Electronic spectroscopy of propadiene (allene) by electron impact

The electron impact excitation of propadiene (allene) has been studied experimentally at impact energies of 20, 40, and 60 eV and scattering angles from 6° to 80°. Two transitions with maxima at 4.28 and 4.89 eV are identified as singlet --> triplet excitations. The magnitude of the splitting between these transitions is a measure of the interaction between the two perpendicular pi molecular orbitals. The significance of these triplet excited states in the interpretation of previous electronic energy transfer experiments and sensitized photochemical studies is briefly discussed. Two very weak transitions are observed between 5.0 and 6.5 eV. The angular dependence of the corresponding cross sections indicates that these are spin-allowed but symmetry-forbidden transitions. A weak singlet --> singlet transition is seen with a maximum at 6.74 eV. This feature has also been optically observed and attributed to an electric dipole-allowed χ 1A1-->1 1E transition. This assignment is discussed in the light of the present results. The first strong transition, the χ 1A1-->1 1B2 pi-->pi* transition, appears with a maximum at 7.24 eV. Higher energy-loss features between 7.95 and 10 eV probably involve excitations to Rydberg states. A previously unreported transition to a superexcited state is observed with a maximum at 11.25 eV

Variable angle electron-impact excitation of nitromethane

The electron-impact excitation of nitromethane has been studied at incident electron energies of 25, 55, and 90 eV, at scattering angles from 6° to 80°. The lowest-lying inelastic process which is observed is a previously unreported feature with a maximum intensity at 3.8 eV energy loss. This feature represents at least one singlettriplet transition. It is likely that this 3.8 eV triplet feature plays a central role in the gas phase photolysis of nitromethane. A weak inelastic process with a peak at 4.45 eV has also been observed, as has a strong transition at 6.23 eV. Both of these excitations are well known from optical spectra, and they are generally believed to represent spin-allowed n→π* and π→π* transitions, respectively. Their assignments are discussed in detail. In addition, seven other transitions, several of which have not been reported previously, have been detected in the 7–12 eV energy-loss range. Three of these transitions, at 8.3, 8.85, and 11.73 eV energy loss, are tentatively assigned to Rydberg excitations of increasingly tightly bound electrons into a 3s Rydberg orbital

Electron impact investigation of electronic excitations in furan, thiophene, and pyrrole

The electronically excited states of furan, thiophene, and pyrrole have been studied by electron impact at scattering angles from 0° to 80°, and impact energies of 30 and 50 eV. Low-lying features at 3.99 and 5.22 eV in furan, 3.75 and 4.62 eV in thiophene, and 4.21 eV in pyrrole are identified as singlet --> triplet transitions. The locations and, for furan and thiophene, the energy splittings of these excitations suggest that they are analogous to the lowest pi --> pi* singlet --> triplet transitions in benzene, and that these heterocycles have appreciable aromatic character. A weak feature observed in pyrrole at 5.22 eV is attributed to an optically forbidden singlet --> singlet transition. In all three molecules, transitions to several superexcited states are observed

Electronic spectroscopy of s-trans 1,3-butadiene by electron impact

The electron impact excitation of 1,3-butadiene has been studied experimentally at impact energies of 20, 35, and 55 eV and scattering angles from 10° to 85°. The energy and angular dependences of the cross section ratios are used to identify the nature of the excited states. Two transitions with maxima at 3.22 and 4.91 eV are identified as singlet-->triplet transitions. Comparison with theoretical calculations indicates that these are due to the 13Bu and 13Ag states, respectively. Their significance for the photochemistry of this molecule is discussed. The optically allowed X-tilde 1Ag --> 11Bu(N --> V1) transition is observed with a maximum at 5.92 eV. An additional transition appears between 6.9 and 7.8 eV with vibrational features at 7.09, 7.28, and 7.46 eV. The optical absorption in this region was originally attributed to a 1A1 state of the s -cis molecule and subsequently to a Rydberg state or to a 1Ag state of the s -trans molecule. On the basis of intensity arguments and the angular dependence of the cross section ratios, we suggest that it may instead be due to the X-tilde 1Ag --> 21Bu transition of the s -trans molecule. Rydberg transitions are observed at 8.00 and 8.18 eV. Two broad transitions are also seen beyond the first ionization potential with maxima at 9.50 and 11.00 eV. The results of this study are in good agreement with recent ab initio configuration interaction (CI) calculations, and give support to the analysis of the valence excited states in terms of a "molecules-in-molecules" approach. This is consistent with recent interpretations of the resonance energy and reactivity of this molecule and differs from the older classic model of extensive delocalization in the pi electron system

Excited electronic states of cyclohexene, 1,4-cyclohexadiene, norbornene, and norbornadiene as studied by electron-impact spectroscopy

