ALBERT

Notes: A coplanar electron spectrometer has been constructed for angle-resolved electron energy loss measurements. The experimental arrangement and performance of the spectrometer are described. Absolute absorption transition probabilities (or generalized oscillator strengths) of valence-shell (7–70 eV) electronic transitions of SF6 have been determined as a function of energy loss and momentum transfer at an impact energy of 2.5 keV. New nondipole transitions have been observed at nonzero momentum transfer. Together with the term values obtained from previous dipole electron energy loss and photoabsorption measurements, the momentum transfer dependence of the transition probabilities can be used to provide tentative assignments of the observed nondipole valence-shell transitions of SF6. Despite the complexity of the electronic structure of SF6, the present work demonstrates the feasibility of angle-resolved electron energy loss spectroscopy for investigating nondipole phenomena and related electron-induced excitation processes.

Notes: Electron-momentum-specific ionization energy (IE) spectra of cis-, trans-, and iso-butene were obtained from 8 to 44 eV ionization energy using noncoplanar symmetric (e,2e) spectroscopy and compared with the corresponding photoelectron spectra obtained using monochromated synchrotron radiation at 100 eV photon energy. In the independent particle picture, the intensity variation of a particular ionic state as a function of the electron momentum in an (e,2e) IE spectrum is characteristic of electron momentum density of the ionized orbital. Under favorable conditions, the (e,2e) IE spectra measured at two relative azimuthal angles (0° and 8°) can be used to provide a unique characterization of the dominant orbital composition of a particular ionic state. Together with ab initio calculations, these spectra have been used to clarify the nature of some of the ionic states found in the photoelectron spectroscopic studies. In particular, the presence of prominent many-body structures in the inner-valence region (≥20 eV) for all three butene isomers indicates the general breakdown of Koopmans' ionization picture in this region. The nature of these many-body states is inferred from their electron momentum dependence in the (e,2e) IE spectra and from recently published configuration-interaction calculations.

Notes: Absolute generalized transition probabilities or generalized oscillator strengths (GOSs) of valence-shell electronic transitions of CF4 and CCl4 have been determined as functions of energy loss and momentum transfer (i.e., the Bethe surfaces) using angle-resolved electron energy loss spectroscopy (EELS) at an impact energy of 2.5 keV. A low-lying feature at 7.2 eV was observed for CCl4 and was attributed predominantly to electronic transitions from the Cl 3p nonbonding orbitals (2t1, 7t2, and 2e) to a C–Cl σ* antibonding orbital (7a1), based on the result of a single-excitation configuration interaction excited-state calculation. The experimental GOS profile of this low-lying feature was found to have a shape characteristic of a mixture of dipole-allowed and nondipole transitions with relative maxima at momentum transfers of 0 and ∼0.9 a.u., respectively. GOS profiles of other low-lying discrete transitions below the first ionization edge at 12.6 and 13.7 eV in CF4 and at 8.7 and 9.7 eV in CCl4 were also determined and found to have secondary minima and maxima, in addition to the strong maximum at zero momentum transfer. These excitation features were assigned mainly to Rydberg transitions originated from the nonbonding highest occupied molecular orbitals (HOMOs) with t1 symmetry and the second HOMOs with t2 symmetry. The extrema in the GOS profiles have been discussed by considering the spatial overlap of the initial-state and final-state orbital wave functions. Tentative assignments for the rest of the valence-shell energy-loss features of CF4 (5–200 eV) and CCl4 (5–150 eV) were also inferred from the term values reported previously.

Notes: Absolute transition probabilities or generalized oscillator strengths (GOSs) of valence-shell electronic transitions of CF3Cl, CF2Cl2, and CFCl3 as functions of energy loss and momentum transfer (or Bethe surfaces) have been determined using angle-resolved electron energy loss spectroscopy (EELS) at an impact energy of 2.5 keV. Low-lying electronic excitation features in the energy loss region of 6.8–8.1 eV are observed. Using the results of single-excitation configuration interaction excited-state calculations, we show that these excitation features can be attributed predominantly as electronic transitions from the Cl 3p nonbonding (n) orbitals to a C–Cl σ* antibonding orbital (i.e., n→σ* transitions), some of which may lead to dissociation of the C–Cl bond. Moreover, the absolute GOS profiles of these low-lying excitation features have been determined for the first time. In particular, the GOS profile of the n→σ* feature of CF3Cl at 7.7 eV has been found to have a shape characteristic of a quadrupole transition. On the other hand, the GOS profiles of analogous excitation features for CF2Cl2 and CFCl3 are found to have more complicated structures. The combined profiles of the GOSs of these n→σ* excitation features in the CF4−mClm (m=1–4) series indicate increased dipole component with the number of chlorine atoms. The possible mechanisms and significance of this trend in the GOSs of these n→σ* transitions have been discussed. Furthermore, the GOS profiles of low-lying preionization-edge Rydberg transitions (that originate from the Cl 3p nonbonding orbitals) are also determined, and found to contain not only strong maxima at zero momentum transfer, which are characteristic of predominant dipole-allowed interactions, but also weak secondary maxima (and minima). The nature of these secondary extrema in the GOS profiles is discussed by considering the spatial overlaps of the initial-state and final-state orbital wave functions. Finally, we provide new tentative assignments for other valence-shell energy loss features of CF3Cl, CF2Cl2, and CFCl3 using the ionization potentials and term values reported in the literature.

