Vibrational fine structure on the core level photoelectron lines of small polyatomic molecules

JOURNAL OF ELECTRON SPECTROSCOPY and Related Phenomena

ELSEVIER

Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 19-27

Invited paper

Vibrational fine structure on the core level photoelectron lines of small polyatomic molecules M. Neeb 1, B. Kempgens, A. Kivim~iki2, H.M. K6ppe, K. Maier, U. Hergenhahn, M.N. Piancastelli3, A. R/idel, A.M. Bradshaw Fritz-Haber-lnstitut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany

Abstract The C ls and O ls photoelectron spectra of CH4, CD4, CF4, C2H6 and C O 2 have been measured with high photon energy resolution. Vibrational fine structure on the main line (single hole state) could be resolved for all molecules except CF4. Even in the case of C2H6 where several modes are expected to couple, two prominent progressions are visible. The fine structure on the C 1s line of CO2 results, as expected, from the totally symmetric stretching mode, whereas the O I s main line is dominated by the antisymmetric stretching mode of au symmetry due to vibronic coupling. © 1998 Elsevier Science B.V. Keywords: Core level photoelectron spectroscopy; Soft X-ray synchrotron radiation; Vibronic coupling

1. Introduction In the photoionization process a photon interacts with a bound electron, promoting it into the continuum if the energy of the photon exceeds the ionization potential of the system. A n analysis of the kinetic energy of these emitted electrons gives the photoelectron spectrum, which is probably the most important source of information on molecular photoionization. In molecules the vibrational excitation can also accompany photoionization, in which case part o f the photon energy is then stored as vibrational energy in the remaining molecular ion. If the experimental resolution is sufficiently high to resolve the vibrational 'energy losses', a fine structure on the Permanent address: ForschungszentrumJiilich GmbH, Institut fiir Festk6rperforschung, 52425 Jiilich, Germany. 2 Permanent address: Department of Physical Sciences, University of Oulu, 90570 Oulu, Finland. 3 Permanent address: Department of Chemical Sciences and Technologies, University "Tor Vergata", 00133 Rome, Italy.

photoelectron line will be evident. In the case of core level photoelectron spectroscopy this may be difficult to o b s e r v e - - i n contrast to the valence r e g i o n - - o w i n g to a combination o f the core hole lifetime broadening and insufficient photon energy resolution in the soft X-ray regime. Indeed, with one or two prominent exceptions [ 1,2], the vibrational fine structure has only recently been resolved in core level photoelectron spectra [3-18]. Further complications arise in resolving and interpreting the vibrational fine Structure o f polyatomic molecules when more than one vibrational mode is excited. A n electronic transition, however, normally takes place on a much shorter time scale than the vibrational period of the nuclei. The F r a n c k - C o n d o n principle can thus be applied to the evaluation of the transition amplitudes: by making use of the B o r n Oppenheimer approximation, the dipole operator o f the ionizing photon field can then be considered as acting only on the electronic parts of the molecular wavefunctions, w h i l e the vibrational parts are

0368-2048/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved PH S0368-2048(97)00262-4

20

M. Neeb et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 19-27

combined in the form of an overlap integral. In this situation, only the totally symmetric vibrations are excited, which simplifies the vibrational fine structure for polyatomic molecules and, in the absence of calculations, increases the chances of identifying the components of the fine structure. If the BornOppenheimer approximation fails, however, as has sometimes been observed in valence level photoionization, different selection rules apply and non-totally symmetric modes may also be allowed via vibronic coupling. Photoelectron spectroscopy in the soft X-ray range 200-700 eV, in which the binding energies of the 1s orbitals of most second-row elements occur, requires a monochromator resolving power of better than 103 in order to separate the vibrational fine structure on core level lines. A resolving power of this order of magnitude has been achieved in recent years on spherical grating and plane grating monochromators at several synchrotron radiation sources. In combination with undulator radiation, these types of monochromator also provide sufficient photon flux in order to measure photoelectron spectra at typical gas target pressures between 10 3 and 10 -4 mbar. In the present paper we report C 1s and O l s photoelectron spectra of several small polyatomic molecules (CH4, CO4, CF4, C2H6 and CO2). With the exception of CF4, the vibrational fine structure is resolved, or partly resolved, on the main photoelectron lines of all molecules studied. The spectra of CH 4 and CzH 6 have been re-measured with slightly higher resolution than in our previous publications [6,11]. Furthermore, it has been found that the O ls spectrum of CO2 cannot be described within the Born-Oppenheimer approximation [15].

