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A reinvestigation of the vibrational structure and the orbital assignments in the photoelectron bands of cyclopropane
Journal of Electron Spectroscopy and Related Phenomena 125 (2002) 57–68 www.elsevier.com / locate / elspec
A reinvestigation of the vibrational structure and the orbital assignments in the photoelectron bands of cyclopropane a, b c D.M.P. Holland *, L. Karlsson , K. Siegbahn a
Synchroton Radiation Department, Daresbury Laboratory, Daresbury, Warrington, Cheshire WA4 4 AD, UK b Department of Physics, Uppsala University, Box 530, SE-75121 Uppsala, Sweden c ¨ 1, SE-75121 Uppsala, Sweden ESCA-LASER Laboratory, Institute for Material Science, Box 534, Regementsvagen Received 5 November 2001; received in revised form 17 January 2002; accepted 31 January 2002
Abstract The valence shell photoelectron spectrum of cyclopropane has been studied using synchrotron and HeI radiation. The photoelectron asymmetry parameters and branching ratios have been determined using monochromated synchrotron radiation in the photon energy range 15–120 eV. The spectral behaviour of the asymmetry parameters associated with the outer valence orbitals has been used to propose an assignment for the valence shell electronic configuration. The experimental features observed in the inner valence energy region of the photoelectron spectrum have been compared with theoretical predictions. The high resolution HeI excited spectrum has allowed vibrational structure to be observed in the outer valence photoelectron bands, and in two of the bands these features can be assigned to progressions involving the n2 (a 19 ) and n11 (e9) ˜ 2 E9 photoelectron band, which exhibits a doublet structure due to Jahn–Teller interaction, displays an modes. The X extended vibrational progression in the doubly degenerate n11 (e9) mode. 2002 Elsevier Science B.V. All rights reserved. Keywords: Photoelectron spectroscopy; Photoelectron angular distributions; Electronic assignments; Vibrational structure; Cyclopropane
1. Introduction The electronic structure and the bonding characteristics of the three-membered ring compound cyclopropane (C 3 H 6 ) have been investigated extensively, with much of the interest arising from the strained nature of the carbon–carbon bonds. Both of these properties have been examined in photoelectron spectroscopic studies using NeI [1], HeI [1–5], HeII [4,6,7] and synchrotron [8] radiation as excitation sources. Nevertheless, in spite of these efforts, *Corresponding author. Tel.: 144-1925-603-425; fax: 144-1925603-124. E-mail address: [email protected] (D.M.P. Holland).
the outer valence shell molecular orbital sequence remains uncertain, and vibrational analyses of the structure observed in the photoelectron bands have yet to be reported. Moreover, only limited information is available concerning the inner valence orbitals. All these issues will be addressed in the present experimental study which uses HeI and synchrotron radiation to record the entire valence shell photoelectron spectrum. In anticipation of our conclusions regarding the electronic configuration, the ground state molecular orbital sequence of cyclopropane can be written as (using D3h symmetry) 2 2 4 4 (1a 19 )2 (1e9)4 (2a 91 )2 (2e9)4 (1a 99 2 ) (3a 9 1 ) (1e0) (3e9)
0368-2048 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 02 )00039-7
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The photoelectron spectrum of cyclopropane reveals four electronic bands, with binding energies less than 18 eV, which may be associated with the outer valence shell orbitals. Ionisation from the outermost orbital gives rise to a photoelectron band displaying a doublet structure, and this has been attributed to Jahn–Teller splitting. The photoelectron band associated with the 1e0 orbital exhibits weak vibrational features, but no evidence of Jahn–Teller interactions. In the binding energy range between 15 and 18 eV two overlapping bands, which must be due to ionisation from the 3a 19 and the 1a 299 orbitals, are discernible in the photoelectron spectrum. However, the question as to which orbital should be attributed to which band has been the subject of much debate, although the results from all theoretical studies [2,5,6,9–14] indicate that the 1a 299 orbital is the more tightly bound of the two. Some of the early molecular orbital assignments were based upon variations observed in the photoelectron band intensities as a function of photon energy. Such variations, at a particular photoelectron detection angle, depend upon a proper knowledge of the asymmetry parameters, and the present results show that the two photoelectron bands associated with the 3a 19 and the 1a 299 orbitals possess significantly different b -parameters. This was not fully appreciated when the early interpretations were being suggested. The electronic structure of cyclopropane has also been investigated using electron momentum spectroscopy [14–17]. In the first [16] of the two studies conducted by Banjavcic et al., the experimental results indicated that the symmetries of the two bands lying in the binding energy range 15–18 eV were s-type, whilst their calculations predicted that one of the bands should be s-type and the other p-type. von Niessen et al. [14] questioned the validity of some of the conclusions reached by Banjavcic et al. [16,17], and undertook a similar electron momentum spectroscopy study to obtain spectra with improved statistics. Their experimental data indicated that the peaks at 15.7 and 16.6 eV possessed p-type and s-type symmetry, respectively. This would lead to the peak at 15.7 eV being assigned to the 1a 299 orbital and the higher energy peak to the 3a 19 orbital, in accordance with the proposals of Evans et al. [3] and of Schweig and Thiel [4]. Although the outer valence electronic structure of
cyclopropane has been studied extensively, the features arising from ionisation of the inner valence (2e9 and 2a 19 ) orbitals have received much less attention. HeII radiation has been used to record the two prominent peaks occurring in the binding energy region up to 30 eV [7], and the entire inner valence region has been investigated, albeit with limited resolution and poor statistics, using electron impact [14,16,17]. In the present study, monochromatic synchrotron radiation has been used to record the complete valence shell photoelectron spectrum, and the results indicate that correlation satellites occur at high binding energies, in agreement with theoretical predictions.
