ABSTRACT
Because electronic energy levels are more widely spaced than vibrational and rotational levels, the study of electronic spectroscopy encompasses many of the previously discussed concepts concerning rotational and vibrational spectra. In electronic spectra of gas-phase molecules, analysis of rovibrational transitions within an electronic absorption or emission band can provide useful structural information, particularly when the pure rotational or vibrational spectrum is forbidden. In condensed phases, discrete rotational structure is not observed, but vibrational transitions contributing to the linewidth are often resolved. For example, the electronic absorption spectrum of benzene, shown in Figure 11.1, displays vibrational structure which will be analyzed in Section 11.5.3. In larger molecules, the high density of vibrational states serves to blur the vibrational structure. In solution, the line-broadening influence of the solvent may prevent individual vibronic transitions from being resolved, but they contribute to the linewidth nonetheless. Consider the absorption spectra of I2, shown in Figure 11.2. In the vapor phase, vibrational transitions are resolved as separate peaks. In solution, however, the vibrational transitions of I2 continue to contribute to the breadth of the spectrum, but are not resolved as distinct features. In this chapter, we will concentrate on the analysis of vibrational contributions to electronic spectra. These hold the key to determining the difference in equilibrium geometries of the ground and excited electronic states. Electronic spectra can also reveal some of the features of the excited state potential surface, as well as dynamics such as dissociation, isomerization, and radiationless decay.