2. E x p e r i m e n t a l

Photoelectron spectra were measured on the X1B undulator beamline [19] at the National Synchrotron Light Source, Brookhaven. Prior to the experiments the mechanically ruled blazed grating (800 l mm l, fused silica blank, Au coating) had been replaced by a holographically manufactured laminar grating (800 1 mm -I, silicon blank, Au coating). Fig. 1 shows the photocurrent Io as measured on a gold-coated copper

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Fig. I. Photon flux vs. photon energy measured on the X I B monochromator behind the exit slit. The dotted curve is the result obtained with the former (blazed) grating. The undulator gap was 39.5 ram, which corresponds to the first harmonic at 305 eV. The full curve is the result obtained with the new, laminar grating at the same gap setting. The dash-dotted curve is the measurement with the new grating, but with a gap of 56.5 mm, corresponding to the first harmonic at the oxygen edge at 550 eV. Note that the entrance slit during the tests with the new grating was approximately a factor of six narrower than for the previous measurement. For the same slit width the photon flux at the C Is edge ( 3 0 0 e V ) is approximately the same as before; at photon energies higher than 5 5 0 e V (O Is) the flux is a factor of 5 - 1 0 greater.

plate behind the exit slit immediately before (dotted line) and after (full line) the change of grating. The normalized intensity (nA mA~,nlg s- l) is displayed on the ordinate while the grating has been scanned over the X 1 undulator wavelength range. The maximum in efficiency of the grating has shifted to higher energy, allowing a more effective use of the undulator radiation up to about 1 keV. The C ls photoelectron spectra were measured typically with a photon energy resolution between 50 and 65 meV and an electron energy resolution of --60 meV. The uncertainty in the values of the photon bandwidth is + 10 meV. An angle-resolving magic-angle cylindrical mirror analyser (CMA) [20] has been used as the energy dispersing element for the photoelectrons. The O ls photoelectron spectrum of CO2 was recorded with a photon energy resolution of about 100meV; the kinetic energy resolution was 105 meV. All measurements were normalized with respect to the photon flux and target gas pressure.

M. Neeb et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 19-27

3. Results and discussion Figs. 2, 4 - 6 show the C ls (O Is) main photoelectron lines of C H 4 and C D 4 , C F 4 , C 2 H 6 and CO2 respectively. The spectra are displayed on a kinetic energy scale and the accumulated electron counts are shown on the ordinate. To keep the ionization crosssection as high as possible, the photon energy was chosen in each case to be close to threshold. A non-linear least squares fit procedure, which takes care of the intrinsic as well as experimental line broadening, has been applied to all spectra in order to extract the contributions to the vibrational fine structure [7]. The full line through the experimental data points (circles) in each spectrum is the fitted curve which includes the instrumental broadening of the monochromator (Gaussian) and the experimentally determined line profile of the electron energy analyser. In contrast to the full line, the dotted lines

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Kinetic energy (eV) Fig. 2. The C ls main line photoelectron spectra of (a) CH 4 and (b) CD4. The experimental data are shown as circles. The photon bandwidths were --50 meV and --65 meV respectively. The electron energy resolutions were 60 meV and 70 meV respectively. The fit (full line) through the data shows the summation of the individual dashed lines after convolution with the instrumental profiles.

