kaindl, Phys. Rev. A 54, 3095 (1996); R. P. Madden, D. L. Ederer, and K. Codling, Phys. Rev. 177, 136 (1969). [24] M. S. Pindzola and D. C. Griffin, Phys. Rev. A 36, 2628 (1987). [25] R. L. Simons and H. P. Kelly, Phys. Rev. A 22, 625 (1980). [26] U. Fano and J.W. Cooper, Phys. Rev. 137, A1364 (1965).

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