Optical spectra molecular parameters from

One of the most interesting and important results of the study was to show how the molecular constants change as the vibrational quantum number v increases. This behaviour is presented in table 8.10. The electron spin-spin and rotational constant values came, initially, from the analysis of the optical electronic spectrum [47], although the values of the spin-spin constants for different vibrational levels were refined by the analysis of the radiofrequency spectrum. The nuclear hyperfine parameters are obtained solely from the magnetic resonance experiments. We will discuss the significance of these constants in the following subsection. 

The popularity of the SOS methods in calculations of non-linear optical properties of molecules is due to the so-called few-states approximations. The sum-over-states formalism defines the response of a system in terms of the spectroscopic parameters, like excitations energies and transition moments between various excited states. Depending on the level of approximation, those states may be electronic or vibronic or electronic-vibrational-rotational ones. Under the assumption that there are few states which contribute more than others, the summation over the whole spectrum of the Hamiltonian can be reduced to those states. In a very special case, one may include only one excited state which is assumed to dominate the molecular response through the given order in perturbation expansion. The first applications of two-level model to calculations of j3 date from late 1970s [93, 94]. The two-states model for first-order hyperpolarizability with only one excited state included can be written as ... 

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