Leif Åsbrink

On the Photoelectron Spectrum of O2

The photoelectron spectrum of molecular oxygen has been studied in the range 12-28 eV with the 584 A and 304 A He lines, and the O2+ states have been measured accurately. Due to the high resolution of our apparatus we have been able to observe the A2 Πu state which has been considered as absent in photoelectron spectroscopy work. Its ionization energy is 17.045 eV. Our result is compared with spectroscopic observations of the second negative band system A2Πu → X2Πg. The relative intensity of the A2Πu state is in good agreement with a recent calculation by Dixon and Hull.

A modification of INDO for interpretation of photoelectron spectra

Abstract It is possible to attain improved agreement between photoelectron-spectroscopic ionzation potentials and theoretical orbital energies simultaneously for all electrons of some hydrocarbons by a suitable parametrization of INDO in its modified form, MINDO. The changed parameters concern mainly the resonance integrals Hμν for which the separate cases of interaction are treated separately. The new procedure gives orbital energies in acceptable agreement with the photoelectron spectra of benzene, methane, ethane, and ethylene. As the procedure might be useful for photoelectron-spectroscopic studies of hydrocarbons it will be referred to as SPINDO (Spectroscopic-Potentials-adjusted-INDO).

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.

Photoelectron and electron impact spectrum of HCN

Abstract The electronic structures of HCN and DCN have been determined by examining high resolution He(I) photelectron spectra of HCN and DCN, He(II) photoelectron spectrum of HCN, and the electron impact energy loss spectra of HCN and DCN. The present investigation supports an earlier assignment of the orbital sequence in HCN. New vibrational data are presented and the Rydberg series and valence transitions are reinvestigated. The adiabatic ionization energies for the 1π and 5σ orbitals in HCN are found to be 13.607 ± 0.002 eV and 14.011 ± 0.003 eV respectively. As mentioned above the investigation of the Rydberg series indicated that the first IP at 13.607 eV is the 1π ionization and the second IP at 14.011 eV is the 5σ ionization. A comparison of the experimental and theoretical intensity ratio between the two first PES progressions also supports this assignment. It is further supported by the fact that in the second IP the ν 3 vibration frequency is not changed as much as it is in the first IP, which is in agreement with the PES of N 2 and CO. The analysis of the bending vibrations also supports this ordering of the orbitals. The same orbital assignment has recently been proposed by Frost et al. 5 , using a comparison with the HCP photoelectron spectrum. The present paper supports their assignment of orbitals and (00 0 0)-(00 0 0) transitions. There are, however, some disagreements concerning the vibrational analysis. This is probably due to the fact that the HCN spectrum of Frost et al. 5 revealed less structure than ours. As indicated by Figure 5 there is possibly still more structure to be revealed.

Rydberg series of small molecules VIII. Photoelectron spectroscopy and electron spectroscopy of NO2

The photoelectron spectrum of NO2 has been measured with high resolution up to 27.5 eV and interpreted by use of molecular orbital theory, taking especially the vibrational structure into account. The electron impact energy loss spectrum has been measured with electron energy 100 eV. The spectrum above 6.5 eV has been interpreted as due to Rydberg transitions and comparison with spectroscopic measurements have been made.

Interpretation of photoelectron spectra of hydrocarbons using semi-empirical SCF MO calculations

It is possible to attain improved agreement between photoelectron-spectroscopic ionization potentials and theoretical orbital energies simultaneously for all electrons in small hydrocarbons by a suitable parametrization of INDO in its modified form, MINDO. The changed parameters concern mainly the resonance integrals, in which the interaction is treated differently for different situations of overlap. The new procedure will be referred to as SPINDO (Spectroscopic-Potentials-adjusted INDO) or, due to its preliminary status, better as SPINDO/1. The photoelectron spectra of hydrocarbons of different types (allene, cyclopropane, norbornadiene, naphthalene, and styrene) are discussed. The conformation of styrene is determined.

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.

Electronic structure of sulphur compounds VI. Photoelectron spectra of some simple thiocarbonyl compounds

Abstract The photoelectron spectra of a number of simple thiones including thiofenchone, thiocarbonates and thioamides have been recorded, and the bands corresponding to ionization energies in the range 7.5–13 eV have been interpreted using vibrational fine structure, comparison of He-I and He-II spectra, and CNDO/S calculations. In all compounds the non-bonded orbital mostly localized on the sulphur atom has the lowest ionization potential. The position of the bonding π and σ orbitals show a marked sensitivity to the nature of the substituents on the thiocarbonyl sulphur atom.

Cyclopentadiene and fulvene, studied by photoelectron spectroscopy and a spindo calculation

Abstract The electronic structures of cyclopentadiene and fulvene have been calculated by use of the spectroscopic-potentials-adjusted INDO (SPINDO). The orbital energies agree well with the ionization energies from the photoelectron spectra, which indicates the usefulness of the new procedure. The molecular orbitals are discussed.

On the Relation Between Semi-Empirical MO Methods and Rigorous Methods in Quantum Chemistry

Semi-empirical molecular orbital theory is explained in terms of the exact solution to the electronic Schrodinger equation. The necessity is pointed out for a parametrization which simulates the effect of a large orbital basis. The non-uniqueness of parameters is discussed using results obtained by Davidson and Ponec. The present work is meant as an introduction to an accompanying paper on the HAM/4 parametrization of atomic energies.