Threshold photoelectron spectroscopy of HBr and DBr

Chemical Physics 238 Ž1998. 133–151

Threshold photoelectron spectroscopy of HBr and DBr A.J. Yencha

a,)

, A.J. Cormack

b,1

, R.J. Donovan b, K.P. Lawley b, A. Hopkirk c , G.C. King d

a

d

Department of Chemistry, State UniÕersity of New York at Albany, Albany, NY 12222, USA b Department of Chemistry, The UniÕersity of Edinburgh, Edinburgh EH9 3JJ, UK c CLRC Daresbury Laboratory, Daresbury, Warrington WA4 4AD, UK Department of Physics and Astronomy, Schuster Laboratory, Manchester UniÕersity, Manchester M13 9PL, UK Received 3 October 1997

Abstract The threshold photoelectron spectra of HBr and DBr have been recorded with varying resolution Ž3–20 meV. over a wide photon energy range Ž11.5–31.5 eV. using synchrotron radiation and a penetrating-field electron spectrometer, encompassing both the outer- and inner-valence ionization regions. Extensive vibrational structure has been observed in the XŽ2 P i . band systems for both isotopomers in the Franck–Condon gap region, between the XŽ2 P i . and AŽ2 Sq . states, that is attributed to the resonance autoionization of 5ss, 6ss and 7ss 1 Sq Rydberg states. Resonance autoionization is also found to play a role in the vibrational bands within the AŽ2 Sq . system that is associated with both non-predissociated and predissociated vibrational levels of the AŽ2 Sq . state. This is evident by the observed increased broadening of these vibrational bands in comparison to those formed in conventional photoelectron spectroscopy obtained at a similar resolution. Photoionization of HBr and DBr in the inner-valence ionization region between 23 and 29 eV shows four sets of pairs of very similar, vibrationally resolved band systems that are identified as being due to satellite ion states and a single, vibrationally resolved, band system assigned to the ‘main-line’ 4ssy1 ionization. Absolute vibrational numbering in these systems was greatly facilitated by having the isotopomer threshold photoelectron spectra. Evidence for the direct excitation of repulsive ion states in the 17.5–23.0 eV range has been found. q 1998 Elsevier Science B.V. All rights reserved.

1. Introduction The electronic states of HBrq and DBrq have been extensively investigated by absorption spectroscopy w1,2x, emission spectroscopy w3–13x, electron impact energy-loss spectroscopy w14–17x, photoionization ion w18–20x and electron w21,22x yield spectroscopy, photoelectron spectroscopy ŽPES. )

Corresponding author. E-mail: [email protected] Present address: Electron Tubes, Bury St., Ruislip, Middlesex, England HA4 7TA, UK. 1

w23–37x, Penning ionization electron spectroscopy ŽPIES. w27,33,38,39x, resonance enhanced multiphoton ionization–photoelectron spectroscopy ŽREMPI– PES. w40x, pulsed-field ionization–zero kinetic energy ŽPFI–ZEKE. electron spectroscopy w41,42x, optical–optical double resonance dissociative ion mass spectrometry w43x and by theory w13,18,30,32,44–46x; yet these ions continue to attract considerable attention because of the complicated nature of their electronic potentials due largely to spin–orbit effects. This manifests itself in the photoabsorption dynamics of the molecules where autoionization and neutral

0301-0104r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 1 0 4 Ž 9 8 . 0 0 2 8 0 - 8

134

A.J. Yencha et al.r Chemical Physics 238 (1998) 133–151

dissociative processes compete with direct ionization. In addition, these effects can be felt in the predissociation dynamics of excited bound neutral and bound ionic states and in the electrostatic couplings between neutral potentials and between ionic potentials. In this regard, threshold photoelectron spectroscopy ŽTPES. provides an excellent means for probing some of these competing processes because its resonance nature affords a view of indirect ionization Žresonance autoionization. processes, in addition to direct ionization processes, that are often obscured or are not capable of detection by other spectroscopic means. In order to assess the impact of such indirect ionization processes in the photoabsorption dynamics of molecules in the ionization energy region it is very helpful to have available conventional photoelectron spectra of comparable electron resolution for comparison. We have embarked on systematic studies of the ionization dynamics in the halogen acid series using the threshold photoelectron spectroscopic method. Two of the major advantages of this method are the relatively high resolution obtainable Žcompared with conventional PES. being typically in the 5–6 meV energy range, and the large photon energy range that can be scanned using tunable synchrotron light sources. Previously we have published the results of the TPES of HCl, DCl w47x and HI w48x over the wide photon energy range covering both outer- and inner-valence ionization of the molecules. We report here on the TPES of HBr and DBr from the onset of ionization up to a photon energy of ; 31.5 eV. To our knowledge, no previous TPE study has been performed on either of these molecules.

2. Experimental The TPE spectra reported here were obtained by a method that has been described in detail previously w49–51x, and thus only a brief account of the method will be presented. Continuum synchrotron light emanating from the Synchrotron Radiation Source of the Daresbury Laboratory was dispersed by a 5 m normal incidence vacuum monochromator ŽMcPherson: beamline 3.2.. Upon exiting the monochromator the

light was focused through a 1 mm bore by 35 cm in length glass capillary that conducted the light to the center of the vacuum apparatus housing the threshold photoelectron spectrometer and an effusive sample gas stream. Threshold electron spectra were obtained using a penetrating-field electron spectrometer w50,51x tuned to accept near threshold electrons Ž- 20 meV, although ; 95% of the electrons detected are within 3 meV of threshold. by maximizing the detected electron signal for the argon doublet ionization peaks at 15.759 eV Ž2 P3r2 . and 15.937 eV Ž2 P1r2 ., while at the same time minimizing the detected electron signal from the autoionization structure found in this energy range. The energy resolution achieved for the TPE spectra presented here varied from ; 3 to ; 20 meV as determined by observing the Arq Ž2 P1r2 . line at 15.937 eV photon energy and was basically controlled by the photon resolution which varied smoothly with wavelength in an approximately inverse linear fashion Ži.e., D ErE f constant.. TPE spectra covering a large photon energy range were obtained by joining together multiple, properly normalized sub-spectra with sufficient energy overlap to ensure an accurate matching of the energy and intensity scales. The sub-spectra energy ranges, the number of spectra recorded, the step sizes and resolutions were as follows. For HBr, the complete TPE spectrum consisted of the following joined sub-spectra energy ranges, number of spectra recorded, the step sizes and resolutions. From 11.500 to 13.404 eV, four spectra were obtained Ž1 meV steps. at a resolution of 6.0 meV. From 13.405 to 15.222 eV, and from 15.223 to 16.905 eV, two sets of three spectra were obtained Ž1 meV steps. at a resolution of 6.0 meV. From 16.905 to 31.5 eV, eight spectra were obtained Ž10 meV steps. at a resolution of 13.0 meV. For DBr, the complete TPE spectrum consists of the following sub-spectra energy ranges, number of spectra recorded, the step sizes and resolutions. From 11.550 to 15.226 eV, three spectra were obtained Ž2 meV steps. at a resolution of 6.0 meV: from 15.227 to 16.945 eV, three spectra Ž1 meV steps. at a resolution of 5 meV and from 16.953 to 31.515 eV, one spectrum Ž10 meV steps. at a resolution of 12.0 meV were obtained. All sub-spectra were corrected for the variation in the photon flux as a function of energy and the decay of the synchrotron current prior to sum-