21

are not convoluted with the instrumental line shapes, so that the intrinsic broadening due to the core hole lifetime and post-collision interaction (PCI) are observable. The dotted lines, therefore, have higher maxima than in the experimental data. After convoluting the sum of the dotted lines by the experimental functions, the full line through the data points is obtained. More details concerning the fit routine can be found in Ref. [7]. Fig. 2 shows the C Is photoelectron spectra of C H 4 and CD4 measured at photon energies of 310.3 eV and 310.1 eV respectively. Both have a well-resolved vibrational fine structure with a maximum at v' = 0. (Only in the case of C 1s photoionization of CO in the near-threshold region have we so far observed a vibrational fine structure with a maximum at v' = 1. This is due to non-Franck-Condon effects and/or autoionization [5,6,9].) In CH4 (Fig. 2(a)) three vibrational quanta up to v' = 2 are excited, whereas in the isotopic species the v' = 3 component is evident from a shoulder at 18.5 eV kinetic energy. The main difference between Fig. 2(a) and Fig. 2(b) is the vibrational spacing: as obtained from the fit, the energies for C H 4 and C D 4 a r e 396 -+ 2 meV and 284 _+ 2 meV respectively. There are nine possible normal modes of vibration: l'l which is totally symmetric (al), 1'2 which is twofold degenerate (e) as well as 1"3 and 1'4 which are threefold degenerate (t2). The C - H stretch (1' l) in the neutral ground state of C H 4 ( C D 4 ) has an energy of 361 meV (258 meV) [21] and is most likely the mode giving rise to the vibrational fine structure. The energies of the 1'2 and 1'4 modes are considerably lower: 189 meV (131 meV) and 162 meV (123 meV) [21]. The 1'3 mode in the neutral molecule has a very similar frequency to pt: 374 meV (280 meV) [21], although it is not expected to contribute to the fine structure to any large extent. Neglecting anharmonicity, the frequency ratio of 0.717 for the vibrational energies in the two isotopomers is very close to that expected from the inverse ratio of the square-root of the reduced masses. We note that a similar value has been obtained for the corresponding ratio associated with the ls ~ 3p(t2) Rydberg resonance in the absorption spectrum [22]. In our earlier study [11], at somewhat poorer resolution, we deduced a lifetime of 83( -+ 10) meV; the present measurements indicate that this value may be several millielectron-volts too low.

22

M. Neeb et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 19-27

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Fig. 3. Potential energy curves for the ground states and C Is-ionized final states of CH 4 and CD 4 along the C - H symmetric stretching coordinate. As the same portions of the potential curves are reached in core ionization, the C ls photoelectron line of the deuterated species shows one more vibrational line due to the decreased vibrational energy. (After Kempgens [25].)

The vibrational fine structure on the C ls photoelectron line of C H 4 appears to be a unique case amongst polyatomic molecules in that it is dominated by a single progression with a large energy spacing. It was resolved many years ago using monochromatized A1 Kc~ radiation by Gelius et al. [1]. The vibrational fine structure in the CD4 spectrum has been resolved with zero-electron-kinetic-energy (ZEKE) spectroscopy [23], although a reliable determination of the spectral parameters was not possible owing to a strongly distorted PCI line shape at the ionization threshold. Conventional photoelectron spectra of CH4 as a function of photon energy in the nearthreshold region have been measured by K6ppe and coworkers [7,11], but, as noted above, at somewhat poorer resolution than in Fig. 2. The photon energy dependence of the angular asymmetry, partial crosssection and the core level satellite structure were also analysed. The recent paper of Osborne et al. [14] contains a C H 4 spectrum at a similar resolution to that of Fig. 2(a). From a Franck-Condon analysis [24] the C - H bond length is found to be contracted by 0.048 _+ 0.002 A with respect to the neutral ground state. The equilibrium bond length of C Is-ionized C H 4 is thus 1.038 -+ 0.002 A. As the same parts of the potential curve are reached in the core-ionized states of CH4

and C D 4 (i.e. the Franck-Condon regions are the same) the C ls photoelectron line of C D 4 shows one more vibrational state due to the decreased vibrational energy. Fig. 3 shows schematically the ground state and the C Is-ionized potential energy curves for CH4 and C D 4. The C 1s photoelectron line of CF4 measured at a photon energy of 313.4 eV is given in Fig. 4. The overall peak envelope is much narrower than those of C H 4 and CD4 in Fig. 2; no vibrational fine structure can be resolved and the profile suggests that the vibrational progression is very short. Owing to the heavier

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M. Neeb et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 19-27