2. Experimental apparatus and procedure
2.1. Synchrotron radiation excited photoelectron spectra The photoelectron spectra were recorded using a hemispherical electron energy analyser and synchrotron radiation emitted by the Daresbury Laboratory storage ring. Detailed descriptions of the monochromator [18] and the experimental procedure [19] have been reported. The electron analyser could be rotated about an axis which coincided with the photon beam, thus enabling photoelectron angular distributions to be measured. The photoionisation differential cross-section in the electric dipole approximation, assuming randomly oriented targets, and electron analysis in a plane perpendicular to the photon propagation direction, can be expressed in the form
F
ds stotal b ] 5 ]] 1 1 ]s3P cos 2u 1 1d 4p 4 dV
G
where stotal is the angle integrated cross-section, b is the photoelectron asymmetry parameter, u is the photoelectron ejection angle relative to the major (horizontal) polarisation axis and P is the degree of linear polarisation of the incident radiation. At each photon energy, photoelectron spectra were recorded at u 508 and u 5908, thus allowing the asymmetry parameter to be determined once the polarisation had been deduced. The degree of polarisation was determined by recording Ar 3p and He 1s photoelectron spectra as a function of photon energy, and
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using the well established b -parameters for these gases [19]. Fig. 1 displays the photoelectron spectrum of cyclopropane recorded at u 508 with a photon energy of 90 eV. The spectra were analysed by dividing the binding energy range into the regions specified in Table 1, and the results are presented in the form of photoelectron asymmetry parameters and b -independent branching ratios. The branching ratio for a specific region is defined as the intensity in that particular region divided by the sum of the intensity in all the energetically accessible regions.
2.2. HeI excited photoelectron spectra The HeI excited spectra were recorded in Uppsala using a photoelectron spectrometer equipped with a double-focussing electron energy analyser. The spectrometer resolution was |8 meV at best. The helium radiation used for ionisation was generated in a source based on a DC-glow gas discharge confined inside a quartz capillary. The main component was the HeIa resonance line at 21.22 eV. The detector consisted of two microchannel plates mounted together in a chevron arrangement and a phosphor screen by which electron pulses were transferred into light pulses for read-out to a computer system via a CCD-camera. The spectrometer and detection system have been described elsewhere [20]. The sample gas was obtained commercially with a purity of better than 98%, and was transferred, via a gas handling system and a needle valve, to the gas cell used for photoionisation. Energy calibration of the spectra was made by a procedure described by Maripuu et al. [21]. It is based on the simultaneous recording of the spectrum from a mixture of the sample gas, argon and krypton using the Ar 3p 1 / 2,3 / 2 lines at 15.9372 and 15.7596 eV, respectively, as well as the Kr 4p 1 / 2,3 / 2 lines at 14.6656 and 13.9997 eV, respectively, as reference lines [22].
3. Results and discussion
3.1. Overall spectrum Fig. 1 displays the complete valence shell photoelectron spectrum of cyclopropane together with the results from the ADC(3) calculations performed by
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von Niessen et al. [14]. The agreement between the experimental data and the theoretical predictions is generally good, particularly in the outer valence region where it is apparent that the single particle model of ionisation is valid [23]. According to the ADC(3) calculations, ionisation from each of the four outer valence orbitals gives a main-line with a pole strength P|0.9 and no significant accompanying satellites. The calculated binding energies are also in good agreement with the experimental values listed in Table 2. The photoelectron bands associated with the outer valence orbitals will be described in greater detail in Section 3.3. Ionisation from the 2e9 and 2a 19 orbitals gives rise to two prominent peaks in the photoelectron spectrum with maxima occurring at 19.63 and 26.30 eV. Similar features were reported by Potts and Streets [7] with binding energies of 19.5 and 26.5 eV. Fig. 1 shows that both of these strong peaks are broad and structureless, and exhibit a tail towards high binding energy. Another very broad feature, centred at 32.2 eV, can probably be attributed to correlation satellites originating from the 2a 19 orbital. For the 2e9 orbital, the molecular orbital model of ionisation still appears to be reasonably valid, with the ADC(3) calculations predicting a main-line at 19.89 eV with a pole strength of 0.82 [14], in good agreement with the experimental results. Furthermore, a weak satellite is predicted to occur at a binding energy of 22.19 eV and this feature may, in part, be responsible for the high energy tail. For the innermost 2a 19 orbital the single particle approximation no longer holds, and this results in the strongest feature, occurring at 26.88 eV, having a pole strength of only 0.53 [14]. Nevertheless, the predicted binding energy of this line is in fair agreement with the maximum observed at 26.30 eV. A fairly strong satellite, with a pole strength of 0.08, is calculated to lie at 27.4 eV [14], and may, in part, account for the high energy tail evident in the experimental spectrum. Although the ADC(3) computations seem to be successful in describing the major features occurring in the inner valence region, the broad feature observed around 32 eV remains unexplained. On the basis of the experimental spectrum it appears that the breakdown of the molecular orbital model of ionisation is more severe for the 2a 19 orbital than is predicted theoretically. This may also be true, although to a lesser extent, for the 2e9
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Fig. 1. Upper frame: The valence shell photoelectron spectrum of cyclopropane recorded at a photon energy of 90 eV. Lower frame: A theoretical spectrum of cyclopropane obtained using the ADC(3) approach, as reported by von Niessen et al. [14]. The relative intensities plotted are the pole strengths.
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Table 1 Energy regions used in the analysis of the photoelectron spectra recorded with synchrotron radiation Region
Binding energy range (eV)
Encompassed states
1 2 3 4 5 6
9.50–12.17 12.17–15.00 15.00–16.40 16.40–18.10 18.40–23.00 24.00–29.20
˜ 2 E9 (3e9)21 X 21 ˜ 2 (1e0) A E0 ˜ 2 A 91 (3a 91 )21 B 21 ˜ 2 (1a 299 ) C A 992
orbital, because the ADC(3) calculations do not predict sufficient satellite structure between |22 and 40 eV to match the photoelectron intensity observed in the experimental spectrum. It is possible that this represents a case where it is necessary to include 3-holes–2-particles-excited configurations in the calculations in order to explain the experimental data. Further theoretical investigations of this point would be of interest.