A.J. Yencha et al.r Chemical Physics 238 (1998) 133–151

ming and catenation. Higher-resolution Ž; 3 meV. TPE spectra of HBr and DBr were recorded over the limited energy range of 11.58–12.08 eV Žsee below.. The energy scales of the spectra were calibrated by measuring the TPE spectra of the lowest ionization potentials of the rare gases He through Xe. The pressure of sample gas in the vacuum system as a whole was generally of the order of Ž8–10. = 10y5 Torr. All sample gases were degassed at liquidnitrogen temperature just prior to use to ensure the complete removal of any hydrogen gas that may have been present due to the decomposition of the sample within the storage cylinder. HBr was supplied by Linde Gas UK ŽStoke on Trent, UK.. DBr was obtained from Argo International ŽEssex, UK.. All spectra were recorded at the ambient temperature Ž; 300 K..

3. Results and discussion 3.1. OÕerÕiew and general considerations The valence electron configuration of HBr Žand DBr. is Ž4ss . 2 Ž4ps . 2 Ž4pp . 4 and the ground electronic state is therefore a 1 Sq state, using the Ž L, S . coupling scheme, although it is understood that this notation is used only for convenience since it is well established that spin–orbit coupling is strong in molecules with a heavy atom, such as bromine w52x. Removal of a single electron from each of the two outermost molecular orbitals Ž4pp .y1 and Ž4ps .y1 leads to the formation of the ground XŽ2 P i . and first-excited AŽ2 Sq . states of HBrq, and the simple molecular orbital picture of ionization is found applicable w44x. However, removal of an electron from the inner-valence molecular orbital Ž4ss .y1 leads to a complete breakdown of the molecular orbital picture with the formation of numerous satellite ion states due to correlation effects w44x. Thus, the results for outer- and inner-valence ionization will be treated separately below. The complete TPE spectra for HBr and DBr from 11.5 to ; 31.5 eV are shown in Fig. 1 and the ionic states are identified. As can be seen, the general appearance of the two spectra is very similar as would be expected for molecules with the same overall electronic structure. However, the relative intensities of the observed systems vary some-

135

what between the two isotopomers. In the inner-valence Ž4ss .y1 ionization region the intensity for DBr ionization is about a factor of 2 greater than that for HBr, for which we have no explanation. A similar trend was observed in the TPES of HCl and DCl, but in that case, the intensities differed by about a factor of 4 w47x. Another obvious observation from an inspection of the spectra in Fig. 1 Žsee lower traces in each panel. is that almost all of the intensity resides in the lowest vibrational band of the ground ionic state system. This will be discussed in detail below. 3.2. Outer-Õalence ionization 3.2.1. High-resolution TPE spectra of HBr r DBr in the 11.58–12.08 eV range The TPE spectra for HBr and DBr in this energy range are shown in Fig. 2 at a resolution - 3 meV Ža resolution of 3.0 meV was determined from the Ar Ž2 P1r2 . ion line at 15.937 eV in adjacent calibration scans.. The energy scale in these spectra was calibrated by comparison with other spectra recorded over a much wider energy range that were themselves calibrated by observing the argon ion doublet lines at 15.759 and 15.937 eV. These TPE spectra display the Õqs 0 and 1 vibrational bands of the XŽ2 P 3r2 . subsystem and the Õqs 0 vibrational band of the XŽ2 P 1r2 . subsystem. There is a clear indication of resolved rotational structure, especially on the low-energy side of the Õqs 0 band in both HBrq and DBrq. It is noteworthy that the intensity of the Õqs 0 vibrational band of the two V components ŽF1 and F2 . of the XŽ2 P i . system differ considerably, whereas in conventional He I PES they are nearly statistical Ži.e., F1rF2 s 1.01 w33x.. The branching ratio here is found to be F1rF2 s 2.30 for HBrq and F1rF2 s 2.60 for DBrq. This difference in intensity is attributable to autoionization processes that preferentially populate rotational levels of the F1 component. Two examples of such processes can be seen in the prominent peak features on the high-energy side of the Õqs 0 band of the F1 component in both HBrq and DBrq Žsee Fig. 2.. These peaks are identified as the 7p and 7dd Rydberg resonances converging on the Jqs 1r2, Õqs 0 rovibrational level of the XŽ2 P 1r2 . ion sub-state based on the very

136

A.J. Yencha et al.r Chemical Physics 238 (1998) 133–151

Fig. 1. Complete threshold photoelectron spectra of HBr and DBr showing the regions of the XŽ2 P i ., AŽ2 Sq . and 4ssy1 states of HBrq and DBrq. The energy resolution in these spectra in different energy regions are given in Figs. 3, 4 and 6.

high-resolution Ž"1 cmy1 . VUV laser photoionization electron yield study of Irrgang et al. w21x. We determined quantum defects of 0.502 Ž n) s 6.498. and 0.141 Ž n) s 6.859. for these resonances, respectively, in the case of HBrq based on the convergence limit of 11.9932 eV w21x. These features represent the enhanced population of rotational states in the Õqs 0 vibrational level of the XŽ2 P 3r2 . ion sub-state as a result of spin–orbit resonance autoionization of the w2 P 1r2 x 7p and w2 P 1r2 x 7dd Rydberg states. These same features in DBrq are marked with asterisks in the lower panel of Fig. 2. A broader feature is also observed on the high-energy tail of the Õqs 1 vibrational band of the XŽ2 P 3r2 . subsystem in DBrq Žsee lower panel in Fig. 2. that we tentatively identify as the w2 P 1r2 x 14d Rydberg resonance. Unfortunately, the resolution in these spectra

is insufficient to enable any meaningful rotational assignment, but they clearly show the effect of autoionization processes that contribute to the formation of the Õqs 0 vibrational band in the F1 component in both HBrq and DBrq and probably to the Õqs 1 vibrational band in the F1 component in DBrq. 3.2.2. TPE spectra of HBr r DBr in the 11.5–17.0 eV range The TPE spectra of hydrogen bromide and deuterium bromide in the outer-valence ionization region Ž11.5–17.0 eV. are shown in Fig. 3 at a resolution of ; 6.0 meV, as measured at 15.759 eV. This energy region contains the XŽ2 P i . and AŽ2 Sq . band systems of HBrq and DBrq that are familiar in conventional He I PES w23–37x, but these systems have not been observed previously by TPES. As can

