Andreas Schmelzer

Fluorine and the fluoroderivatives of acetylene and diacetylene studied by 30.4 nm He(II) photoelectron spectroscopy

Abstract The 30.4 nm He(II) photoelectron spectra of fluorine(F 2 ), of acetylene and diacetylene and their fluorosubstituted derivatives have been recorded. The use of He(II) radiation allowed the confirmation of the conjectured B∼ 2 Σ + g state of the fluorine radical cation at 21.1 eV. The spectra of the fluoroacetylenes and -diacetylenes are characterized by a small number of distinct bands. The assignment is readily obtained on qualitative considerations only. We have examined these spectra with special attention to the qualitative rules known as “perfluoro-effect”. In addition, the experimental ionization energies have been compared to calculations performed with the following methods: many-body Greens functions, STO-3G, HAM/3 and CNDO/S.

The photoelectron spectrum of butatriene

Abstract Photoelectron spectra of butatriene (I), tetradeuterobutatriene (II), tetramethylbutatriene (III) and tetra- t -butyl- butatriene (IV) are reported and discussed. The four main bands of the spectra of I and II are assigned to ionization from the 1b 3g , 2b 3u , 1b 2u and 1b 2g molecular orbitals on the basis of molecular orbital calculations (ab initio with STO-3G and 4-31G bases and semi-empirical SPINDO) all of which lead to the electron configuration… (1b 2g ) 2 (1b 2u ) 2 (2b 3u ) 2 (1b 3g ) 2 . The first three bands of III and the first two of IV correspond to 2b 3g , 5b 3u , 2b 2u and 7b 3g , 12b 3u molecular orbitals respectively. The spectra of I and II also show an additional band which cannot be assigned to direct ionization of an occupied molecular orbital in the ground state configuration. it is suggested that this may arise from a cation state… (1b 2g ) 2 (1b 2u ) 2 (2b 3u ) 2 (2b 2u ) 2 B 2u . Such a state is correlated with the 2 B 3g cationic ground state by internal rotation but corresponds to a local maximum in the rotational potential. Some simple model calculations are presented of Franck-Condon factors for the internal motion corresponding to these states. These show a composite band of the general type observed. However, the observed vibrational spacings indicate that significant carbon skeletal as well as torsional changes take place on ionization.

Predissociation of the molecular oxygen cations 16O2+ and 18O2+ studied by photoelectron-photoion coincidence spectroscopy

Branching ratios for the fragmentation of 16O2+ and 18O2+ molecular cations initially prepared in their b−4Σg− ν = 0–5 and B2Σg − ν = 0–5 vibronic states by direct ionization (He(1α)radiation) are reported. For the b−4∑g− ν = 0–5 vibronic states of 16O2+, the extent of competition between the emission process b−4Σg− — a−4Πu and predissociation is found to be in good agreement with the literature. New data for the b −4Σg− electronic state of 18O2+ are obtained. The ν = 3–5 vibrational levels of the B2Σg− electronic state of both isotopic species are shown to decay competitively via the fragmentation channels O+ (4S)+O(3P and O− (4S)+O(1D. The suggestion that at the ionization energy of the B2Σg− ν = 4 vibronic state of 16O2+ a curve-crossing between the 2Σg− and presumably a 4Πg potential curve takes place is corroborated.

The predissociation of molecular oxygen cations O2+(B̃ 2Σg−)

Abstract Using the fixed wavelength (He-Iα) photoelectron-photoion coincidence technique, we observed that molecular oxygen cations generated in their (B 2 Σ g − ν = 4 vibronic state decay competitively via the fragmentation channels O + ( 4 S)+O( 3 P) and O + ( 4 S) + O( 1 D), the ratio being [O + ( 4 S) + O( 3 P)]:[O + ( 4 S)+O( 1 D)]=0.75:0.25.

The ionisation energies of bent and twisted double bonds. Part I

Abstract The energies of the π orbital and the other molecular orbitals of ethylene have been calculated, using the ab initio STO-3G method, as a function of the three out-of-plane internal coordinates using a 33 complete factorial design. The analysis yields the significant terms needed for a complete description of the influence of the out-of-plane deformations of ethylene on its π−1 ionisation energies. In agreement with experimental evidence obtained from the photoelectron spectra of strained and twisted substituted ethylenes, it is found that the effect of nonplanar distortion upon the π-electron orbital energy is insignificant. This lack of sensitivity is due both to the small sizes of the individual contributions associated with each of the three internal coordinates and, in addition, to the compensation of such effects having opposite sign and/or the cross-terms between pairs of internal coordinates.