9 - Interaction of Radiation and Matter and Electronic Spectra

Abstract

This chapter starts with the time-dependent Schrödinger equation and based on the semiclassical theory provides a detailed account of interaction of radiation and matter culminating into electronic transitions of valence electrons, core electrons and Rydberg transitions, and interaction between the electronic and vibrational motions. After having derived the Fermi's Golden rule, selection rules for transitions in diatomic and polyatomic molecules have been worked out. Electronic spectra of polyatomic molecules have been treated in detail and relationship established between the oscillator strength and intensity of absorption bands. Quantum mechanical treatment of the Franck–Condon principle for vibronic coupling has been given and used to understand intensity distribution in vibrational band intensities in electronic transitions. In order to predict the energy, properties, and reactivity of molecules in their excited states and electronic transitions, CIS (configuration interaction-singles) method based on Hartree–Fock theory and time-dependent density functional theory have been described with illustrative examples. A comparative account of methods used for the study of Rydberg transitions and core electronic transitions has also been given.