3.2. Photoelectron angular distributions and branching ratios The photoelectron asymmetry parameters and branching ratios for cyclopropane, corresponding to the energy regions specified in Table 1, are plotted in Figs. 2 and 3, respectively. Keller et al. [8] have previously carried out angular distribution measurements on the outer valence orbitals in the photon energy range 13–28 eV. In the region of overlap, the two sets of measurements display the same spectral
behaviour, although in the threshold region the present values for the b -parameters associated with 3e9 and the 1e0 orbitals lie slightly higher than those reported by Keller et al. The asymmetry parameters corresponding to regions 1, 2 and 4 exhibit a similar photon energy dependence, rising from a fairly low value close to threshold and reaching b |1.5 at high energy. Previous work has demonstrated that regions 1 and 2 can be associated with the 3e9 and the 1e0 orbitals, respectively. The asymmetry parameter for region 3 displays a completely different spectral behaviour compared to those for regions 1, 2 and 4. Fig. 2 shows that the b -value for region 3 is greater than unity at threshold, and that it remains high throughout the photon energy range covered in the present experiment. Moreover, compared to the asymmetry parameters for regions 1, 2 and 4, that for region 3 displays only a relatively small change (|0.4) as a function of energy. The asymmetry parameters for regions 5 and 6 are typical of those associated with molecular orbitals composed primari-
Table 2 Assignments of photoelectron bands, electron binding energies, and vibrational energies and assignments of the valence shell photoelectron spectrum of the cyclopropane molecule Electronic state
Vertical binding energy a (eV)
Predicted energy (eV) from the ADC(3) calculations [14]
Vibrational mode
˜ 2 E9 (3e9)21 X 21 ˜ 2 (3e9) X E9 ˜ 2 E0 (1e0)21 A 21 ˜ 2 9 (3a 1 ) B A 91 (1a 992 )21 C˜ 2 A 992 ˜ 2 E9 (2e9)21 D (2a 91 )21 E˜ 2 A 91 (CI state) 2 A 91
10.47 11.25 12.96 15.77 16.66 19.63 26.30 32.2
10.88 10.88 13.12 15.91 16.93 19.89 b 26.88 b
n11 (e9)
60
n2 (a 91 )
130
a b
Energy corresponding to the maximum intensity of the photoelectron band. Main CI component.
Vibrational energy (meV)
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Fig. 2. The photoelectron asymmetry parameters of cycloproprane associated with the energy regions specified in Table 1.
ly of atomic s-type orbitals. The b -value is reasonably high (|1) at threshold and increases to 1.6–1.8 at high energy. The orbital assignments for the photoelectron bands in regions 3 and 4 have been discussed ever since Basch et al. [2] reported the original HeI excited spectrum and associated bands 3 and 4 with the 3a 19 and the 1a 299 orbitals, respectively. Later, Schweig and Thiel [4], based upon their studies using HeI and HeII radiation, proposed that the ordering should be reversed. This reversal was subsequently supported by Evans et al. [3]. Leng and Nyberg [1] determined the asymmetry parameters for
bands 3 and 4 using NeI and HeI radiation, and concluded that the original assignment of Basch et al. [2] was indeed correct. The original assignment was also used by Keller et al. [8], although the emphasis of their study was aimed at the Jahn–Teller splitting observed in the first band, and not the assignments of the higher energy bands. In contrast to these apparently contradictory results from experimental studies, the theoretical work [2,5,6,9–14] has been consistent in predicting that bands 3 and 4 should be identified with the 3a 19 and the 1a 99 2 orbitals, respectively. Set against this uncertain background, the recent
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Fig. 3. The photoelectron branching ratios of cyclopropane associated with the energy regions specified in Table 1.
electron momentum spectroscopy results obtained by von Niessen et al. [14] showed that the momentum distribution for band 3 was consistent with that of a transition having p-like symmetry, whilst the distribution for band 4 was consistent with a transition having s-like symmetry. Hence, on the basis of their experimental data, bands 3 and 4 were identified with the 1a 299 and the 3a 19 orbitals, respectively, even though this assignment was in contradiction to their own ADC(3) calculations. The angle resolved photoelectron spectra shown in Fig. 4 help to explain some of the difficulties encountered in interpretations based upon band intensities observed in HeI or HeII excited spectra measured at one specific detection angle. The spectra
in Fig. 4 encompass the outer valence orbitals, and it can be seen that the relative intensities of bands 3 and 4 depend markedly on both the detection angle and the incident photon energy. The present data yield b -parameters for band 3 of 1.19 and 0.69, at photon energies of 21 and 40 eV, respectively, whilst the corresponding values for band 4 are 0.85 and 1.42. This large variation in the b -parameters was not fully appreciated in the early studies. If the analogy of ionisation from s-type or p-type orbitals is applied to the present data, then the consistently high value of the asymmetry parameter associated with region 3 suggests that an s-type orbital is involved, because it is well established that ionisation from an atomic s orbital results in b 52.
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Fig. 4. Photoelectron spectra of cyclopropane recorded at photon energies of 21 and 40 eV, and at u 508 and u 5908. For each photon energy, the relative intensities at u 508 and u 5908 are shown correctly. However, the relative intensity at one photon energy to that at the other energy is arbitrary.
This indicates that band 3 should be associated with the 3a 19 orbital, in accordance with theory, but in disagreement with the apparently convincing experimental data reported by von Niessen et al. [14]. We are unable to put forward an explanation for this discrepancy. Theoretical calculations of the photoelectron asymmetry parameters would allow the present proposals to be placed on a firmer footing. On the experimental side, an electron momentum spectroscopy study performed at an improved resolution could help clarify the issue. In the work performed by Banjavcic et al. [16,17] and by von Niessen et al. [14] the resolution was insufficient to separate the bands due to the 3a 19 and the 1a 99 2 orbitals, and deconvolution procedures had to be employed. A study in which these two bands could be separated might help resolve this uncertainty. The photoelectron branching ratios for the valence
shell orbitals are shown in Fig. 3. The branching ratios for the inner valence features increase relative to those for the outer valence orbitals as the photon energy increases. This behaviour is due to the relative variations in the atomic carbon 2s / 2p crosssections as a function of photon energy. At high energies the cross-sections associated with p-type orbitals decrease faster than those associated with s-type orbitals. Thus, since the inner valence orbitals consist primarily of s-type orbitals their branching ratios become more significant at high energies.