A.J. Yencha et al.r Chemical Physics 238 (1998) 133–151

137

Fig. 2. High-resolution Ž- 3 meV. threshold photoelectron spectra of HBr and DBr covering the Õqs 0 and 1 vibrational bands of the XŽ2 P 3r 2 . subsystem and the Õqs 0 vibrational band of the XŽ2 P 1r2 . subsystem of HBrq and DBrq. Several Rydberg resonances are identified: the asterisks in Žb. correspond to the Rydberg states identified in Ža. as w2 P 1r 2 x 7p and w2 P 1r2 x 7dd.

be seen in Fig. 3, the XŽ2 P i . system exhibits a long vibrational progression in both HBrq and DBrq, essentially disappearing at the onset of the AŽ2 Sq . band system. Since direct photoionization of HBr from its ground state is known to populate only vibrational levels up to Õqs 2 in the ground state of the ion, as, for example, is found in the He I PES of

HBr w33x, the extended vibrational structure observed in these TPE spectra must be accessed by the indirect mechanism of resonance autoionization of super-excited Rydberg states lying in the Franck–Condon ŽFC. gap region between the ground and firstexcited states of the molecular ion. The observation of extended vibrational structure in the first FC gap

138

A.J. Yencha et al.r Chemical Physics 238 (1998) 133–151

Fig. 3. A portion of the threshold photoelectron spectra covering the XŽ2 P i . and AŽ2 Sq . band systems of HBrq and DBrq with the assignment of the observed vibrational progressions. The energy resolution in these spectra is ; 5–6 meV. The insert in the upper panel is a reproduction of the AŽ2 Sq . band system of HBrq positioned so as to approximately represent the energy location and general appearance of the w2 Sq x 5ss Rydberg state in HBr. The half-tick marks in the AŽ2 Sq . band systems represents the predissociated vibrational levels that were not used in the vibrational fit procedure Žsee text..

region in molecular ion spectra produced by the threshold photoelectron method is now very common and has lead to significantly improved spectroscopic constants for the ground state ions of a large number

of systems. The observed vibrational band head energy positions for both spin–orbit components of the XŽ2 P i . system for HBrq and DBrq are given in Tables 1 and 2, respectively. These data were ana-

A.J. Yencha et al.r Chemical Physics 238 (1998) 133–151

139

Table 1 Y Observed and calculated vibrational band head positions Žin eV. for the transitions: eyq HBrq ŽX 2 P i , Õq . § HBrŽX 1 Sq, Õ s 0. and the 2 2 experimental branching ratios: P 3r 2r P 1r2 2

Õq

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 a

P 3r 2 , transition energy

2

P 1r2 , transition energy

obs.a

calc.

obs.a

calc.

11.673 11.963 12.244 12.513 12.772 13.019 13.256 13.483 13.698 13.903 14.099 14.284 14.458 14.623 14.778 14.922 15.057 15.183

11.673 11.964 12.244 12.513 12.772 13.019 13.256 13.483 13.698 13.904 14.099 14.284 14.458 14.623 14.778 14.922 15.058 15.183

12.001 12.292 12.572 12.841 13.099 13.346 13.583 13.808 14.023 14.228 14.422 14.606 14.779 14.940 15.092 15.234

12.002 12.292 12.572 12.841 13.099 13.346 13.582 13.808 14.023 14.228 14.422 14.605 14.778 14.941 15.092 15.234

2

P 3r2 r2 P 1r2

2.30 1.23 0.48 0.35 0.92 7.30 4.73 0.36 0.31 0.98 3.76 5.13 1.38 1.25 1.78

Uncertainty "0.002 eV.

lyzed using the modified third-order vibrational Dunham expression given in Eq. Ž1. w47x: G i Ž ÕX . s Ý Yn , 0 r i Ž ÕX q 12 .

n

,

Ž 1.

n

where G i Ž ÕX . is the term value for vibrational level ÕX , Yn,0 is a Dunham parameter w53x, and r i s'm Ž HBr . r m i , where m i is the reduced mass of the relevant isotopomer Ži.e., HBr or DBr.. Spectroscopic constants derived using Eq. Ž1. from the combined analysis of the HBrq XŽ2 P i . and DBrq XŽ2 P i . data are given in Table 3 together with comparable spectroscopic constants from the literature. Calculated vibrational band head energy positions based on the above analysis procedure are given in Tables 1 and 2 for HBrq and DBrq, respectively. From an inspection of the lower traces in each panel in Fig. 3, we see that there are two aspects of the intensity of the XŽ2 P i . system that are of note. The first is that almost all of the intensity resides in the Õqs 0 band of the XŽ2 P 3r2 . subsystem. The maximum peak height of the Õqs 0 band in the HBrq XŽ2 P 1r2 . subsystem is misleading because of

the contribution to this peak height by the partially unresolved Õqs 1 band of the XŽ2 P 3r2 . subsystem. The reason for the disproportionate intensity of the Õqs 0 band of the XŽ2 P 3r2 . subsystem has already been given above as being due to enhanced rotational intensity as a result of spin–orbit autoionization processes between the ground vibrational levels of the two spin–orbit components. The second aspect of the intensity of note in Fig. 3 is the oscillating intensity distributions found in the vibrational bands in the FC gap region in both HBrq XŽ2 P i . and DBrq XŽ2 P i .. There appears to be three lobes between ; 12.4 and ; 15.2 eV in each TPE spectrum with intensity maxima of the lobes occurring at roughly 13.0, 14.0 and 15.0 eV. We associate these three lobes in each spectrum with three Rydberg states converging respectively on the AŽ2 Sq . state of the HBr and DBr ions. In the case of the TPES of HCl and DCl three similar lobe distributions were found in the first FC gap region that were tentatively identified as being due to nss 1 Sq Rydberg states for n s 4–6 w47x. In order to demonstrate that the distributions in Fig. 3 have their origin in Rydberg states converging on the AŽ2 Sq . ion state, we have

A.J. Yencha et al.r Chemical Physics 238 (1998) 133–151

140

Table 2 Y Observed and calculated vibrational band head positions Žin eV. for the transitions: eyq DBrq ŽX 2 P i , Õq . § DBrŽX 1 Sq, Õ s 0. and the 2 2 experimental branching ratios: P 3r 2r P 1r2 Õq

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 a b

2

P 3r 2 , transition energy

2

2

P 1r2 , transition energy

obs.a

calc.

obs.a

calc.

