Photoelectron spectra, interpretation

The photoelectron spectra of pyridazine have been interpreted on the basis of many-body Green s function calculations both for the outer and the inner valence region. The calculations confirm that ionization of the first n-electron occurs at lower energy than of the first TT-electron (79MI21201). A large number of bands in the photoelectron spectrum of 3,6-diphenylpyridazine in stretched polymer sheets have been assigned to transitions predicted... 

Robin et al.162 investigated the photoelectron spectrum of unsubstituted cyclo-propenone and interpreted its results with the aid of Gaussian-type orbital calculations of double-zeta quality for the electronic ground state using the experimentally established133 geometry of cyclopropenone. 

The photoelectron spectrum of the dihydrogen molecule was described and interpreted in terms of molecular orbital theory. 

The photoelectron spectrum of 4//-pyran (5) was measured446 and interpreted by various MO methods.56,466... 

Photoelectron spectra of unsubstituted 1,2,4-triazine, its three monomethyl derivatives, two dimethyl derivatives and the trimethyl derivative have been measured by Gleiter (77MI21900). The He(I) photoelectron spectrum of the parent 1,2,4-triazine (1) is characterized by a single band at 9.6 eV followed by a complicated band system of several overlapping peaks between 11 and 13 eV. A further composite band is found at 15 eV. Methyl substitution seems to help in the resolution of the peaks. The experimental PE spectra were interpreted by comparison with the result of MO calculations and empirical correlation procedures. In Figure 1 the correlation of the first five bands of the 1,2,4-triazine (1) and its three methyl derivatives are given. 

A DZ-quality calculation on HN03 has been reported and used in the interpretation of the photoelectron spectrum.107... 

The staggered rotamer is predicted to be more stable, in agreement with experiment for B2C14 but not for B2F4, but in the latter case the error in the barrier is only ca. 3 kJ, so it is possible that a more extended basis set would correct this. The wave functions were analysed in detail and used to interpret the photoelectron spectrum of B4C14. 

The problem of the static and dynamic behavior of a core hole is intimately connected with the question of how the core hole was created. The energy level spectrum of the core hole is uniquely given by the energy level spectrum of the residual ion but the intensity distribution is determined by the excitation process. In order to interpret a photoelectron spectrum we therefore have to understand the photoionization process which gave rise to the photoelectrons. The purpose of this section is to discuss in simple terms the physics of. the photoionization process and to demonstrate how the various contributions to the photoelectron spectrum can arise. A more formal discussion is presented in Section 3. 

The photoelectron spectrum was initially interpreted (29e) in terms of trapped valencies but a theoretical study by Hush (67) has shown that a delocalized description is equally consistent with the data available so far. Infrared data are still under discussion, but the Raman spectrum has been interpreted (29f) in terms of localization. It does seem clear that the rate of valence interchange according to Eq. (1) is rapid compared with the NMR time scale (29b) the possibility remains that the rate may be comparable with the frequencies of certain bond vibrations. 

The development of gas phase ultraviolet photoelectron spectroscopy (UPS) has proceeded rapidly in the last twelve years. Turner first showed that the process of photoionisation could be used to generate a photoelectron spectrum giving a direct measurement of atomic or molecular ionisation potentials (7). The early results of the Oxford group led to the acceptance of two simple guide lines in the interpretation of such spectra. Firstly the electron kinetic energies were considered to satisfy a modified Einstein relation ... 

This calm was shattered by the first direct measurement of the singlet-triplet gap of methylene by Lineberger in 1976. Laser photoelectron spectrometiy of CH2 provided a spectrum interpreted as transitions to singlet and triplet CH2. Despite some difficulty in arriving at the 0-0 transition, Lineberger concluded that was equal to 19.6 kcal mol", far above the theoretical (or previous experimental) predictions. However, since this was the first direct observation of the gap, it was considered to be the most accurate determination yet. 

When the polymer backbone contains a large number of substituents, or long side chains, it is clear that its valence band photoelectron spectrum will contain a lot of peaks each side group will bring its own fingerprint in the XPS spectrum, and the interpretation of the data will be very difficult. An example of this limitation has been found in the study of polydiacetylenes, which until now were not synthesized with sufficiently short side chains to reduce the number of bands (molecular orbitals) appearing in the valence spectrum (, 25). 

If we ignore for the moment this deeper ionization, the 584 A photoelectron spectrum of the alkali halides should be very simple— ir and a peaks, separated by a fraction of an electron volt, with the additional proviso that the tt orbital is split due to spin-orbit interaction. The detailed structure of the photoelectron spectrum, and its interpretation, have however been the subject of some controversy. Experimentally, the dispute concerns the presence of one, two or three peaks in the valence region. Those who advocate three peaks (14) interpret them as 3/2 1/2 approach ( ) has been to... 

Additional information on electronic structure may be obtained from the x-ray emission spectra of the SiOj polymorphs. As explained in Chapter 2, x-ray emission spectra obey rather strict selection rules, and their intensities can therefore give information on the symmetry (atomic or molecular) of the valence states involved in the transition. In order to draw a correspondence between the various x-ray emission spectra and the photoelectron spectrum, the binding energies of core orbitals must be measured. In Fig. 4.12 (Fischer et al., 1977), the x-ray photoelectron and x-ray emission spectra of a-quartz are aligned on a common energy scale. All three x-ray emission spectra may be readily interpreted within the SiO/ cluster model. Indeed, the Si x-ray emission spectra of silicates are all similar to those of SiOj, no matter what their degree of polymerization. Some differences in detail exist between the spectra of a-quartz and other well-studied silicates, such as olivine, and such differences will be discussed later. 

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