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F ligands the vibrational frequency is expected to decrease by a factor of about x/r~ compared with CH4 (assuming the same force constant), from which we roughly estimate a vibrational spacing for the totally symmetric stretching mode of 90 meV. (A similar, crude estimate can be obtained by simply taking the frequency of the symmetric C - F stretch in the neutral ground state which has a value of 120 meV [21].) A 90 meV spacing has been used as the basis for the fit in Fig. 4, which shows a very small second vibrational component. The lifetime broadening is considerably lower than that in C H 4 (in agreement with the prediction of Coville and Thomas [26]), but it is not possible to determine a reliable value, Tetrahedral molecules have only one totally symmetric mode and, in the case of CH4, the large energy spacing gives rise to a very pronounced fine structure, The situation is quite different for polyatomic molecules with two or more totally symmetric modes, Fig. 5 shows the C ls photoelectron line of C2H6 which belongs to the D3d point group and contains two equivalent carbon atoms connected by a single bond. Within the dipole approximation all three totally symmetric modes could appear in the photoelectron spectrum. In fact, the latter looks similar to that of CH4, except for a shoulder on each main structure which results from a second progression with a quite different energy spacing. The fit gives vibrational spacings for the two progressions of 407( --5) and 176( _ 5)meV. Osborne et al. [14] give fit values of 404( _ 5) meV and 185( --- 5) meV respectively. The first is assigned to the C - H stretching

mode (/)l) which is --10% higher than the energy of /)1 in the neutral ground state (366 meV) [27]. The second we assign either to the dipole-allowed C H 3 deformation mode (/)2), or to the C - C stretching mode (/)3), or to contributions from both. Using Hartree-Fock calculations Bozek et al. [18] have recently shown that the C - H scissor and C - C stretching modes (designated /)3 and /)2)both contribute to the lower energy progression in ethene. As in the present case of ethane, both modes are actually mixtures of C - C stretch and C - H bend. In the ground state of ethane )'e and /)3 have energies of 172 meV and 123 meV respectively, so that an assignment solely in terms of/)3 would mean an unlikely increase of --40% in its energy upon ionization. Although there is no obvious indication of non-totally symmetric modes, their presence cannot be excluded. In particular, the asymmetric C - H stretch, /)5, might be expected to contribute to the high energy progression via vibronic coupling (see the discussion on CO2 below), in the same way as has been suggested for the/)li mode in ethene [28]. It is certain that vibronic coupling plays a role in the photoabsorption spectrum of the latter molecule [29-31]. In the photoelectron spectrum, however, the same problem is encountered as in Fig. 5: the mode is expected to have a frequency very similar to the totally symmetric C - H stretch and thus to be unresolvable in a fitting procedure without prior knowledge. Formally at least, the CH3 deformation mode /)6 may also be present due to vibronic coupling. The linear COe molecule is the simplest polyatomic

24

M. Neeb et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 19-27

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molecule investigated here; the point group is D~ h and the C atom lies at the centre of inversion. Four normal modes exist: one totally symmetric stretching mode (vl), one antisymmetric stretching mode (v2) and two degenerate bending modes (1P3,~4). Only the coupling of vl is dipole-allowed. Fig. 6(a) shows the C ls spectrum of CO2 measured at a slightly different photon energy than in our earlier publication (311 eV instead of 313 eV) [15]. Three vibrational quanta are partly resolved on the C Is photoelectron line with the maximum at v' = 0. The fit predicts one vibrational mode with a spacing of 161( _+ 7 ) m e V which we assign to the totally symmetric C - O stretching mode vl. The lifetime broadening is 78( + 15) meV. The vibrational energy is slightly lower than that in the neutral ground state (167 meV). This is not the case in the tetrahedral molecules discussed above and may be connected with a more effective shielding of the core hole by the non-bonding electrons on the O ligands as well as with the stronger double bond. The situation is quite different for oxygen

photoionization. The corresponding O l s photoelectron line is shown in Fig. 6(b) on the same relative energy scale as in Fig. 6(a). Here, the vibrational fine structure is more pronounced because of a considerably larger spacing of 307( ___ 3) meV. The observed frequency is very close to that of the au antisymmetric stretching mode in the neutral ground state (291 meV). We thus have an example of vibronic coupling, i.e. the Born-Oppenheimer approximation does not hold, and a non-totally symmetric vibration is excited. The reason is the presence of the two equivalent O atoms which lead to lag and lau combinations of the ls orbitals with a very small energy splitting of 1.5 meV [32]. The lag and lau orbitals are then coupled via the au mode. The effect of the nearly degenerate O ls orbitals is shown schematically in Fig. 7. When ionization occurs at the outer end of the molecule, the nuclei are subject to asymmetric forces and all three begin to move relative to the former inversion point. This asymmetric motion removes the equivalence of the two O atoms and the Og and au orbitals split under the influence of the broken symmetry (C~v) into l a and 2a orbitals. Owing to the symmetry lowering the core hole becomes dynamically localized. However, since it is not possible to decide if the ionization takes place on the right atom or on the left atom, we have to consider a superposition of two asymmetric motions. The full symmetry of the resulting vibronic eigenstates of the ion therefore remains D~h and the localization of the core hole has to be considered as a dynamic process. In ab initio calculations [33] a static localization of the core hole has been used to describe the symmetry breaking, an assumption which results in a dipole-allowed antisymmetric vibration without consideration of vibronic coupling. Although the O 1s photoelectron line profile has been well reproduced in these calculations, this approach (also used by Bozek et al. [18] for C2H4) could be considered formally as incorrect. This is, however, not obvious when comparison is made with the photoelectron spectrum. On the other hand, a static localization of the core hole would be inadequate in the calculation of soft X-ray emission spectra where it has been shown that the parity selection rule still holds in the presence of a core hole [13]. Simple group theory shows that vibronic coupling does not occur in tetrahedral molecules when

