Wayne M. Flicker

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 π → π* 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.

Low energy, variable angle electron-impact excitation of 1,3,5-hexatriene☆

Abstract A mixture of cis and trans 1,3,5-hexatriene has been studied by electron impact at incident electron energies of 20 eV, 40 eV, 50 eV, and 70 eV, at scattering angles from 0° to 80°, and with effective energy resolutions in the range from 0.05 eV to 0.15 eV. Singlet → triplet transitions with maximum intensities at 2.61 eV and 4.11 eV are observed. The lowest energy spin-allowed excitation which can be detected is the electric dipole-allowed X 1 Ag → 1 1Bu transition (in the notation appropriate for the trans isomer). No evidence has been found for a spin-allowed but symmetry-forbidden X 1 Ag → 2 1Ag excitation in the vicinity of 4.4 eV transition energy. Many other spin-allowed excitations are observed in the 6–11 eV energy-loss region, and the correlation between these features and those observed in high resolution ultraviolet absorption spectra and other electron-impact spectra is discussed.

Singlet → triplet transitions in methyl-substituted ethylenes

Abstract The low energy electron impact energy-loss spectra of ethylene and its six methylated derivatives have been studied at impact energies from 15 eV to 80 eV, and at scattering angles from 10° to 80°. In each molecule, the N → T singlet → triplet transition has been detected with a maximum intensity at an energy loss which shifts from 4.32 eV in ethylene to 4.10 eV in tetramethylethylene. No features which can be assigned to the N → T R singlet → triplet Rydberg transition or the N → R * (π → σ * ) singlet → singlet Rydberg transition were detected.

Triplet states of furan, thiophene, and pyrrole

Abstract Low-lying triplet electronic states have been detected in furan, thiophene, and pyrrole by the method of variable-angle, electron-impact spectroscopy. Singlet → triplet transitions occur with maximum intensity at 3.99 eV and 5.22 eV in furan, 3.75 eV and 4.62 eV in thiophene, and 4.21 eV in pyrrole. A weak transition at 5.22 eV in pyrrole is assigned as the lowest observed singlet → singlet excitation in that molecule.

Electron-impact investigation of excited singlet states in 1,3-butadiene☆

Abstract Electron-impact energy-loss spectra of 1,3-butadiene have been obtained at impact energies from 35 eV to 90 eV, at a scattering angle of 0°, and at a

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.

Triplet states in 1,3-butadiene

Abstract The electron impact excitation spectrum 0f 1,3-butadiene has been studied at 20, 35 and 55 eV impact energies and scattering angles of 10° to 80°. Two low lying states are observed with maxima at 3.2 and 4.9 eV, and are identified as the 3Bu state and 3Ag state respectively

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.

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.