Gerhard Bieri

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.

Multimode Jahn–Teller and pseudo‐Jahn–Teller effects in BF+3

The ionic states of BF3 exhibit striking vibronic coupling effects. These effects are investigated using ab initio calculated ionization energies and vibronic coupling constants. The two Jahn–Teller active modes show interference effects in all bands of E symmetry. In the 2A1′ ionic state single quanta of a degenerate vibration are excited. They borrow their intensities from the adjacent 2E′ state by means of a pseudo‐Jahn–Teller interaction. The symmetry of BF+3 in its nondegenerate ground state is predicted to be lowered due to two‐mode pseudo‐Jahn–Teller interaction with the second excited state. This interaction is surprisingly strong and leads to an interesting vibronic structure. The vibrational fine structure in all experimental spectra is reproduced satisfactorily. The earlier discrepancies in assigning the three lowest lying ionic states are explained.

Spectra of p-Benzoquinone, studied with ham/3

Abstract Photoelectron spectra of p -benzoquinone and tetrafluoro- p -benzoquinone have been measured by He II up to 25 eV and interpreted using the semi-empirical MO method HAM/3. The order of the four highest occupied MOs has been obtained as n n π π from a HAM study of the methylated compounds. The electron affinities have been calculated. Finally, the accuracy of these studies has been checked by a study of the singlet and triplet excitation of p -benzoquinone.

The He(II) photoelectron spectra of the fluorosubstituted ethylenes and their analysis by the Green's function method

Abstract The 30.4 nm He(II) photoelectron spectra of the fluorosubstituted ethylenes have been recorded. The assignment of all main bands is obtained from many-body Greens function calculations. The results from the semi-empirical HAM/3 method lead to a nearly identical assignment. For the mono- and difluoroethylenes, an unambiguous interpretation of the spectra can be established from empirical considerations alone. The set of spectra has been reexamined with respect to the “perfluoro-effect” rules. Also discussed are the ionisation energies as a function of the geminal FCF bonding angle and the similarity of the spectra of the cis and trans isomers of 1,2-difluoroethylene. Additional weak bands were detected in the energy region 21–24 eV in all spectra and were attributed to “shake-up” transitions on the basis of 2ph-Tamm-Dancoff Greens function calculations. The orbital model of ionisation breaks down for the ionisation out of the F(2s) and C(2s) orbitals in general. The calculations reveal a charge transfer character of the excitations accompanying ionisation from the C(2s) orbitals.

à 2A1 → X̃ 2B1 emission spectrum of cis-1,2-difluoroethylene cation and the non-radiation decay of the à states of the haloethylene cations in the gaseous phase

Abstract The results of the search for the A → X radiative relaxation of haloethylene cations in the gaseous phase are reported. Only in the case of cis-1,2-difloroethylene cation was an emission spectrum detected. It is identified as the A 2 A 1 → X 2 B 1 band system on the basis of photoelectron spectroscopic measurements. An assignment of the emission bands yields the vibrational frequencies of four of the A 1 fundamentals (under C 2 , symmetry) for the X state and one for the A state. Well resolved Ne(I) photoelectron spectra of cis- and trans-1,2-difluoroethylene are presented, from which some vibrational frequencies for these cations in the X and A states are also obtained. The lifetimes of cis-1,2-difluoroethylene cation in the lowest vibrational levels of the A 2 A 1 state have been measured. The decay of this cation is unusual as these levels are depleted both by, radiative, and pathways leading to fragment ions (C 2 HF + ). The lack of detectable emissions with other fluoro-, chloro- and bromo-ethylene cations is discussed and the likely symmetries of the A states are proposed.

HNC+ radical cation studied by charge-exchange mass spectrometry

Abstract The fragment ion with m/e 27 from CH 3 NH 2 or CH 3 NC has the HNC + configuration rather than the tautomeric HCN + structure. This has been deduced from its recombination energy of 12.5 ± 0.2 eV, which has been determined from a comparative investigation of charge-exchange reactions studied in a double mass spectrometer.

Cyanogen fluoride: A photoelectron-spectroscopic investigation

The photoelectron spectrum of cyanogen fluoride (FCN) has been recorded, using He(I) and He(II) radiation. The spectrum can be unambiguously assigned through a correlation with the known photoelectron-spectroscopic data of the other cyanogen halides XCN, X = Cl, Br, I. The observed electronic states of the radical cation FCN+ are: A = 2Π32, 12; A = 2Σ+; B = 2Π32, 12; C = 2Σ+.

Emission spectra of the cations of cyanodiacetylene, methylcyanodiacetylene and ethylcyanodiacetylene in the gaseous phase. Ã(π–1)→text-decoration:overlineX(π–1) band systems

The emission spectra of the cations of cyanodiacetylene, methylcyanodiacetylene and ethylcyanodiacetylene have been detected using low energy electron beam excitation. The emission band systems lie in the 550-670 nm wavelength region and are assigned to the A2Π→text-decoration:overlineX2Π, A2E→text-decoration:overlineX2E and A2A″→text-decoration:overlineX2A″ electronic transitions of these three cations, respectively. This follows from the photoelectron spectra of these compounds, which have also been obtained, and from their assignment. The vibrational frequencies (± 10 cm–1) of four or five of the totally symmetric fundamentals of the three cyanodiacetylene cations in the text-decoration:overlineX(π–1) states have been inferred from the vibrational analyses of the emission spectra. These frequencies are supplemented by values (± 160 cm–1) obtained for the A(π–1) states from the photoelectron spectra. The lifetimes of the title cations in the lowest vibrational level of their A states have been measured as 15 ± 2, 8 ± 2 and ⩽ 6 ns, respectively.

A photoelectron spectroscopic investigation of the electronic structure of trimethylsilylhaloacetylenes

Abstract The photoelectron spectra (He I excitation) of trimethylsilylacetylene M(H) and of the four trimethylsilylhaloacetylenes M(X) with X = F, Cl, Br, I hav