Robert P. Frueholz

The low lying states of ammonia; generalized valence bond and configuration interaction studies☆

Abstract The nine lower singlet and triplet states of ammonia were studied using ab initio generalized valence bond (GVB) and configuration interaction (CI) wavefunctions. The eight lowest (vertical) excited states were found to be Rydberg in nature. Vertical excitation energies were determined to be (in eV): 6.14 and 6.37 for n → 3s; 7.86 and 7.88 for n → 3p x,y , and 7.87 and 8.15 for n → 3p z (in each case the triplet energy is first). These results are in excellent agreement (0.06 eV average error) with the observed electron impact excitation energies (in eV), 6.39 (A 1 A″ 2 ), 7.93 (B 1 E″), and 8.26 C 1 A′ 1 ). The small singlet triplet energy splittings are consistent with those expected for Rydberg-like excited states. We find no vertical transition consistent with the small peak at 4.4 eV apparently observed in threshold electron impact experiments.

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.

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.

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.

Excited electronic states of ketene

Abstract The electron-impact energy-loss spectrum of ketene has been measured at impact energies of 30 eV and 50 eV, and scattering angles varying from 5° to 80°. Transitions have been observed at 3.7, 5.3, and 5.86 eV. These values are in excellent agreement with recent high quality ab initio calculations. They are assigned, respectively, to an n → π* X 1 A 1 → 1 A 2 transition, a π → π* X 1 A 1 → 3 A 1 transition, and an n → 3s singlet → singlet Rydberg transition. The results do not preclude the existence of an X 1 A 1 → 3 A 2 transition near 3.6 eV underlying the low-energy side of the X 1 A 1 → 1 A 2 excitation. However, no evidence is found for the presence of a triplet state with maximum intensity transition energy lower than 3.4 eV.

Electronic spectroscopy of UF6 and WF6 by electron impact

The electron-impact excitation spectra of uranium hexafluoride (UF6) and tungsten hexafluoride (WF6) have been studied experimentally at impact energies of 30, 50, and 75 eV and at scattering angles from 5° to 80°. Eleven features in UF6 are observed with maxima at 3.26, 4.2, ~4.7, 5.8, 7.0, 7.86, 9.26, 11.01, 11.75, 12.5, and 13.2 eV. Four features in WF6 are observed with maximum intensity at 7.25, 7.9, 8.5, and 9.85, in good agreement with optical work. In addition, three previously unobserved features in WF6 at 11.75, 12.6, and 13.5 eV are reported. Similarity between the spectra of UF6 and WF6 suggests that the primary contribution to the absorption intensity in UF6 above 5.8 eV and in WF6 results from charge transfer transitions from fluorine p orbitals to metal d orbitals. Tentative assignments based in part on recent theoretical studies are made.

Doublet→quartet transitions in nitric oxide as detected by electron‐impact spectroscopy

Vibronic bands in NO are investigated using electron beam excitation. The beams had energies of 25, 35, and 50 eV. (AIP)

Electronic spectroscopy of 1,3,5,7‐cyclooctatetraene by low‐energy, variable‐angle electron impact

The electron‐impact energy loss spectrum of 1,3,5,7‐cyclooctatetraene has been measured at electron impact energies of 30, 50, and 75 eV, and scattering angles varying from 5° to 80°. Three transitions with maxima at 3.05, 4.05, and 4.84 eV are identified as singlet → triplet excitations. The significance of the lowest lying of these triplet states in the quenching process of dye laser solutions (in particular rhodamine 6G) is discussed and an exciplex mechanism for triplet quenching is suggested. Singlet→singlet transitions are observed at 4.43, 6.02, and 6.42 eV. These spin‐allowed transitions have been observed optically and are assigned as ? 1A1 →1A2, ? 1A1→1E, and ? 1A1→1E excitations. Three new, singlet → singlet transitions are observed at 6.99, 8.41, and 9.05 eV and are tentatively assigned as the ? 1A1→1B2, ? 1A1→1E, and ? 1A1 →1E, π→π* excitations. Several superexcited features between 10 and 15 eV have been observed and are believed to involve excitations to autoionizing Rydberg states.

Singlet→triplet transitions in C≡N containing molecules by electron impact

The electron‐impact excitation spectra of hydrogen cyanide (HCN), acetonitrile (CH3CN), malononitrile [CH2(CN)2], propionitrile (C2H5CN), and butyronitrile (C3H7CN) have been studied experimentally at impact energies of 25, 50, and 75 eV and at scattering angles from 5° to 80°. Results for hydrogen cyanide are in excellent agreement with previous work. Previously unobserved singlet→triplet transitions of acetonitrile, propionitrile, and butyronitrile are reported. Also, the first study of the electronic spectrum of malononitrile is reported. Tentative assignments for transitions observed are reported.

Vibronic structure of the second triplet state of 1,3,5‐hexatriene

Electron impact spectroscopy is used to study the vibronic structive of the second triplet state of 1,3,5‐hexatriene.(AIP)