3.3. Studies using HeI radiation Fig. 5 shows the overall HeI excited outer valence shell spectrum of cyclopropane, together with the proposed orbital assignments. The innermost band, associated with the 2e9 orbital, can be observed in
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Fig. 5. The overall HeI excited photoelectron spectrum of cyclopropane covering the binding energy range 9–20 eV.
the binding energy range 19–20 eV and has a distorted shape due to a high background of low kinetic energy electrons. At lower binding energies the bands are well resolved and the spectrum resembles the synchrotron radiation excited counterpart plotted in Fig. 1, but is better resolved. As has already been discussed in the Introduction, the first band, identified with the degenerate 3e9 orbital, is widely split into two components due to a Jahn– Teller instability which reduces the molecular symmetry from D3h to C2v (or possibly even to Cs ). At this lower symmetry the degenerate 2 E9 electronic state is split into two non-degenerate states, and these are revealed by the two photoelectron bands with intensity maxima at 10.47 and 11.25 eV. The second and the fifth photoelectron bands, appearing with maximum intensity at binding energies of 13.0 and 19.6 eV, respectively, are also associated with degenerate states which are susceptible to Jahn–Teller instability. However, the effect is evidently smaller in these two cases. Nevertheless, a broadening of the bands can be inferred, and this could correspond to a small splitting of, at most, 0.5 eV between the two components. These observations agree qualitatively with the orbital energies predicted by Buenker and Peyerimhoff [24], where the split-
ting of the 2e9 and 1e0 orbitals is distinctly smaller than that for the 3e9 orbital, on lowering the symmetry. The first component of the 3e9 band shows extensive vibrational structure which can be seen in the detailed recording plotted in Fig. 6. The main structure forms a regular progression with a spacing of |60 meV, and probably corresponds to excitations of the Jahn–Teller active n11 (e9) vibrational mode which has an energy of 107 meV in the neutral ground state [25]. This assignment assumes a substantial lowering, by |44%, of the vibrational energy upon ionisation, but since the 3e9 orbital strongly affects the molecular bonding such a large change in energy is not unlikely. The n11 vibrational mode corresponds to a mixed ring-deforming and CH 2 wagging motion, which is consistent with the expected symmetry lowering upon ionisation. The widths of the individual lines in the progression discernible in Fig. 6 vary somewhat, and this indicates that other vibrational modes, in addition to n11 , are being excited, although the distortions are not sufficient to enable further assignments to be made. Moreover, the vibrational structures close to the band onset are irregular, which could also reflect excitations of other modes. However, an alternative
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Fig. 6. A detail of the HeI excited photoelectron spectrum of cyclopropane showing the outermost of the two Jahn–Teller components of the 3e9 (s) band. A vibrational progression in the n11 (e9) mode is marked, starting at 9.93 eV. The average spacing between the lines is 60 meV.
explanation of these irregularities could be that the lowest degenerate vibronic energy levels are split by a potential barrier which locks the molecule into a state of symmetry lower than D3h , where internal rotation by the n11 mode is effectively prevented. Such an effect can be explained by second order perturbation theory and requires strong vibronic coupling in order to be observable in an experimental spectrum. An example can be seen in the photoelectron spectrum of H 2 S [26]. However, the large splitting, of more than 1 eV, between the two major ˜ 2 E9 state suggests Jahn–Teller components in the X that the coupling could indeed be of such a magnitude. Further theoretical investigations into this matter would be of interest, especially since evidence of second order Jahn–Teller coupling in photoelectron spectra is sparse. The vibrational structure seems to be restricted to the lower energy Jahn–Teller component, with the band corresponding to the higher energy component being essentially structureless (Fig. 5). This is somewhat surprising because resonances with fairly regular spacings have been predicted for the higher energy Jahn–Teller component in C3v type molecules [27,28]. It is possible that the absence of structure is due to the resonances having very short
lifetimes. Such lifetimes result in line broadening, and this, in turn, tends to wipe out discrete structure. The photoelectron band at 13 eV, corresponding to the 1e0 orbital, is essentially structureless, but shows some step-like changes that could reflect vibrational excitations. If this is the case, then the potential energy surface is not entirely repulsive and it is likely that the vibrational excitations will be very complicated, leading to overlapping structures which cannot be resolved. Vibrational structure can also be discerned in the photoelectron band, associated with the 3a 19 orbital, occurring in the binding energy range 15–16 eV. The band seems to begin with a step-like structure at 15.2 eV which could thus correspond to the adiabatic binding energy. The remaining features at higher binding energies are unresolved and no assignments can be made. It is possible that many vibrational modes are excited, leading to a complicated unresolved spectral pattern. The photoelectron band at |16.5 eV, ascribed to the 1a 299 orbital, shows a pronounced vibrational progression with energy separations of 130 meV. Since the electronic state is non-degenerate, these excitations should be associated with one of the totally symmetric modes. Energetically, two modes
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are possible; the n3 mode corresponding primarily to C 3 motion with an energy of 147 meV, and the n2 mode corresponding mainly to CH 2 motions with an energy of 186 meV [25]. A decrease in the vibrational energy by only 12%, as would be obtained for n3 excitations, is typical for non-bonding or weakly bonding orbitals, whereas a decrease of |30%, as would be obtained for n2 excitations, fits better with observations for bonding orbitals. Since both of the orbitals (3a 19 or 1a 299 ) that could give rise to this photoelectron band apparently have bonding character, an assignment in terms of the n2 mode seems reasonable. This interpretation is also consistent with the observation of a broad photoelectron band which indicates that there is a substantial change in geometry upon ionisation, as expected for a bonding orbital. Strong excitations of the n2 mode are expected for an orbital localised primarily on the CH 2 groups, and this therefore provides further support for assigning the band to the 1a 299 orbital, rather than to the alternative 3a 19 orbital which has a mixed CH 2 and C–C character [29]. This interpretation agrees with that reached from the angular distribution studies (Section 3.2).
4. Summary The valence shell photoeletron spectrum of cyclopropane has been studied using synchrotron and HeI radiation. Photoelectron asymmetry parameters have been measured in the energy range 15–120 eV and, based upon their spectral behaviour, the valence shell electronic configuration of cyclopropane should be written as (using D3h symmetry) 2 2 4 4 (2a 19 )2 (2e9)4 (1a 99 2 ) (3a 9 1 ) (1e0) (3e9)
Ionisation from the outermost orbital gives rise to a photoelectron band displaying a doublet structure, due to a Jahn–Teller instability, with the splitting between the two components being 0.78 eV. In addition, the band exhibits a regular vibrational progression attributable to the n11 mode. The asymmetry parameter for region 3 displays a completely different spectral behaviour compared to those for regions 1, 2 and 4. The consistently high value of the asymmetry parameter associated with
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region 3 suggests than an s-type orbital is involved. This indicates that band 3 corresponds to the 3a 91 orbital, in agreement with theoretical work [2,5,6,9– 14], and that band 4 arises from the 1a 299 orbital.