M. Neeb et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 19-27

25

no symmetrybreaking O=C=O

O--e--Q Q--e-O cls 01s=~

lCg,l~u

Fig. 7. Vibronic coupling and dynamic core hole localization upon core ionization of the two equivalent O ls orbitals in CO> Left: ionization without vibronic coupling would lead to a non-localized core hole. Right: the core hole is dynamically localized in the antibonding 2a orbital owing to the asymmetric motion of the nuclei. Owing to the non-adiabatic coupling process between the lag, lau and the au vibrational modes, however, the total symmetry is still conserved, but the au mode also appears in the vibrational fine structure. ionization occurs at the s orbital of the central atom. In the tetrahedral compounds of Si it will, however, appear if the 2p orbital is ionized [3] owing to a J a h n - T e l l e r splitting of the triply degenerate level. Similar to the O atoms in CO2, the two C atoms in C 2H6 are also equivalent and, as we have noted above, vibronic coupling may also be present in the C l s photoelectron spectrum. That it is not readily apparent may be due to a stronger interaction of the two C 1s orbitals than between the two O ls orbitals in CO2, leading to an energy splitting which is somewhat larger. Coupling of the resulting Cru and Og levels by a vibrational mode then occurs less readily. Domcke and Cederbaum [32] first predicted non-adiabatic coupling to the antisymmetric vibrational mode in the O ls core ionization spectrum of CO2 20 years ago. Although they correctly ascertained the dominance of the antisymmetric mode, the totally symmetric mode still had some intensity in their calculations. This may be explained by relaxation effects which were neglected in the one-electron approximation [32]. Relaxation effects are known to be much more important in core ionization than in valence ionization and thus to play an important role in determining the strength of vibronic coupling [34]. The doubly degenerate bending mode cannot be excited in photoionization (not even via vibronic

coupling) since the O Is-derived 'molecular' orbitals do not have 7r symmetry. This mode can only be observed in photoabsorption if a core electron is excited into a 7r orbital, as shown in the C l s angle-resolved ion-yield absorption spectrum [35]. The line width due to lifetime broadening in the O l s -1 state of CO2 is 165( - 10) meV, which is considerably larger than that in the C 1S-1 state. F r o m a F r a n c k - C o n d o n analysis the change in equilibrium bond length along° the ou coordinate is found to be 0.055( _+ 0 . 0 0 5 ) A [15], corresponding to a shortening/lengthening of the C - O bond by 0.039( - 0 . 0 0 4 ) A .

4. Conclusions The C l s and O l s main photoelectron lines have been measured for the five polyatomic molecules CH4, CD4, CF4, C2H6 and CO2 at high spectral resolution. Vibrational fine structure up to v' = 3 has been resolved for all molecules except CF4; the peak m a x i m u m is at v' = 0 in each case. The tetrahedral molecules CH4, CD4 and CF4 cannot exhibit vibronic coupling and the dipole-allowed totally symmetric mode a p p e a r s to be the only one excited upon C 1 s ionization. In C2H 6 two vibrational progressions are