The excited electronic states of cyclohexene, 1,4-cyclohexadiene, norbornene (bicyclo[2.2.1]-2-heptene), and norbornadiene (bicyclo-[2.2.1]-2,5-heptadiene) have been studied by electron impact at scattering angles from 5° to 80°, and impact energies of 30 and 50 eV. Low-lying features with intensity maxima at 4.24 eV in cyclohexene and 4.10 eV in norbornene are identified as singlet --> triplet transitions. Similar features in the spectra of 1,4-cyclohexadiene and norbornadiene extending from 3.4 to 5.4 eV and 2.9 to 4.5 eV, respectively, are believed to result from superposition of two low-lying singlet --> triplet transitions in each molecule. In norbornadiene these features have estimated intensity maxima at 3.4 and 3.9 eV, while in 1,4-cyclohexadiene they appear to be more highly overlapped, yielding a single intensity maximum at 4.29 eV. The singlet --> singlet excited state spectra of these molecules are discussed from the point of view of a model in which ethylene units interact via through-bond and through-space effects. In each of these four molecules, transitions to several superexcited states are observed

Electronic spectroscopy of 1,3-cyclopentadiene, 1,3-cyclohexadiene and 1,3-cycloheptadiene by electron impact

The electronic spectra of three conjugated cis-dienyl systems, 1,3-cyclopentadiene, 1,3-cyclohexadiene, and 1,3-cycloheptadiene have been investigated using electron-impact spectroscopy. Spectra were obtained at impact energies ranging from 20 to 75 eV and scattering angles from 5° to 80°. A single singlet --> triplet transition was observed for each molecule at 3.10, 2.94, and 2.99 eV, respectively. Information on the Franck–Condon envelopes was obtained for these transitions. The N-->V1, N-->V2, and V3, and several Rydberg transitions were also observed in each substance. Some previous unreported superexcited states lying above the first ionization potential were detected

Electronic spectroscopy of benzene and the fluorobenzenes by variable angle electron impact

Electron-impact spectra of benzene and 11 fluorine-substituted derivatives have been obtained at impact energies of 75, 50, and either 25 or 30 eV, and scattering angles from 5° to 80°. Each molecule shows an absorption maximum at about 3.9 eV corresponding to a singlet-->triplet, pi-->pi*, transition. In benzene, fluorobenzene, o- and m-difluorobenzene, and 1,3,5-trifluorobenzene, an additional singlet-->triplet excitation was detected at about 5.7 eV. Three singlet-->singlet transitions analogous to the 4.90, 6.20, and 6.95 eV benzene excitations are seen in each of the fluorine-substituted molecules. The more highly substituted compounds exhibit an additional singlet-->singlet transition, which we designate as the C band system, that is most clearly observed in the hexafluorobenzene spectrum, where it has a peak at 5.32 eV. We briefly discuss the effects on relative transtion intensities due to the different molecular symmetries of the various fluorobenzenes. We also report numerous superexcited states for each molecule studied

Electron-impact excitation of low-lying electronic states in CS2, OCS, and SO2

Variable angle, electron-impact energy-loss spectra of CS2, OCS, and SO2 have been obtained at incident electron energies of 25, 40, and 70 eV for CS2, and 30 and 55 eV for OCS and SO2. Singlet-->triplet excitations are observed with peaks at transition energies of 3.36 eV in CS2, 4.94 eV in OCS, and 3.40 eV in SO2. A feature which peaks at 3.65 eV in CS2 is observed to have singlet-->triplet character. The CS2 and OCS spectra do not confirm the existence of several spin-forbidden transitions reported in solid phase ultraviolet absorption studies of these molecules. In SO2, no evidence is found of transitions to the 1 3A2 and 1 3B2 states, believed to lie near the well-known ? 3B1 state

Electronic spectroscopy of trans-azomethane by electron impact

The electron impact excitation of trans-azomethane (i.e., trans-dimethyl diazine CH3–N–N–CH3) has been studied by both trapped electron (TE) and differential electron scattering (DES) techniques. The nature of the excited state in each of several transitions has been identified by the energy and angular dependences of the excitation cross section. Two previously unreported singlet-->triplet transitions are observed with maxima at 2.75 and 4.84 eV. Theoretical calculations on the parent compound, trans-diimide (H–N=N–H), suggest that these are the χ 1Ag-->1 3Bg (produced by excitation of an electron from an n + molecular orbital to a pi* molecular orbital) and the χ 1Ag-->1 3Bu (pi-->pi*) transitions, respectively. The χ 1Ag-->1 1Bg (n + -->pi*) transition is observed with a peak at 3.50 eV in the DES studies. A strong peak at 6.01 eV in the TE spectra appears as a weak shoulder in the DES studies and is interpreted as either a symmetry-forbidden or Rydberg-like singlet-->singlet transition. Allowed singlet-->singlet features overlap each other in the transition energy range from 6 to 10 eV. Peaks are seen in the DES spectra at 6.71, 7.8, and 9.5 eV and in the TE spectrum at 8.0 eV. Several significant differences between the TE and the DES spectra are analyzed on the basis of the different nature of the two experiments