Acknowledgements We are grateful for financial support from the Engineering and Physical Sciences Research Council and the Swedish Natural Science Research Council.
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A reinvestigation of the vibrational structure and the orbital assignments in the photoelectron bands of cyclopropane a, b c D.M.P. Holland *, L. Karlsson , K. Siegbahn a
Synchroton Radiation Department, Daresbury Laboratory, Daresbury, Warrington, Cheshire WA4 4 AD, UK b Department of Physics, Uppsala University, Box 530, SE-75121 Uppsala, Sweden c ¨ 1, SE-75121 Uppsala, Sweden ESCA-LASER Laboratory, Institute for Material Science, Box 534, Regementsvagen Received 5 November 2001; received in revised form 17 January 2002; accepted 31 January 2002
Abstract The valence shell photoelectron spectrum of cyclopropane has been studied using synchrotron and HeI radiation. The photoelectron asymmetry parameters and branching ratios have been determined using monochromated synchrotron radiation in the photon energy range 15–120 eV. The spectral behaviour of the asymmetry parameters associated with the outer valence orbitals has been used to propose an assignment for the valence shell electronic configuration. The experimental features observed in the inner valence energy region of the photoelectron spectrum have been compared with theoretical predictions. The high resolution HeI excited spectrum has allowed vibrational structure to be observed in the outer valence photoelectron bands, and in two of the bands these features can be assigned to progressions involving the n2 (a 19 ) and n11 (e9) ˜ 2 E9 photoelectron band, which exhibits a doublet structure due to Jahn–Teller interaction, displays an modes. The X extended vibrational progression in the doubly degenerate n11 (e9) mode. 2002 Elsevier Science B.V. All rights reserved. Keywords: Photoelectron spectroscopy; Photoelectron angular distributions; Electronic assignments; Vibrational structure; Cyclopropane
1. Introduction The electronic structure and the bonding characteristics of the three-membered ring compound cyclopropane (C 3 H 6 ) have been investigated extensively, with much of the interest arising from the strained nature of the carbon–carbon bonds. Both of these properties have been examined in photoelectron spectroscopic studies using NeI [1], HeI [1–5], HeII [4,6,7] and synchrotron [8] radiation as excitation sources. Nevertheless, in spite of these efforts, *Corresponding author. Tel.: 144-1925-603-425; fax: 144-1925603-124. E-mail address: [email protected] (D.M.P. Holland).
the outer valence shell molecular orbital sequence remains uncertain, and vibrational analyses of the structure observed in the photoelectron bands have yet to be reported. Moreover, only limited information is available concerning the inner valence orbitals. All these issues will be addressed in the present experimental study which uses HeI and synchrotron radiation to record the entire valence shell photoelectron spectrum. In anticipation of our conclusions regarding the electronic configuration, the ground state molecular orbital sequence of cyclopropane can be written as (using D3h symmetry) 2 2 4 4 (1a 19 )2 (1e9)4 (2a 91 )2 (2e9)4 (1a 99 2 ) (3a 9 1 ) (1e0) (3e9)
0368-2048 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 02 )00039-7
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The photoelectron spectrum of cyclopropane reveals four electronic bands, with binding energies less than 18 eV, which may be associated with the outer valence shell orbitals. Ionisation from the outermost orbital gives rise to a photoelectron band displaying a doublet structure, and this has been attributed to Jahn–Teller splitting. The photoelectron band associated with the 1e0 orbital exhibits weak vibrational features, but no evidence of Jahn–Teller interactions. In the binding energy range between 15 and 18 eV two overlapping bands, which must be due to ionisation from the 3a 19 and the 1a 299 orbitals, are discernible in the photoelectron spectrum. However, the question as to which orbital should be attributed to which band has been the subject of much debate, although the results from all theoretical studies [2,5,6,9–14] indicate that the 1a 299 orbital is the more tightly bound of the two. Some of the early molecular orbital assignments were based upon variations observed in the photoelectron band intensities as a function of photon energy. Such variations, at a particular photoelectron detection angle, depend upon a proper knowledge of the asymmetry parameters, and the present results show that the two photoelectron bands associated with the 3a 19 and the 1a 299 orbitals possess significantly different b -parameters. This was not fully appreciated when the early interpretations were being suggested. The electronic structure of cyclopropane has also been investigated using electron momentum spectroscopy [14–17]. In the first [16] of the two studies conducted by Banjavcic et al., the experimental results indicated that the symmetries of the two bands lying in the binding energy range 15–18 eV were s-type, whilst their calculations predicted that one of the bands should be s-type and the other p-type. von Niessen et al. [14] questioned the validity of some of the conclusions reached by Banjavcic et al. [16,17], and undertook a similar electron momentum spectroscopy study to obtain spectra with improved statistics. Their experimental data indicated that the peaks at 15.7 and 16.6 eV possessed p-type and s-type symmetry, respectively. This would lead to the peak at 15.7 eV being assigned to the 1a 299 orbital and the higher energy peak to the 3a 19 orbital, in accordance with the proposals of Evans et al. [3] and of Schweig and Thiel [4]. Although the outer valence electronic structure of
cyclopropane has been studied extensively, the features arising from ionisation of the inner valence (2e9 and 2a 19 ) orbitals have received much less attention. HeII radiation has been used to record the two prominent peaks occurring in the binding energy region up to 30 eV [7], and the entire inner valence region has been investigated, albeit with limited resolution and poor statistics, using electron impact [14,16,17]. In the present study, monochromatic synchrotron radiation has been used to record the complete valence shell photoelectron spectrum, and the results indicate that correlation satellites occur at high binding energies, in agreement with theoretical predictions.