26

M. Neeb et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 19-27

observed. The progression with the h i g h e r e n e r g y spacing is assigned to the totally s y m m e t r i c C - H stretch with a possible contribution f r o m an a s y m metric C - H stretch due to vibronic coupling. T h a t with the l o w e r e n e r g y spacing is ascribed either to the C - C m o d e , or to the CH3 d e f o r m a t i o n m o d e or to a m i x t u r e of both. CO2 is the first case w h e r e vibrational fine structure has been r e s o l v e d on an O l s line. T h e latter is d o m i n a t e d by the a n t i s y m m e t r i c cru m o d e , which b e c o m e s a l l o w e d through vibronic c o u p l i n g i n v o l v i n g the nearly d e g e n e r a t e o x y g e n c o r e orbitals. T h e C i s s p e c t r u m contains only the totally symmetric m o d e . T h e existence o f e q u i v a l e n t atoms is a necessary c o n d i t i o n for vibronic c o u p l i n g b e t w e e n 1 s orbitals; the effect also p r o v i d e s a m e c h a n i s m for d y n a m i c c o r e h o l e localization. As a perspective, w e note that the nuclear d y n a m i c s in systems h a v i n g m o r e than two e q u i v a l e n t centres, has b e e n discussed theoretically [36]. H o p e f u l l y , they will h a v e b e e n fully investigated e x p e r i m e n t a l l y in less than 20 years f r o m now.

Acknowledgements W e thank L.S. C e d e r b a u m , H. K6ppel, J. S c h i r m e r and T.D. T h o m a s for stimulating discussions. Part o f this w o r k has b e e n financed by the D e u t s c h e Fors c h u n g s g e m e i n s c h a f t . O n e o f the authors ( A K ) w o u l d also like to a c k n o w l e d g e support f r o m the R e s e a r c h C o u n c i l for the Natural S c i e n c e s o f the A c a d e m y of Finland. T h e N a t i o n a l S y n c h r o t r o n L i g h t Source at B r o o k h a v e n National L a b o r a t o r y is supported by the U.S. D e p a r t m e n t o f E n e r g y u n d e r Contract No. D E - A C 0 2 - 7 6 C H 0 0 0 1 6 .

References [1] U. Gelius, S. Svensson, H. Siegbahn, E. Basilier, AFaxiil,~., K. Siegbahn, Chem. Phys. Lett. 28 (1974) 1. [2] L. Asplund, U. Gelius, S. Hedman, K. Helenelund, K. Siegbahn, P.E.M. Siegbahn, J. Phys. B: At. Mol. Phys. 18 (1985) 1569. [3] J.D. Bozek, G.M. Bancroft, J.N. Cutler, K.H. Tan, Phys. Rev. Lett. 65 (1990) 2757. [4] R.G. Cavell, K.H. Tan, Chem. Phys. Lett. 197 (1992) 161. [5] K.J. Randall, A.L.D. Kilcoyne, H.M. K6ppe, J. Feldhaus, A.M. Bradshaw, J.-E. Rubensson, W. Eberhardt, Z. Xu, P.D. Johnson, Y. Ma, Phys. Rev. Lett. 71 (1993) 1156.

