Photoelectron spectroscopy band vibrational structure

For a nonradiative lifetime of 10 13s, the linewidth will be 50 cm-1, causing the rotational structure to disappear and the band to become diffuse. For a shorter nonradiative lifetime, 10 15s, the vibrational structure disappears and the spectrum becomes similar to a continuous spectrum. As spectral lines become broader and broader, their intensity is spread out and becomes lost in the background. However, low-resolution techniques, such as photoelectron spectroscopy or electron impact spectroscopy, can enable detection of strongly predissociated bands (see Table 7.2). 

Experimental evidence for the vibrational structure of XHX transition states has been provided by photoelectron spectroscopy of XHX- anions with X = Cl, Br, and I (134,160-163). This technique, by inducing photodetachment of an electron from the XHX" anions, probes the Franck-Condon region, which is believed for these systems to include geometries in the vicinity of the transition state region for the neutral systems. Spectral bands have been interpreted as evidence for trapped-state resonances associated with asymmetric stretch-excited levels of the transition state (160-163), and they are in general agreement with synthetic photoelectron spectra calculated from the scattering computations of Schatz (17-19). In recent experimental spectra (158,162), more closely spaced oscillations have been observed these are apparently related to rotational thresholds as described by Schatz. 

Vibronic-coupling theory has been a well established area of research since many years. The basic elements of the theory are the concept of dia-batic electronic states, the normal-mode description of vibrational motion, and the application of symmetry selection rules to derive appropriate model Hamiltonians. The applications of vibronic-coupling theory cover the full range of molecular spectroscopy, including, in particular, optical absorption and emission and photoelectron spectroscopy. Typical spectroscopic phenomena associated with vibronic interactions are the appearance of nominally forbidden electronic bands, the excitation of nontotally symmetric modes, or unusual and complex vibronic fine structures of electronic spectra. A fairly comprehensive and up-to-date exposition of vibronic-coupling theory is provided by the monograph of Bersuker and Polinger. ... 

The purpose of this chapter is to provide an overview of a rather wide array of experimental techniques that can tell us about the electronic structure of molecules. Some of these techniques, such as photoelectron (PE) spectroscopy, which is based on Einstein s photoelectric effect, are generally applied to gas-phase molecules. They can give high-resolution spectra, providing information about molecular vibrations and even, in a few cases, rotations. At the other end of the scale, UV/vis spectroscopy, particularly as applied to transition-metal complexes in solution, involves broad bands, and although it is an important and widely-used method, the information it gives is limited. Emission spectroscopy of transition-metal compounds has also become important. 

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