2. Experimental apparatus and procedure
2.1. Synchrotron radiation excited photoelectron spectra The photoelectron spectra were recorded using a hemispherical electron energy analyser and synchrotron radiation emitted by the Daresbury Laboratory storage ring. Detailed descriptions of the monochromator [18] and the experimental procedure [19] have been reported. The electron analyser could be rotated about an axis which coincided with the photon beam, thus enabling photoelectron angular distributions to be measured. The photoionisation differential cross-section in the electric dipole approximation, assuming randomly oriented targets, and electron analysis in a plane perpendicular to the photon propagation direction, can be expressed in the form
F
ds stotal b ] 5 ]] 1 1 ]s3P cos 2u 1 1d 4p 4 dV
G
where stotal is the angle integrated cross-section, b is the photoelectron asymmetry parameter, u is the photoelectron ejection angle relative to the major (horizontal) polarisation axis and P is the degree of linear polarisation of the incident radiation. At each photon energy, photoelectron spectra were recorded at u 508 and u 5908, thus allowing the asymmetry parameter to be determined once the polarisation had been deduced. The degree of polarisation was determined by recording Ar 3p and He 1s photoelectron spectra as a function of photon energy, and
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using the well established b -parameters for these gases [19]. Fig. 1 displays the photoelectron spectrum of cyclopropane recorded at u 508 with a photon energy of 90 eV. The spectra were analysed by dividing the binding energy range into the regions specified in Table 1, and the results are presented in the form of photoelectron asymmetry parameters and b -independent branching ratios. The branching ratio for a specific region is defined as the intensity in that particular region divided by the sum of the intensity in all the energetically accessible regions.
2.2. HeI excited photoelectron spectra The HeI excited spectra were recorded in Uppsala using a photoelectron spectrometer equipped with a double-focussing electron energy analyser. The spectrometer resolution was |8 meV at best. The helium radiation used for ionisation was generated in a source based on a DC-glow gas discharge confined inside a quartz capillary. The main component was the HeIa resonance line at 21.22 eV. The detector consisted of two microchannel plates mounted together in a chevron arrangement and a phosphor screen by which electron pulses were transferred into light pulses for read-out to a computer system via a CCD-camera. The spectrometer and detection system have been described elsewhere [20]. The sample gas was obtained commercially with a purity of better than 98%, and was transferred, via a gas handling system and a needle valve, to the gas cell used for photoionisation. Energy calibration of the spectra was made by a procedure described by Maripuu et al. [21]. It is based on the simultaneous recording of the spectrum from a mixture of the sample gas, argon and krypton using the Ar 3p 1 / 2,3 / 2 lines at 15.9372 and 15.7596 eV, respectively, as well as the Kr 4p 1 / 2,3 / 2 lines at 14.6656 and 13.9997 eV, respectively, as reference lines [22].
3. Results and discussion
3.1. Overall spectrum Fig. 1 displays the complete valence shell photoelectron spectrum of cyclopropane together with the results from the ADC(3) calculations performed by
59
von Niessen et al. [14]. The agreement between the experimental data and the theoretical predictions is generally good, particularly in the outer valence region where it is apparent that the single particle model of ionisation is valid [23]. According to the ADC(3) calculations, ionisation from each of the four outer valence orbitals gives a main-line with a pole strength P|0.9 and no significant accompanying satellites. The calculated binding energies are also in good agreement with the experimental values listed in Table 2. The photoelectron bands associated with the outer valence orbitals will be described in greater detail in Section 3.3. Ionisation from the 2e9 and 2a 19 orbitals gives rise to two prominent peaks in the photoelectron spectrum with maxima occurring at 19.63 and 26.30 eV. Similar features were reported by Potts and Streets [7] with binding energies of 19.5 and 26.5 eV. Fig. 1 shows that both of these strong peaks are broad and structureless, and exhibit a tail towards high binding energy. Another very broad feature, centred at 32.2 eV, can probably be attributed to correlation satellites originating from the 2a 19 orbital. For the 2e9 orbital, the molecular orbital model of ionisation still appears to be reasonably valid, with the ADC(3) calculations predicting a main-line at 19.89 eV with a pole strength of 0.82 [14], in good agreement with the experimental results. Furthermore, a weak satellite is predicted to occur at a binding energy of 22.19 eV and this feature may, in part, be responsible for the high energy tail. For the innermost 2a 19 orbital the single particle approximation no longer holds, and this results in the strongest feature, occurring at 26.88 eV, having a pole strength of only 0.53 [14]. Nevertheless, the predicted binding energy of this line is in fair agreement with the maximum observed at 26.30 eV. A fairly strong satellite, with a pole strength of 0.08, is calculated to lie at 27.4 eV [14], and may, in part, account for the high energy tail evident in the experimental spectrum. Although the ADC(3) computations seem to be successful in describing the major features occurring in the inner valence region, the broad feature observed around 32 eV remains unexplained. On the basis of the experimental spectrum it appears that the breakdown of the molecular orbital model of ionisation is more severe for the 2a 19 orbital than is predicted theoretically. This may also be true, although to a lesser extent, for the 2e9
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Fig. 1. Upper frame: The valence shell photoelectron spectrum of cyclopropane recorded at a photon energy of 90 eV. Lower frame: A theoretical spectrum of cyclopropane obtained using the ADC(3) approach, as reported by von Niessen et al. [14]. The relative intensities plotted are the pole strengths.
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Table 1 Energy regions used in the analysis of the photoelectron spectra recorded with synchrotron radiation Region
Binding energy range (eV)
Encompassed states
1 2 3 4 5 6
9.50–12.17 12.17–15.00 15.00–16.40 16.40–18.10 18.40–23.00 24.00–29.20
˜ 2 E9 (3e9)21 X 21 ˜ 2 (1e0) A E0 ˜ 2 A 91 (3a 91 )21 B 21 ˜ 2 (1a 299 ) C A 992
orbital, because the ADC(3) calculations do not predict sufficient satellite structure between |22 and 40 eV to match the photoelectron intensity observed in the experimental spectrum. It is possible that this represents a case where it is necessary to include 3-holes–2-particles-excited configurations in the calculations in order to explain the experimental data. Further theoretical investigations of this point would be of interest.