11.677 11.886 12.089 12.289 12.481 12.668 12.850 13.026 13.197 13.363 13.523 13.677 13.827 13.971 14.111 14.244 14.373 14.497 14.615 14.728 14.837 14.940 15.039 15.132 15.222

11.676 11.886 12.090 12.288 12.481 12.668 12.850 13.026 13.197 13.363 13.523 13.677 13.827 13.971 14.110 14.244 14.373 14.497 14.615 14.728 14.837 14.940 15.039 15.132 15.221

12.005 12.214 12.418 12.616 12.808 12.995 13.177 13.353 13.523 13.688 13.848 14.002 14.152 14.296 14.434 14.567 14.694 14.816 14.933 15.045 15.152 15.253

12.005 12.214 12.418 12.616 12.808 12.995 13.177 13.353 13.523 13.689 13.848 14.003 14.152 14.295 14.434 14.567 14.694 14.817 14.934 15.045 15.152 15.253

P 3r2r2 P 1r2

2.60 2.02 6.47 0.72 0.47 0.07 0.55 b b b b b b

0.77 2.02 2.20 1.54 1.12 b b b b

Uncertainty "0.002 eV. Unable to determine due to one or both peaks being unresolved.

inserted in the upper panel of Fig. 3 the AŽ2 Sq . band system intensity profile shifted in energy such that its maximum intensity Že.g., Õqs 2. corresponds to the maximum in the lobe intensity Že.g., at the Õqs 3, XŽ2 P 1r2 . vibrational band.. Clearly the two intensity profiles are very similar. We have tentatively identified the autoionizing state as the w2 Sq x 5ss 1 Sq Rydberg state. To our knowledge this Rydberg state in HBr and DBr has not been energy located in any other study. However, the w2 Sq x 5ss 3 q S Rydberg state has been assigned in an electron energy-loss study of HBr and DBr giving band origins at 11.893 and 11.910 eV, respectively w17x. This would yield, in the case of HBr, a singlertriplet energy separation of 0.634 eV Ž12.527–11.893., based on our estimate of the position of the origin for the singlet Rydberg state. The splitting between the w2 Sq x 5ss 1 Sq and 3 Sq states arises from

electron exchange between the Rydberg electron and the unpaired electron in the bonding s-orbital of HBrq. Denoting the exchange integral by K s :5s , an inspection of the dominant configurations shows that the splitting is 2 K s :5s . We can estimate the expected magnitude of this exchange energy from selected term values of the 5s Rydberg state of atomic Br. The energy difference between the states w3 P2 x 5s: V s 5r2 and w1 D 2 x 5s: V s 5r2 differs from that between the Brq core states 2 P2 and 1 D 2 by K 4p:5s . Inserting the appropriate atomic term values gives K 4p:5s s 2486 cmy1 . If the assumption is made that the molecular valence s orbital is very similar to an atomic Br 4p orbital, the 1 Sqy3 Sq splitting in the 5s molecular Rydberg state would be expected to be 4972 cmy1 Ž0.616 eV.. We thus assign the autoionizing state as the w2 Sq x 5ss 1 Sq Rydberg state. An extension of this reasoning leads us to the conclusion

A.J. Yencha et al.r Chemical Physics 238 (1998) 133–151

141

Table 3 Summary of spectroscopic constants Žin eV. derived from analyses of the HBr and DBr TPES data presented here and from literature data Ionization energy a

State 2

q

XŽ P 3r 2 .

11.673Ž2. 11.672Ž4. 11.666Ž2. 11.645Ž5. 11.6668Ž1. c 11.6668Ž1. d 11.66689Ž12. c

q

XŽ2 P 1r 2 .

12.001Ž2. 11.979Ž5. 11.9973Ž1. e 11.99734Ž12. e

q

AŽ2 Sq .

15.298Ž2. 15.295Ž4. 15.296Ž2. 15.290Ž1–2. 15.288 15.288Ž5.

HBr

HBr

HBr

DBrq XŽ2 P 3r 2 .

q

DBr

2

XŽ P 1r 2 .

DBrq AŽ2 Sq .

11.677Ž2. 11.673Ž5. 11.668Ž2. 12.005Ž2. 12.002Ž5.

15.320Ž2. 15.284Ž5.

10 3ve x e

ve .b

10 5ve ye

Ref.

.b

1.56Ž5. b

this work w33x w19x w26x w40x w21x w42x w54x

0.58Ž5. b

this work w26x w21x w42x w54x

0.3024Ž1

y5.60Ž1

0.30270

y5.877

0.30144Ž9. b

y5.46Ž1. b

0.30270

y5.877

0.1729Ž8. b

y4.9Ž3. b

0.1741

y4.674

0.21518Ž7. b

y3.987Ž9. b

0.21502

y2.740 .b

0.21447Ž6

y3.887Ž9. b

0.21502

y2.740

0.1231Ž6. b

y3.5Ž2. b

0.12388

y2.368

this work w33x w1x w35x w34x w26x w54x 1.11Ž3. b

this work w26xa w19x w54x

0.42Ž4. b

this work w26x w54x this work w26x w54x

a

From the measured Õqs 0 peak position unless otherwise specified. Calculated from the analysis of the combined HBr and DBr vibrational data. See text. Y Y c Value for HBrq XŽ2 P 3r 2 , Õqs 0, Jqs 3r2. § HBr XŽ1 Sq, Õ s 0, J s 0. transition. d From the onset of ionization determined by extrapolation to zero extraction field. Y Y e Value for HBrq XŽ2 P 1r 2 , Õqs 0, Jqs 3r2. § HBr XŽ1 Sq, Õ s 0, J s 0. transition. b

that the three autoionizing states responsible for the extended vibrational structure in the first FC gap region in HBrq and DBrq are the w2 Sq x nss 1 Sq Rydberg states with n s 5–7. In Tables 1 and 2 are given the 2 P 3r2r2 P 1r2 intensity branching ratios for all of the assigned vibrational structure observed in the XŽ2 P i . band systems of HBrq and DBrq, respectively. The intensities Žareas. of the individual vibrational bands were

determined using a Lorentzian curve fitting program ŽMicrocal Software, Northampton, MA.. As can be seen in Tables 1 and 2, the branching ratios vary widely. A similar behavior has been found in the XŽ2 P i . band systems of HClq and DClq observed in TPES studies of these molecules w47x. Clearly these branching ratios reflect the mechanism by which the vibrational bands are formed. As discussed above, in the FC region Ž Õqs 0–2 in HBrq . the