[6] H.M. K6ppe, A.L.D. Kilcoyne, J. Feldhaus, A.M. Bradshaw, J. Chin. Chem. Soc. 42 (1995) 255. [7] H.M. Kiippe, A.L.D. Kilcoyne, J. Feldhaus, A.M. Bradshaw, J. Electron Spectrosc. Relat. Phenom. 75 (1995) 97. [8] S.J. Osborne, A. Ausmees, S. Svensson, A. Kivimiiki, O.-P. Sairanen, A. Naves de Brito, H. Aksela, S. Aksela, J. Chem. Phys. 102 (1995) 7317. [9] H.M. K6ppe, B. Kempgens, A.L.D. Kilcoyne, J. Feldhaus, A.M. Bradshaw, Chem. Phys. Lett. 260 (1996) 223. [10] B. Kempgens, A. Kivimiiki, M. Neeb, H.M. K6ppe, A.M. Bradshaw, J. Feldhaus, J. Phys. B: At. Mol. Opt. Phys. 29 (1996) 5389. [11] H.M. K/Sppe, B.S. Itchkawitz, A.L.D. Kilcoyne, J. Feldhaus, B. Kempgens, A. Kivim/iki, M. Neeb, A.M. Bradshaw, Phys. Rev. A 53 (1996) 4120. [12] M.R.F. Siggel, C. Field, L.J. Saethre, K.J. BCrve, T.D. Thomas, J. Chem. Phys. 105 (1996) 9035. [13] P. Glans, K. Gunnelin, P. Skytt, J.-H. Guo, N. Wassdahl, J. Nordgren, A..~gren, F.Kh. Gel'mukhanov, T. Warwick, E. Rotenberg, Phys. Rev. Lett. 76 (1996) 2448. [14] S.J. Osborne, S. Sundin, A. Ausmees, S. Svensson, LJ. Saethre, O. Svaeren, S.L. Sorensen, J. V6gh, J. Karvonen, S. Aksela, A. Kikas, J. Chem. Phys. 106 (1997) 1661. [15] A. Kivimgki, B. Kempgens, K. Maier, H.M. K6ppe, M.N. Piancastelli, M. Neeb, A.M. Bradshaw, Phys. Rev. Lett. 79 (1997) 998. [16] B. Kempgens, H. K6ppel, A. Kivim/iki, M. Neeb, L.S. Cederbaum, A.M. Bradshaw, Phys. Rev. Lett., in press. [17] L.J. Saethre, O. Svaeren, S. Svensson, S. Osborne, T.D. Thomas, J. Jauhiainen, S. Aksela, Phys. Rev. A 55 (1997) 2748. [18] J. Bozek, T. X. Carroll, J. Hahne, L.J. Saethre, J. True, T.D. Thomas, Phys. Rev. A, in press. [19] K.J. Randall, J. Feldhaus, W. Erlebach, A.M. Bradshaw, W. Eberhardt, Z. Xu, Y. Ma, P.D. Johnson, Rev. Sci. Instrum. 63 (1992) 1367. [20] J. Feldhaus, W. Erlebach, A.LD. Kilcoyne, K.J. Randall, M. Schmidbauer, Rev. Sci. Instrum. 63 (1992) 1454. [21] T. Shimanouchi, Tables of Molecular Frequencies, Consolidated Volume I, NSRDS-NBS, National Bureau of Standards, Washington, 1972, p. 39. [22] G. Remmers, M. Domke, G. Kaindl, Phys. Rev. A 47 (1993) 3085. [23] P.A. Heimann, L.J. Medhurst, M.R.F. Siggel, D.A. Shirley, C.T. Chen, Y. Ma, F. Sette, Chem. Phys. Lett. 183 (1991) 234. [24] B. Kempgens, K. Maier, A. Kivim~iki,H.M. K6ppe, M. Neeb, M.N. Piancastelli, U. Hergenhahn, A.M. Bradshaw, J. Phys. B, in press. [25] B. Kempgens, Ph.D. thesis, Free University of Berlin, 1997. [26] M. Coville, T.D. Thomas, Phys. Rev. A 43 (1991) 6053. [27] G. Herzberg, Molecular Spectra and Molecular Structure II. Infrared and Raman Spectra of Polyatomic Molecules, Van Nostrand Reinhold Company, New York, 1945. [28] B. Kempgens, A. Kivim/iki, B.S. Itchkawitz, H.M. K6ppe, M. Schmidbauer, M. Neeb, K. Maier, J. Feldhaus, A.M. Bradshaw, J. Electron Spectrosc. Relat. Phenom., in press.

M. Neeb et aL/Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 19-27

[29] F.X. Gadea, H. K6ppel, J. Schirmer, L.S. Cederbaum, K.J. Randall, A.M. Bradshaw, Y. Ma, F. Sette, C.T. Chen, Phys. Rev. Lett. 66 (1991) 883. [30] B. Kempgens, B.S. Itchkawitz, K.J. Randall, J. Feldhaus, A.M. Bradshaw, H. K6ppel, F.X. Gadea, D. Nordfors, J. Schirmer, L.S. Cederbaum, Chem. Phys. Lett. 246 (1995) 347. [31] H. K6ppel, F.X. Gadea, G. Klatt, J. Schirmer, L.S. Cederbaum, J. Chem. Phys. 106 (1997) 4415.

27

[32] W. Domcke, L.S. Cederbaum, Chem. Phys. 25 (1977) 189. [33] D.T. Clark, J. Miiller, Chem. Phys. 23 (1977) 429. [34] W. Domcke, L.S. Cederbaum, Chem. Phys. Lett. 31 (1975) 582. [35] S. Adachi, N. Kosugi, E. Shigemasa, A. Yagishita, J. Chem. Phys. 107 (1997) 4919. [36] H.D. Schulte, L.S. Cederbaum, J. Chem. Phys. 103 (1995) 698.