3.2. Photoelectron angular distributions and branching ratios The photoelectron asymmetry parameters and branching ratios for cyclopropane, corresponding to the energy regions specified in Table 1, are plotted in Figs. 2 and 3, respectively. Keller et al. [8] have previously carried out angular distribution measurements on the outer valence orbitals in the photon energy range 13–28 eV. In the region of overlap, the two sets of measurements display the same spectral
behaviour, although in the threshold region the present values for the b -parameters associated with 3e9 and the 1e0 orbitals lie slightly higher than those reported by Keller et al. The asymmetry parameters corresponding to regions 1, 2 and 4 exhibit a similar photon energy dependence, rising from a fairly low value close to threshold and reaching b |1.5 at high energy. Previous work has demonstrated that regions 1 and 2 can be associated with the 3e9 and the 1e0 orbitals, respectively. The asymmetry parameter for region 3 displays a completely different spectral behaviour compared to those for regions 1, 2 and 4. Fig. 2 shows that the b -value for region 3 is greater than unity at threshold, and that it remains high throughout the photon energy range covered in the present experiment. Moreover, compared to the asymmetry parameters for regions 1, 2 and 4, that for region 3 displays only a relatively small change (|0.4) as a function of energy. The asymmetry parameters for regions 5 and 6 are typical of those associated with molecular orbitals composed primari-
Table 2 Assignments of photoelectron bands, electron binding energies, and vibrational energies and assignments of the valence shell photoelectron spectrum of the cyclopropane molecule Electronic state
Vertical binding energy a (eV)
Predicted energy (eV) from the ADC(3) calculations [14]
Vibrational mode
˜ 2 E9 (3e9)21 X 21 ˜ 2 (3e9) X E9 ˜ 2 E0 (1e0)21 A 21 ˜ 2 9 (3a 1 ) B A 91 (1a 992 )21 C˜ 2 A 992 ˜ 2 E9 (2e9)21 D (2a 91 )21 E˜ 2 A 91 (CI state) 2 A 91
10.47 11.25 12.96 15.77 16.66 19.63 26.30 32.2
10.88 10.88 13.12 15.91 16.93 19.89 b 26.88 b
n11 (e9)
60
n2 (a 91 )
130
a b
Energy corresponding to the maximum intensity of the photoelectron band. Main CI component.
Vibrational energy (meV)
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Fig. 2. The photoelectron asymmetry parameters of cycloproprane associated with the energy regions specified in Table 1.
ly of atomic s-type orbitals. The b -value is reasonably high (|1) at threshold and increases to 1.6–1.8 at high energy. The orbital assignments for the photoelectron bands in regions 3 and 4 have been discussed ever since Basch et al. [2] reported the original HeI excited spectrum and associated bands 3 and 4 with the 3a 19 and the 1a 299 orbitals, respectively. Later, Schweig and Thiel [4], based upon their studies using HeI and HeII radiation, proposed that the ordering should be reversed. This reversal was subsequently supported by Evans et al. [3]. Leng and Nyberg [1] determined the asymmetry parameters for
bands 3 and 4 using NeI and HeI radiation, and concluded that the original assignment of Basch et al. [2] was indeed correct. The original assignment was also used by Keller et al. [8], although the emphasis of their study was aimed at the Jahn–Teller splitting observed in the first band, and not the assignments of the higher energy bands. In contrast to these apparently contradictory results from experimental studies, the theoretical work [2,5,6,9–14] has been consistent in predicting that bands 3 and 4 should be identified with the 3a 19 and the 1a 99 2 orbitals, respectively. Set against this uncertain background, the recent
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Fig. 3. The photoelectron branching ratios of cyclopropane associated with the energy regions specified in Table 1.
electron momentum spectroscopy results obtained by von Niessen et al. [14] showed that the momentum distribution for band 3 was consistent with that of a transition having p-like symmetry, whilst the distribution for band 4 was consistent with a transition having s-like symmetry. Hence, on the basis of their experimental data, bands 3 and 4 were identified with the 1a 299 and the 3a 19 orbitals, respectively, even though this assignment was in contradiction to their own ADC(3) calculations. The angle resolved photoelectron spectra shown in Fig. 4 help to explain some of the difficulties encountered in interpretations based upon band intensities observed in HeI or HeII excited spectra measured at one specific detection angle. The spectra
in Fig. 4 encompass the outer valence orbitals, and it can be seen that the relative intensities of bands 3 and 4 depend markedly on both the detection angle and the incident photon energy. The present data yield b -parameters for band 3 of 1.19 and 0.69, at photon energies of 21 and 40 eV, respectively, whilst the corresponding values for band 4 are 0.85 and 1.42. This large variation in the b -parameters was not fully appreciated in the early studies. If the analogy of ionisation from s-type or p-type orbitals is applied to the present data, then the consistently high value of the asymmetry parameter associated with region 3 suggests that an s-type orbital is involved, because it is well established that ionisation from an atomic s orbital results in b 52.
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Fig. 4. Photoelectron spectra of cyclopropane recorded at photon energies of 21 and 40 eV, and at u 508 and u 5908. For each photon energy, the relative intensities at u 508 and u 5908 are shown correctly. However, the relative intensity at one photon energy to that at the other energy is arbitrary.
This indicates that band 3 should be associated with the 3a 19 orbital, in accordance with theory, but in disagreement with the apparently convincing experimental data reported by von Niessen et al. [14]. We are unable to put forward an explanation for this discrepancy. Theoretical calculations of the photoelectron asymmetry parameters would allow the present proposals to be placed on a firmer footing. On the experimental side, an electron momentum spectroscopy study performed at an improved resolution could help clarify the issue. In the work performed by Banjavcic et al. [16,17] and by von Niessen et al. [14] the resolution was insufficient to separate the bands due to the 3a 19 and the 1a 99 2 orbitals, and deconvolution procedures had to be employed. A study in which these two bands could be separated might help resolve this uncertainty. The photoelectron branching ratios for the valence
shell orbitals are shown in Fig. 3. The branching ratios for the inner valence features increase relative to those for the outer valence orbitals as the photon energy increases. This behaviour is due to the relative variations in the atomic carbon 2s / 2p crosssections as a function of photon energy. At high energies the cross-sections associated with p-type orbitals decrease faster than those associated with s-type orbitals. Thus, since the inner valence orbitals consist primarily of s-type orbitals their branching ratios become more significant at high energies.