142

A.J. Yencha et al.r Chemical Physics 238 (1998) 133–151

vibrational bands can be formed by both direct ionization and indirect resonance autoionization of Rydberg states, while in the FC gap region Ž Õq) 2 in HBrq. the vibrational bands can only be formed by indirect resonance autoionization. To a first approximation Že.g., ignoring variations in the transition moments as a function of energy., the overall probability for populating a rovibrational level of the ion in the indirect autoionization process is proportional to the product of the probability of populating a rovibrational level of a Rydberg state by a direct absorption process and the probability of resonant autoionization of that same state to form a rovibrational level of the ion. That is, the overall probability for indirect autoionization is proportional to the product of two FC factors. A more detailed description of resonance autoionization processes in TPES is given elsewhere w47,49x. Because the Rydberg state vibrational bands and the final ion state vibrational bands are both rather narrow and fairly widely spaced Žsee upper panel of Fig. 3., it is understandable that the overall probability for formation of the final ion state vibrational bands can vary widely.

3.2.3. TPE spectra of HBr r DBr in the 15.0–17.0 eV range The AŽ2 Sq . band system in HBrq and DBrq forms a moderately long progression of vibrational bands between 15.0 and 17.0 eV, as shown in Fig. 3. The general appearance of this band system in HBrq is similar to that observed in conventional He I PES w35x, as can be seen in panel Ža. of Fig. 4, where the two different measurement methods are directly compared at a resolution that is nearly identical Že.g., 5 meV in the TPE spectrum vs. 4 meV in the PE spectrum.. This band system in HBrq Žand DBrq. exhibits the classical characteristics of predissociation with a region of several sharp, well-defined vibrational bands followed by a region of broad, diffuse bands. The potential energy curves for all the HBrq states that correlate to the first three dissociation limits, wBrqŽ3 P. q Hx, wBrqŽ1 D. q Hx and wBrŽ2 P. q Hqx , have been thoroughly investigated theoretically w46x. In this ab initio study it was shown that the AŽ2 Sq . state of HBrq Žand DBrq. is predissociated by the three repulsive ion states, w4 Sy, 2 Sy and 4 P x originating from the wBrqŽ3 PJ . q Hx asymp-

totes, by spin–orbit interactions w46x. In addition, the total predissociation lifetimes of individual vibrational levels of the AŽ2 Sq . state of HBrq and DBrq were calculated, as well as the individual contributions to the total predissociation lifetimes due to spin–orbit coupling between the three repulsive ion states and the AŽ2 Sq . state. It was shown that the AŽ2 Sq . state vibrational levels Õqs 0–1 in HBr and Õqs 0–3 in DBrq are stable Žt ) 1 = 10y9 s, see w43x. with respect to predissociation, in agreement with experimental results w3,4,7,43x. Although the AŽ2 Sq . band system in HBrq produced by TPES and PES are similar in general appearance, there are some significant differences in the spectra as displayed in panel Ža. of Fig. 4 and in expanded form in Fig. 5. In order to make the comparison meaningful in both figures the PE spectrum Žreproduced by permission w35x. was shifted in energy by q8 meV so as to have the band heads of the Õqs 0 band coincident in the two types of spectra. The two spectra were also peak-height normalized to the highest peak band Ž Õqs 2. in both spectra. As can be seen in Fig. 4, panel Ža. and in Fig. 5, the peak heights in the first two bands Ž Õqs 0, 1. in each spectrum are essentially identical. In addition, the energy position of the band head in the PE spectrum for Õqs 1 is identical with that of the same band in the TPE spectrum. However, all other vibrational band heads are noticeably shifted relative to each other with the TPE bands peaking at lower energies Žsee Fig. 4, panel Ža. and Fig. 5.. In addition, all of the bands that can be clearly compared Že.g., see the Õqs 0–4 bands in Fig. 5. are broadened in the TPE spectrum as compared with the PE spectrum that cannot be accounted for on the basis of any resolution difference, e.g., compare the similar shape of the partially resolved P branch rotational structure on the low-energy side of the Õqs 0 and 1 bands in the two spectra of Fig. 5. The broadening in the TPE spectrum appears asymmetric with the first two bands broadening more towards higher energy, while all subsequent bands appear to broaden more towards lower energies. There also appears to be a general ‘filling-in’ between vibrational bands in the TPE spectrum Žsee Fig. 5., resulting in substantially increased intensity Žintegrated area. most noticeable for Õqs 3–7 in Fig. 4, panel Ža.. These general spectral characteristics in the HBr

A.J. Yencha et al.r Chemical Physics 238 (1998) 133–151

143

Fig. 4. Separately recorded threshold photoelectron spectra of HBr Žpanel b. and DBr Žpanel c. covering the A2 Sq band system with an energy resolution of 5 meV in both, as measured in adjacent argon TPE calibration spectra and determined at 15.937 eV. In panel Ža. is the same TPE spectrum of HBr with a comparison of the reported He I PE spectrum of HBr at a resolution of 4 meV Žreproduced with permission. w35x. In panels Žb. and Žc. the heavy lines of the peak forms are from a deconvolution peak fit procedure Žsee text..

TPE spectrum are similarly observed in the DBr TPE spectrum shown in Fig. 4, panel Žc.. In addition, similar observations have been made in the TPE spectra of HCl and DCl w47x.

Based on the rotational band analysis of Baltzer et al. w35x for several of the vibrational bands of the AŽ2 Sq . system in their PES study of HBr, numerous rotational branches Žfrom D N s y3 to D N s q3.

