Leif Karlsson

Vibrational and Vibronic Structure in the Valence Electron Spectra of CH3X Molecules (X=F, Cl, Br, I, OH)

Valence electron spectra of the methyl halides and of methyl alcohol have been induced by means of the HeI resonance radiation at 21.22 eV. A large number of new vibrational bands are reported. The observation of these was facilitated by a high instrumental resolving power and fast data acquisition. In most cases the new bands are observed in 1A1?2E transitions but also the electron band of the 1A1?2A1 transition of CH3I exhibits extensive progressions of closely spaced vibrational bands not previously reported. The vibrational structure of the 1A1?2E transitions is affected by vibronic coupling. The structure of the 2e electron band of CH3Cl probably reflects the operation of the Ham effect. Calculations of the band structure including spin-orbit coupling are performed according to a model which is briefly described.

The Jahn-Teller Effect in the Electron Spectrum of Methane

The high resolution He I photoelectron spectrum of the (a12t25)2T2 < - (a12t26)1A1 transition in methane has been recorded and analyzed and a value has been obtained for the adiabataic ionization potential. The electron spectrum resulting from ionization of a t2 electron is a composite of at least three overlapping electronic bands which are separated by several tenths of an eV. These bands are the split components of the 2T2 electronic state of the CH4+ ion which is distorted by strong Jahn-Teller forces. The symmetry of the molecule in these three electronic states is believed to be, in order of increasing energy, D2d<C3e<D2h. The active vibration which causes the Jahn-Teller splitting is the r2(e) mode. The adiabatic ionization potential of methane is certainly below 12.62 eV. Our extrapolated value is approx- 12.51 eV. Four different vibrational progressions have been found, one of which shows vibronic splittings.

Photoelectron spectra of the nucleobases cytosine, thymine and adenine

The complete valence shell photoelectron spectra of cytosine, thymine and adenine have been investigated experimentally and theoretically. Vertical ionization energies and spectral intensities have been evaluated using the many-body Greens function method, thereby enabling theoretical photoelectron spectra to be derived. In cytosine, the influence of tautomers and rotational conformers has been investigated. The calculated spectra display a satisfactory agreement with the experimental data and this has allowed most of the photoelectron bands to be assigned. Photoelectron asymmetry parameters have been determined from angle resolved spectra recorded with synchrotron radiation. The experimental data show that the electronic configuration of the five outer orbitals in cytosine, thymine and adenine is π, σ, π, σ, π. Vertical ionization energies have been measured for all the outer-valence orbitals even though some of the associated bands overlap significantly.

Vibrational and Vibronic Structure in the Valence Electron Spectrum of H2S

The high resolution HeI induced electron spectrum of H2S is presented and the energies of the observed vibrational and vibronic bands are reported. In the 2B1 electron band two progressions of vibrational bands have been measured. In the 2A1 electron band the vibronic structure associated with v2 bending vibrations has been partially resolved. Assignments of the vibronic bands are suggested from a comparison with electronic emission data in terms of the vibrational and angular momentum quantum numbers v2 and K.

The identification of the outer valence shell π-photoelectron bands in furan, pyrrole and thiophene

Abstract The valence shell photoelectron spectra of furan, pyrrole and thiophene have been studied using synchrotron radiation and particular attention has been paid to the bands associated with the π orbitals. In each of these molecules three doubly occupied π-type molecular orbitals (denoted π 1 , π 2 and π 3 ) are formed, and previous work has established that π 2 and π 3 constitute the two outermost orbitals in all three molecules. However, the location of the most tightly bound π 1 orbital within the electronic configuration remains uncertain. In the present work the photoelectron bands due to the π orbitals have been investigated through angular distribution measurements. Photoelectron asymmetry parameters and branching ratios have been determined using monochromated synchrotron radiation in the photon energy range 15–120 eV. The spectral behaviour of asymmetry parameters associated with π orbitals is expected to differ from that of asymmetry parameters associated with σ orbitals. These differences provide an experimental means of distinguishing between the two types of orbitals. The many-body Green’s function method has been employed to evaluate the ionisation energies and pole strengths of all valence states. These theoretical predictions have proved equally important in identifying the π 1 photoelectron bands because they indicate whether ionisation from a particular orbital should give rise to a main-line, or whether the single-particle model has broken down. Interpretations have been proposed for most of the features observed in the inner valence region.

The HeI excited electron spectrum of phosphine : AN EXPERIMENTAL AND THEORETICAL-STUDY

The HeI excited electron spectrum of phosphine (PH3) has been recorded with high resolution. The 2 A 1 band reveals an extensive vibrational progression in the v 2 bending mode with irregular structure at the low binding energy side due to hindered inversion. The existence of a barrier in the PH3 + 2 A 1 ionic state has been confirmed by vibrational energy level and Franck-Condon factor calculations based on CASSCF calculations of the total energy. The equilibrium angle between a P-H bond and the plane of the three hydrogen atoms in this ionic state was determined to be 15·5° and the barrier against inversion to be 160 meV. The adiabatic electron binding energy of this transition ([Xtilde] 2 A 1 ←X 1 A 1) was found to be 9·868±0·005 eV.

The valence electron spectrum of SiF4

Abstract The He I induced valence electron spectrum of SiF 4 is presented and the energies of the bands are reported. The spectrum reveals vibrational structure in three of the four accessible electron bands. Only the band at 19.5 eV has been reported previously to exhibit vibrational progressions.

Inner valence satellite structure in high resolution X-ray excited photoelectron spectra of N2 and CO

The high resolution X-ray excited (hν = 1487 eV) valence photoelectron spectra of N2 and CO are presented. The spectra show a rich line structure down to 65 eV binding energy due to electron correlation effects. Several new structures are observed and new assignments are made from comparisons with calculations, monochromatized He II excited photoelectron spectra and resonance Auger electron spectra.

A theoretical investigation of the U.V. excited 1 A 1→2 A 1 photoelectron spectra of NH3 and ND3

The vibronic spectra of the 1 A 1→2 A 1 U.V. photoelectron transitions in NH3 and ND3 are analysed theoretically. Optimum geometry and potential curves are obtained by means of ab initio configuration interaction calculations. A one-dimensional approach, using the decoupled planar upper state symmetry coordinates and a Manning type function for the double-minimum potentials, is found to account for frequency shifts, anharmonicity and isotope shifts in the 3a 1 electron ionization. The intensities of the photoelectron spectrum are redetermined through integration of the experimental vibronic bands and are furthermore computed within the Franck-Condon approximation. From calculations of hot band progressions the first adiabatic ionization potentials of NH3 and ND3 could be set to 10·073 and 10·12 eV, respectively.

An experimental and theoretical study of the valence shell photoelectron spectrum of allene

Abstract The photoelectron spectrum of allene has been studied using He I, He II and synchrotron radiation as sources for ionisation. In addition to the main bands associated with the single hole states, a complex satellite structure due to many-electron effects has been observed in both the outer and inner valence regions. Two variations of the many-body Greens function technique have been employed to determine the ionisation energies and pole strengths for all valence states, and the results have facilitated the interpretation of the experimental spectra. Photoelectron angular distributions and photoinisation branching ratios have been measured using monochromated synchrotron radiation in the 15–120 eV range. He I and He II excitation have been used to obtain highly resolved spectra, and vibrational structure has been studied in the X 2E, A 2E and B 2A1 ionic states. Excitations of Jahn-Teller active modes have been found to be important in the X 2E and the A 2E states.