3.3. Studies using HeI radiation Fig. 5 shows the overall HeI excited outer valence shell spectrum of cyclopropane, together with the proposed orbital assignments. The innermost band, associated with the 2e9 orbital, can be observed in
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Fig. 5. The overall HeI excited photoelectron spectrum of cyclopropane covering the binding energy range 9–20 eV.
the binding energy range 19–20 eV and has a distorted shape due to a high background of low kinetic energy electrons. At lower binding energies the bands are well resolved and the spectrum resembles the synchrotron radiation excited counterpart plotted in Fig. 1, but is better resolved. As has already been discussed in the Introduction, the first band, identified with the degenerate 3e9 orbital, is widely split into two components due to a Jahn– Teller instability which reduces the molecular symmetry from D3h to C2v (or possibly even to Cs ). At this lower symmetry the degenerate 2 E9 electronic state is split into two non-degenerate states, and these are revealed by the two photoelectron bands with intensity maxima at 10.47 and 11.25 eV. The second and the fifth photoelectron bands, appearing with maximum intensity at binding energies of 13.0 and 19.6 eV, respectively, are also associated with degenerate states which are susceptible to Jahn–Teller instability. However, the effect is evidently smaller in these two cases. Nevertheless, a broadening of the bands can be inferred, and this could correspond to a small splitting of, at most, 0.5 eV between the two components. These observations agree qualitatively with the orbital energies predicted by Buenker and Peyerimhoff [24], where the split-
ting of the 2e9 and 1e0 orbitals is distinctly smaller than that for the 3e9 orbital, on lowering the symmetry. The first component of the 3e9 band shows extensive vibrational structure which can be seen in the detailed recording plotted in Fig. 6. The main structure forms a regular progression with a spacing of |60 meV, and probably corresponds to excitations of the Jahn–Teller active n11 (e9) vibrational mode which has an energy of 107 meV in the neutral ground state [25]. This assignment assumes a substantial lowering, by |44%, of the vibrational energy upon ionisation, but since the 3e9 orbital strongly affects the molecular bonding such a large change in energy is not unlikely. The n11 vibrational mode corresponds to a mixed ring-deforming and CH 2 wagging motion, which is consistent with the expected symmetry lowering upon ionisation. The widths of the individual lines in the progression discernible in Fig. 6 vary somewhat, and this indicates that other vibrational modes, in addition to n11 , are being excited, although the distortions are not sufficient to enable further assignments to be made. Moreover, the vibrational structures close to the band onset are irregular, which could also reflect excitations of other modes. However, an alternative
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Fig. 6. A detail of the HeI excited photoelectron spectrum of cyclopropane showing the outermost of the two Jahn–Teller components of the 3e9 (s) band. A vibrational progression in the n11 (e9) mode is marked, starting at 9.93 eV. The average spacing between the lines is 60 meV.
explanation of these irregularities could be that the lowest degenerate vibronic energy levels are split by a potential barrier which locks the molecule into a state of symmetry lower than D3h , where internal rotation by the n11 mode is effectively prevented. Such an effect can be explained by second order perturbation theory and requires strong vibronic coupling in order to be observable in an experimental spectrum. An example can be seen in the photoelectron spectrum of H 2 S [26]. However, the large splitting, of more than 1 eV, between the two major ˜ 2 E9 state suggests Jahn–Teller components in the X that the coupling could indeed be of such a magnitude. Further theoretical investigations into this matter would be of interest, especially since evidence of second order Jahn–Teller coupling in photoelectron spectra is sparse. The vibrational structure seems to be restricted to the lower energy Jahn–Teller component, with the band corresponding to the higher energy component being essentially structureless (Fig. 5). This is somewhat surprising because resonances with fairly regular spacings have been predicted for the higher energy Jahn–Teller component in C3v type molecules [27,28]. It is possible that the absence of structure is due to the resonances having very short
lifetimes. Such lifetimes result in line broadening, and this, in turn, tends to wipe out discrete structure. The photoelectron band at 13 eV, corresponding to the 1e0 orbital, is essentially structureless, but shows some step-like changes that could reflect vibrational excitations. If this is the case, then the potential energy surface is not entirely repulsive and it is likely that the vibrational excitations will be very complicated, leading to overlapping structures which cannot be resolved. Vibrational structure can also be discerned in the photoelectron band, associated with the 3a 19 orbital, occurring in the binding energy range 15–16 eV. The band seems to begin with a step-like structure at 15.2 eV which could thus correspond to the adiabatic binding energy. The remaining features at higher binding energies are unresolved and no assignments can be made. It is possible that many vibrational modes are excited, leading to a complicated unresolved spectral pattern. The photoelectron band at |16.5 eV, ascribed to the 1a 299 orbital, shows a pronounced vibrational progression with energy separations of 130 meV. Since the electronic state is non-degenerate, these excitations should be associated with one of the totally symmetric modes. Energetically, two modes
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are possible; the n3 mode corresponding primarily to C 3 motion with an energy of 147 meV, and the n2 mode corresponding mainly to CH 2 motions with an energy of 186 meV [25]. A decrease in the vibrational energy by only 12%, as would be obtained for n3 excitations, is typical for non-bonding or weakly bonding orbitals, whereas a decrease of |30%, as would be obtained for n2 excitations, fits better with observations for bonding orbitals. Since both of the orbitals (3a 19 or 1a 299 ) that could give rise to this photoelectron band apparently have bonding character, an assignment in terms of the n2 mode seems reasonable. This interpretation is also consistent with the observation of a broad photoelectron band which indicates that there is a substantial change in geometry upon ionisation, as expected for a bonding orbital. Strong excitations of the n2 mode are expected for an orbital localised primarily on the CH 2 groups, and this therefore provides further support for assigning the band to the 1a 299 orbital, rather than to the alternative 3a 19 orbital which has a mixed CH 2 and C–C character [29]. This interpretation agrees with that reached from the angular distribution studies (Section 3.2).
4. Summary The valence shell photoeletron spectrum of cyclopropane has been studied using synchrotron and HeI radiation. Photoelectron asymmetry parameters have been measured in the energy range 15–120 eV and, based upon their spectral behaviour, the valence shell electronic configuration of cyclopropane should be written as (using D3h symmetry) 2 2 4 4 (2a 19 )2 (2e9)4 (1a 99 2 ) (3a 9 1 ) (1e0) (3e9)
Ionisation from the outermost orbital gives rise to a photoelectron band displaying a doublet structure, due to a Jahn–Teller instability, with the splitting between the two components being 0.78 eV. In addition, the band exhibits a regular vibrational progression attributable to the n11 mode. The asymmetry parameter for region 3 displays a completely different spectral behaviour compared to those for regions 1, 2 and 4. The consistently high value of the asymmetry parameter associated with
67
region 3 suggests than an s-type orbital is involved. This indicates that band 3 corresponds to the 3a 91 orbital, in agreement with theoretical work [2,5,6,9– 14], and that band 4 arises from the 1a 299 orbital.
Acknowledgements We are grateful for financial support from the Engineering and Physical Sciences Research Council and the Swedish Natural Science Research Council.
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