144

A.J. Yencha et al.r Chemical Physics 238 (1998) 133–151

Fig. 5. A part of the TPE and PE spectra of HBr shown in panel Ža., Fig. 4, here given in expanded form. See Fig. 4 caption.

are found to contribute. Similar rotational angular momentum changes were reported by Mank et al. w41x in their rotationally-resolved PFI–ZEKE study of HBr. The same rotational behavior was found in the formation of the AŽ2 Sq . system in HClq via PES w55x. We recently attributed the broadening effects in the TPE spectrum in the AŽ2 Sq . system in

HClq as compared with the PE results for the predissociated levels Ž Õqs 6–13. to enhanced rotational intensity in the observed rotational branches due to resonance autoionization effects w47x. This conclusion was based on rotational branch spectral simulations that involved even larger D N values Žfrom D N s –5 through D N s q4. in some of the vibra-

A.J. Yencha et al.r Chemical Physics 238 (1998) 133–151

tional bands in order to obtain a reasonable fit between the experimental and simulation results w47x. Similar rotational simulations were not found possible in our HBr TPES study because of the somewhat smaller rotational constants in the AŽ2 Sq . state of HBrq. However, it is clear that the same sort of broadening is occurring in the AŽ2 Sq . systems of HBrq and DBrq as occurred in the AŽ2 Sq . systems of HClq and DClq. We thus conclude that resonance autoionization effects contribute in a significant way to the rotational profiles of the predissociated vibrational bands Ž ÕqG 2 and ÕqG 4. in the AŽ2 Sq . system of HBrq and DBrq, respectively. Furthermore, because of the higher-resolution TPE and PE spectra compared in HBr, we can conclude also that resonance autoionization processes contribute to the formation of the non-predissociated vibrational bands in the AŽ2 Sq . systems of HBrq and DBrq and result in increased rotational intensity Žmostly in the R branch.. In order to quantify our observations for further discussion we have deconvoluted the vibrational bands in the AŽ2 Sq . systems of HBrq and DBrq in the following way. Two types of fit procedures were

145

applied to the TPE data shown in Fig. 4. In order to derive the spectroscopic constants for the AŽ2 Sq . state of HBrq and DBrq, the modified second-order vibrational Dunham expression given in Eq. Ž1. was used on the combined observed vibrational band head positions indicated by the full-length tick marks in Fig. 3 for both HBrq and DBrq. The derived spectroscopic constants are given in Table 3 where they are compared with literature values. The other fit procedure used involved deconvoluting the experimental spectra into individual peaks using combined Lorentzian and Gaussian functions in a curve fitting program ŽMicrocal Software, Northampton, MA.. The peak forms used for both HBrq and DBrq were as follows: for the full-tick marked peaks in Fig. 3 a Lorentzian line shape function was used while for the half-tick marked peaks in Fig. 3 a Gaussian line shape function was used. The deconvoluted peaks are shown as the heavier line curves in panels Žb. and Žc. of Fig. 4. Based on these deconvoluted peaks the vibrational band positions, widths ŽFWHM. and intensities Žareas. were determined for inclusion in Tables 4 and 5. The band positions given as observed in Tables 4 and 5 were used to construct the

Table 4 Observed and calculated vibrational band head positions Žin eV., determined band widths ŽFWHM. Žin meV., relative intensities Žband Y areas. and calculated Franck–Condon factors for the transitions: eyq HBrq ŽA 2 Sq, Õq . § HBr ŽX1 Sq, Õ s 0. Õq

0 1 2 3 4 5 6 7 8 9 10 a

Transition energy

Band width

Relative intensity

obs.a

cal.b

TPES c

He I PES d

TPES c

He I PESe

15.298 15.461 15.614 15.734 15.873 16.031 16.168 f 16.345 f 16.455 f 16.561g 16.655 g

15.298 15.461 15.614 15.734 15.873 16.031 16.168 16.345 16.455 16.561 16.655

23.8 23.2 43.6 64.5 157 97 192 85 90 75 g 75 g

11 11 12 22 120

0.288 0.451 1.000 0.750 0.985 0.235 0.342 0.072 0.054 0.024 0.017

0.40 0.80 1.00 0.88 0.84 0.56 0.36 0.18 0.11 0.076 0.036

FCF

0.562 0.993 1.000 0.768 0.502 0.295 0.160 0.082 0.041 0.020 0.010

Uncertainty "0.002 eV unless otherwise noted. Only observed values for Õqs 0–2 used as data in vibrational Dunham fit procedure. All other values listed as observed and calculated were derived from the deconvolution peak fit procedure. c This work. Based on the deconvolution peak fit procedure. d Ref. w35x. Based on the deconvolution peak fit procedure from the present work. e Ref. w35x. f Uncertainty "0.005 eV. g Parameter fixed in deconvolution peak fit procedure. b

A.J. Yencha et al.r Chemical Physics 238 (1998) 133–151

146

Table 5 Observed and calculated vibrational band head positions Žin eV., determined band widths ŽFWHM. Žin meV., relative intensities Žband areas. and calculated Franck–Condon factors for the transiY tions: ey qDBrq ŽA 2 Sq, Õq . § DBrŽX 1 Sq, Õ s 0. Õq

Transition energy obs.

0 1 2 3 4 5 6 7 8

b

15.320 15.438 15.551 15.659 15.744 15.833 15.938 d 16.067 d 16.170 e

cal.

c

15.320 15.438 15.551 15.659 15.744 15.833 15.938 16.067 16.170

Band widtha

Relative intensity a

FCF

21.5 22.5 23.6 27.2 48.1 73 150 100 e 100 e

0.171 0.399 0.532 0.723 0.805 0.645 1.000 0.305 0.249

0.298 0.731 0.998 1.000 0.816 0.567 0.376 0.225 0.125

a

Based on peak fit procedure. Uncertainty "0.002 eV unless otherwise noted. c Only observed values for Õq s 0–3 used as data in vibrational Dunham fit procedure. All other values listed as observed and calculated were derived from a deconvolution peak fit procedure. d Uncertainty "0.004 eV. e Paramater fixed in the deconvolution peak fit procedure. b

vibrational progressions Žladders. shown in Figs. 3 and 4 for the AŽ2 Sq . state of HBrq and DBrq, respectively. Note the irregular spacings between Õqs 4 and 7 in HBrq and between Õqs 5 and 8 in DBrq that is clearly indicative of a perturbed electronic system. In Table 4 the relative intensities of the vibrational bands observed in the AŽ2 Sq . band system in the He I PES of HBr are similar to the calculated FC factors for Õqs 0–2 but begin to deviate in the next two vibrational bands observed, most probably due to predissocation effects. On the other hand, the relative intensities of the vibrational bands observed in the AŽ2 Sq . band system in the TPES of HBr show little correspondence with the calculated FC factors, except for the higher vibrational levels observed where the uncertainty in the intensities becomes significant. This lack of correspondence is likely due to the presence of the two-step ionization mechanism operating in the TPE spectrum. This disparity between observed vibrational intensities and calculated FC factors is even more pronounced in the TPES of DBr as can be see in Table 5. Of particular interest in Table 4 are the vibrational band widths in the TPES and He I PES of HBr. The larger widths