Notes: A nondipole low-lying excitation feature at 8.0 eV in difluorochloromethane (CHF2Cl) has been observed in angle-resolved electron energy loss spectra obtained at 2.5 keV impact energy. Based on the results of a single-excitation configuration interaction calculation, we assign this feature predominantly as electronic transitions from Cl nonbonding 14a' and 7a‘ orbitals [the highest occupied molecular orbital (HOMO) and second HOMO, respectively] to an antibonding σC–Cl* 15a' orbital (the lowest unoccupied molecular orbital), the so-called n→σ* type transitions. The generalized oscillator strength (GOS) profile of this transition has been determined and found to be similar in shape and magnitude to that of a recently reported nCl → σC–Cl* (7e→11a1) transition in CF3Cl [Ying et al., Chem. Phys. Lett. 212, 289 (1993)]. Both GOS profiles are found to have a shape characteristic of a quadrupole transition, with a maximum at momentum transfer of ∼1 a.u. The similarity in the nature of these nCl → σC–Cl* transitions in CHF2Cl and CF3Cl to that of a p-to-p transition in a chlorine atom can be demonstrated qualitatively by using contour maps of the density functions of the dominant initial-state and final-state orbitals generated from ab initio self-consistent field wave functions. The homology of transition-related properties (including the GOS profile) among the nCl → σC–Cl* transitions in "simple'' polyatomic molecules that contain a single highly localized C–Cl bond is also discussed. Furthermore, the calculated potential energy diagram for CHF2Cl along the C–Cl bond direction suggests that electronic transitions from the 14a' and 7a‘ orbitals to the 15a' orbital may lead to dissociation of the C–Cl bond. Such predissociation may represent a possible common consequence of the nCl → σC–Cl* transitions in these monochloro-substituted chlorofluorocarbons and related derivatives.

Notes: The ionization energy spectra of iso-dichloroethylene (iso-C2H2Cl2) at electron momentum of ∼0 and 0.7 a.u. have been obtained by molecular (e,2e) spectroscopy in the symmetric noncoplanar geometry at 1200 eV impact energy. Momentum distributions (MDs) of selected ionic states, including the outervalence states X(3b1)−1 (frontier-orbital state) and G(6b2)−1, and the innervalence states H(9a1)−1, S1(8a1 )−1, S2(5b2)−1, and S3(7a1)−1 as well as an unassigned many-body feature, have also been determined and compared with ab initio calculations using self-consistent-field wave functions of 4-31G, 6-31G, and 6-31++G** basis sets. Combined with our earlier (e,2e) results on the cis- and trans-C2H2Cl2, these MD measurements can be used to investigate the effects of isomeric changes on the valence-shell electronic structure of these disubstituted ethylene derivatives. The sensitivity of the experimental MDs to chemical bonding details can be demonstrated. In addition, "new'' many-body features at 19–22 eV and above ∼26 eV have been observed in our ionization energy spectra, which support the breakdown of Koopmans' theorem as predicted by an earlier Green's function calculation. Furthermore, our MD measurements can be used to attribute the new many-body features observed at 19–22 eV to (9a1)−1 satellite states and to characterize the many-body states above ∼26 eV predominantly as (7a1)−1 satellite states. © 1994 American Institute of Physics.

Notes: The ionization energy (IE) spectra and electron momentum distributions (MDs) of the valence shell of cis-dichloroethylene have been obtained using symmetric noncoplanar (e,2e) spectroscopy. The IE spectra were found to be in good accord with earlier photoelectron data and in qualitative agreement with a literature pole-strength spectrum generated by a Green's function (GF) calculation. In particular, extensive many-body features were observed above ∼22 eV in the IE spectra and found to be consistent with the pole-strength splitting of the three innermost valence states (7a1)−1, (6b2)−1, and (6a1)−1, as predicted by the GF calculation. The measured MDs were compared with ab initio calculations using self-consistent-field wave functions of 4-31G, 6-31G, and 6-31++G** basis sets. The general lack of quantitative agreement between experiment and calculations indicated the need for improved wave functions beyond the 6-31++G** level for the outer-valence orbitals. Moreover, the experimental MDs have provided an unambiguous assignment of the ordering of the D(8b2)−1 and E(2b1)−1 ionic states. A new satellite feature at ∼21 eV was found to have a MD similar to that of the H(7b2)−1 state, confirming the GF result. Furthermore, the many-body features above 26 eV were dominated by s-type angular dependence and could be ascribed predominantly to ionization of the innermost valence orbital 6a1. Finally, the bonding morphology of the valence orbitals was found to be different from other related ethylene derivatives due to ligand substitution effects.