observed in the non-predissociated Õqs 0–1 bands in the TPES study as compared with the PES study probably reflects an increase in the rotational intensities Ždominantly the R branch. within the vibrational bands. As mentioned above, this is likely due to the contribution of indirect resonance autoionization. However, it is noteworthy that the width of the Õqs 2 band increases by a factor of 2 over that of the Õqs 1 band in the TPES study while little change is observed in the PES study. This is significant because it is known experimentally w3,4,7,43x and theoretically w46x that predissociation begins in the Õqs 2 level in the AŽ2 Sq . state of HBrq. Hence, TPES, with its possible dual mode of ionization Ždirect and indirect., appears to be more sensitive towards predissociation than is He I PES that only significantly detects the effect of predissociation in the band broadening in Õqs 3. As proposed in our paper on the TPES of HCl and DCl w47x and supported here, the increased broadening Ži.e., increased intensities of rotational structure. in the predissociated levels in the TPE spectra of HBr Žand DBr. over that observed in the PE spectra of HBr is due to autoionization processes involving, most probably, mixed Žboundrrepulsive. Rydberg and ionic potentials. 3.3. Inner-Õalence ionization The TPE spectra of HBr and DBr in the 17.5 to 31.5 eV range are shown in Fig. 6. This energy region contains the expected 4ssy1 ‘main-line’ ionization system plus numerous anticipated satellite systems. The position of the numbers 1–6 represents the approximate peak maxima of six broad, structureless band systems observed by Adam et al. w34x in an Al K a PES study of HBr Žand DBr. in the energy range of Fig. 6. Their reported maxima correspond closely with the main features in the TPE spectra shown in Fig. 6, taking into account the large difference in spectral resolution achieved in the two studies Ž360 meV in the PES study vs. ; 20 meV in the present TPES study.. The extensive, broad band system identified by the number zero in Fig. 6 was not observed in the PES study of Adam et al. w34x. It actually appears to be made up of several overlapping broad band systems between ; 17.5 and ; 23.0

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147

Fig. 6. Overview of the threshold photoelectron spectra of HBr and DBr in the higher-energy region studied. The energy resolution at 26 eV is ; 20 meV in both spectra. See text for an explanation of the numbered labeling.

eV too poorly resolved with too poor a signal-to-noise ratio to make any positive identification of peak structures. However, a similar broad, complex system of bands has been observed in the TPE spectra of HCl and DCl in the same general energy range, preceding the inner-valence ionization region, in these molecules that was assigned to either the direct excitation to repulsive ion states or indirect autoionization of repulsive Rydberg states converging on the aforementioned repulsive ion states w47x. The presence of such Rydberg states in HBr in this energy region has been proposed to account for the observation of low-energy electrons identified with autoionizing bromine atoms formed in dissociative excitation transfer processes involving metastable He ) Ž2 3 S. Ž19.818 eV. and He ) Ž2 1 S. Ž20.615 eV. atoms in collisions with ground state HBr molecules w33x.

Similar atomic autoionizing structure has been observed in He ) metastable collisions with HCl w56x and HI w57x. The most likely repulsive HBrq ŽDBrq. states to be resident in the FC region in the energy range 17.5–23.0 eV are those associated with the asymptotic limits wBrq Ž3 PJ . q Hx at 15.572 eV for J s 2, wBrq Ž1 D 2 . q Hx at 17.071 eV and wBrŽ2 P3r2 . q Hq. at 18.273 eV based on the calculated potential energy curves in HBrq w46x and the experimental evidence discussed above w33x. Moreover, it is unlikely that the repulsive limbs of the bound states of HBrq ŽDBrq. would be present in the FC region in the energy range 17.5–23.0 eV based on the same calculated energy curves in HBrq w46x Žsee also the potential energy curves given in w33x.. It should be noted that the intensity of the structured features 1–6 shown in Fig. 6 are extremely

148

A.J. Yencha et al.r Chemical Physics 238 (1998) 133–151

weak by comparison with the features at lower energies, e.g., see Fig. 1 for a relative perspective. This is probably due to a combination of factors. One factor is likely to be the low ionization probability for ejecting an electron from the 4ss molecular orbital that is highly localized on the bromine atom while a second factor is probably due to the low probability of producing a threshold electron in a two-electron promotion process leading to satellite ion formation. Also note that in this energy region Ž17.5–31.5 eV. the TPE spectrum of DBr is about twice as intense as that of HBr Žsee Fig. 1.. A similar intensity difference was noted in the TPES of HCl and DCl, except in that case the difference was a factor of four w47x. The structured features 1–4 in Fig. 6 are examined more closely in Fig. 7 where ten lettered vibrational band systems are identified. The

identification of these systems closely parallels similar systems found in the TPES of HCl and DCl w47x. The absolute numbering of the vibrational progressions was greatly facilitated by having the TPE spectra of the two isotopomers. A listing of all the vibrational band head positions identified in Fig. 7 are given in Table 6. Band system E in Fig. 7 in both HBrq and DBrq is similar to the band system E 2 in the TPES of HCl and DCl w47x in that it is a structured system superimposed on the long, high-energy tail of system zero in Fig. 6. It is assigned as a neutral Rydberg state that is detected through resonance autoionization into the continuum ion stateŽs. represented by the band system zero. By analogy with the findings in the TPES of HCl and DCl w47x, we assign its convergence limit to the satellite ion system H 2 from which

Fig. 7. Threshold photoelectron spectra of HBr and DBr in the 4ssy1 inner-valence ionization region showing the probable vibrational assignments of the observed band systems. The energy resolution of the spectra is the same as in Fig. 6.

A.J. Yencha et al.r Chemical Physics 238 (1998) 133–151

149

Table 6 Vibrational band head positionsa Žin eV. for the inner valence Ž4ssy1 . ionization region of hydrogen and deuterium bromide from the present threshold photoelectron work Band system

Õq

HBrq

DBrq

Band system

Õq

HBrq

E

0 1 2 3 4

22.108 22.374 22.624 22.868

22.108 22.296 22.475 22.646 22.812

H2

0 1 2 3 4 5

25.406 25.646 25.906 26.105 26.274

25.433 25.604 25.767 25.932 26.092 26.252

F1

0 1 2 3 4 5 6

23.181 23.461 23.720 23.970 24.219

23.194 23.394 23.584 23.793 23.985 24.177 24.367

I

0 1 2 3

25.996 26.254 26.494 26.732

26.032 26.215 26.383 26.525

J

0 1 2 3 4 5 6

23.720 23.970 24.219 24.448 24.678

0 1 2 3 4 5

26.121 26.378 26.625 26.861 27.095

26.122 26.289 Ž26.476. 26.662 26.822 26.928

K

0 1 2 3

24.574 24.836 25.084

24.593 24.784 24.960 25.135

26.931 27.267 27.527 27.785 28.009 28.228 28.430

26.952 27.131 27.310

G

0 1 2 3 4 5 6

H1

0 1 2 3 4 5

24.888 25.086 25.306 25.536 25.745

24.976 25.145 25.316 25.462 25.606 25.753

L

0 1 2 3

27.042 27.341 27.650 27.927

27.077 27.254 27.428 27.590

F2

a

23.737 23.934 24.114 24.284 24.473 24.664 24.835

DBrq

Uncertainty "0.005 eV.

effective quantum numbers are obtained Ž2.03 in HBr and 2.02 in DBr.. Band systems F1 and F2 in Fig. 7 are similar to the same systems in the TPES of HCl and DCl w47x except that they seem to be better resolved and shifted more relative to each other and to the system G. The long vibrational progressions observed in these satellite ion states implies that their formation results in a substantial change in the equilibrium internuclear distance compared with the ground state of HBr and DBr. This would suggest the involvement of the outer-valence sigma molecular orbital in the two-electron excitationrionization process. How-

ever, there is little evidence that these states are predissociated in contrast to the counterpart systems in HClq and DClq w47x. Band system G in Fig. 7 is assigned to the ‘main-line’ 4ssy1 ionization. The relatively short vibrational progression observed confirms the nonbonding nature of this molecular orbital. As in the TPES of HCl and DCl w47x, this system shows some evidence for predissociation from the broadness of the higher vibrational members of the progression. Band systems H 1 and H 2 , I and J, and K and L are three similar sets of interwoven systems as are systems F1 and F2 . Again, the relatively long vibrational

150

A.J. Yencha et al.r Chemical Physics 238 (1998) 133–151

progressions observed implies the involvement of the outer-valence sigma molecular orbital in the twoelectron excitationrionization process to form these satellite ion states. Unfortunately, the only theoretical study of the satellite ion states in HBr was the early Green’s function calculations of von Niessen et al. w44x that show only three electronic states in the 21–29 eV region. Clearly there is a need for more extensive ab initio calculations to be performed on the satellite ion region in HBrq before any meaningful assignment of the structure in Fig. 7 can be made. It would also be useful to obtain TPE spectra in this spectrally weak region in HBr and DBr at a somewhat higher resolution and with a much better signal-to-noise ratio to more positively identify the vibrational progressions observed in the systems shown in Fig. 7 from which reliable vibrational constants can be derived. 4. Summary and conclusions The threshold photoelectron spectra of HBr and DBr are reported for the first time. They were recorded under medium resolution conditions Ž3–20 meV. using synchrotron radiation and utilizing a penetrating-field electron spectrometer. The electron spectra cover both the outer- and inner-valence ionization regions Ž11.5–31.5 eV. in these molecules. In the outer-valence ionization region the effect of autoionization is clearly observed, for example, in the extended vibrational progressions in the FC gap region between the XŽ2 P i . and AŽ2 Sq . band systems of HBrq and DBrq and in the extensive broadening of the vibrational bands in the AŽ2 Sq . system. Analysis of the vibrationally resolved XŽ2 P i . band system in HBrq and DBrq using a modified isotopic Dunham expression has led to improved spectroscopic constants for this state. The extended vibrational structure observed in the XŽ2 P i . system in both molecular ions is attributed to resonance autoionization of Rydberg states converging on the AŽ2 Sq . state and these are tentatively identified as nss 1 Sq Rydberg states with n s 5–7. Based on this and other TPE studies a propensity for producing threshold photoelectrons from ss Rydberg states in the first FC gap region by resonance autoionization processes appears to be emerging.

Separate high-resolution TPE spectra Ž- 3 meV. of HBr and DBr covering the Õqs 0 and 1 vibrational bands of the XŽ2 P 3r2 . subsystem and the Õqs 0 vibrational band of the XŽ2 P 1r2 . subsystem were recorded. These display partially resolved rotational structure and pronounced rotational features due to spin–orbit resonance autoionization of Rydberg states. The vibrational bands observed in the AŽ2 Sq . system in both HBrq and DBrq for the predissociated levels exhibit broadening beyond that expected for predissociation and this is attributed to autoionization processes, most likely involving mixed Žboundrrepulsive. Rydberg and ionic potentials. The threshold photoelectron spectra of HBr and DBr in the inner-valence ionization region Ž17.5–31.5 eV. exhibit numerous vibrationally resolved ionic systems that are attributed to four sets of pairs of very similar satellite ion systems and a single system representing the ‘main-line’ for 4ssy1 ionization. In addition, an extensive broad band system is observed in both electron spectra that most likely arises from the excitation of repulsive ion states. Threshold photoelectron spectroscopy at the resolution now routinely attainable Ž5–6 meV. is proving to be a very useful method for obtaining new information on the ionic states of molecules and of neutral Rydberg states. Acknowledgements The authors acknowledge financial support for this work by the CLRC Daresbury Laboratory, and one of us ŽAJC. gratefully acknowledges funding of a CASE studentship from the CLRC and EPSRC. We would like to thank Dr. Peter Baltzer for making available his photoelectron spectrum of HBr w35x and for granting permission for our reproducing it here. References w1x D.T. Terwilliger, A.L. Smith, J. Mol. Spectrosc. 50 Ž1974. 30. w2x D.T. Terwilliger, A.L. Smith, J. Chem. Phys. 63 Ž1975. 1008. w3x F. Norling, Z. Phys. 95 Ž1935. 179. w4x M.J. Haugh, K.D. Bayes, J. Phys. Chem. 75 Ž1971. 1472. w5x W.C. Richardson, D.W. Setser, D.L. Albritton, A.L. Schmeltekopf, Chem. Phys. Lett. 12 Ž1971. 349.

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