Notes: The absolute transition probabilities, more commonly known as absolute generalized oscillator strengths (GOSs), have been determined for selected electronic excitations from the C 1s or Cl 2p shell to the lowest-lying σ* orbitals for chlorofluoromethanes CF4−nCln (n=1–4) using angle-resolved electron energy loss spectroscopy at 2.5 keV impact energy. The GOS profiles of these inner-shell transitions were found to have shapes characteristic of predominant dipole-allowed transitions. Furthermore, the proportions of dipole-allowed components of these GOS profiles appeared to increase with successive chlorination for these molecules. In particular, the GOS profile of the C 1s→σC–Cl* [lowest unoccupied molecular orbital (LUMO)] transition of CF3Cl was found to contain relatively more intense higher momentum-transfer (K) components than those of the corresponding features in CF2Cl2 and CFCl3 (and CCl4). The GOS profiles for the Cl 2p3/2,1/2→σC–Cl* transitions of CF3Cl were found to contain a well-defined secondary maximum at K2∼1.2 a.u., characteristic of predominant nondipole interaction, in addition to the primary dipole-dominated peak at zero momentum transfer. Within the present statistics, our GOS measurements have not, however, revealed any discernible difference between the Cl 2p3/2→11a1 and Cl 2p1/2→11a1 features that could be attributed to spin effects. Furthermore, the dipole-dominated GOS profile was found to become more concentrated in the lower momentum transfer region for the Cl 2p3/2→σC–Cl* transition in CCl4 than that in CFCl3. The present work illustrates the possibilities of atomic-site-specific investigation of the excited states by means of GOS measurements of electronic excitations from localized inner-shell initial states.

Notes: Angle-resolved electron energy loss spectroscopy has been used to determine the absolute generalized oscillator strengths (GOSs) of valence-shell electronic transitions of difluorochloromethane (CHF2Cl) and dichlorofluoromethane (CHFCl2) as functions of energy loss and momentum transfer at an impact energy of 2.5 keV. Absolute GOS profiles of the prominent low-lying preionization-edge energy loss features of CHF2Cl and CHFCl2 were determined and found to be consistent with the previous assignments of the underlying transitions made by VUV photoabsorption spectroscopy. In particular, the lowest-lying features at 8.0 eV in CHF2Cl and at 7.5 eV in CHFCl2 have been attributed predominantly to electronic excitations from the Cl 3p nonbonding (n) orbitals to the C–Cl σ* antibonding orbital, in good accord with single-excitation configuration interaction (CI) excited-state calculations. The corresponding GOS profiles of these n(Cl 3p)→σ*(C–Cl) (HOMO→LUMO) transitions revealed an interesting trend of increased dipole character with increasing Cl content, i.e., from an essentially quadrupole-dominated profile, characterized by a maximum at K2∼0.9 a.u., in CHF2Cl to a mixed dipole-quadrupole profile in CHFCl2 and CHCl3. The CI calculations further showed that some of the underlying n(Cl 3p)→σ*(C–Cl) transitions in CHF2Cl, CHFCl2, and CHCl3, like the other chlorofluorocarbons: CF3Cl, CF2Cl2, CFCl3, and CCl4, could lead to dissociation of the C–Cl bond. In addition, the GOS profiles of the remaining low-lying preionization-edge features at 9.8 and 11.2 eV in CHF2Cl and at 9.4, 10.7, and 11.6 eV in CHFCl2 were also determined. These features have been previously assigned as Rydberg transitions originated from the nonbonding HOMOs. In particular, these experimental GOS profiles were found to be dominated by a strong maximum at K=0, which is indicative of strong dipole interactions. The weak secondary maxima observed at K2∼2.8–3.5 a.u. could be interpreted qualitatively in terms of the spatial overlaps between the initial-state and final-state orbital wave functions. Together with our earlier work on CHF3 and CHCl3, the present work on the remaining members of the CHFmCl3−m (m=0–3) series, CHF2Cl and CHFCl2, provides further evidence for the empirical trends on the preionization-edge structures observed in the CFnCl4−n (n=0–4) series. © 1996 American Institute of Physics.