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Green function techniques are used to develop a simple and efficient method towards the calculation of optical absorption, excitation, and dispersed fluorescence spectra of large harmonic polyatomic molecules. The molecular line shapes are expressed in terms of Fourier transforms of appropriate correlation functions which may be explicitly evaluated. Closed expressions are derived for a general harmonic molecule with two electronic states including equilibrium displacements, frequency changes, and Dushinsky rotation, within the Condon approximation. A simple method for extracting the complete set of parameters characterizing the ground and the electronically excited states from supersonic beam absorption and emission spectra is presented. Detailed calculations are performed for a model system with ten vibrational modes, and the sensitivity of the various experimental observables to Dushinsky rotation is analyzed.

A correlation function formulation, based on the Liouville equation for the density matrix, provides a microscopic theory for solvation dynamics and establishes a general fundamental connection between the calculation of rate processes and nonlinear optical processes in solution. The present rate theory requires the calculation of four-point correlation functions of the nonadiabatic coupling, which is formally identical with the calculation of four-wave-mixing processes and the nonlinear susceptibility (3). A novel semiclassical propagation scheme (the Liouville-space generating function, LGF) is developed and used in these calculations. The connection with (3) may allow the direct use of solvent correlation functions obtained from nonlinear optical measurements, in the calculation of molecular rate processes. The present theory interpolates continuously from the adiabatic to the nonadiabatic limits. A new criterion for adiabaticity is derived, and the role of the solvent time scale in inducing the crossover from the nonadiabatic to the adiabatic regimes is clarified. The present results generalize the Kramers theory of isomerization and the Marcus theory of electron transfer in polar solvents. Both static (polarity) interactions, which affect the reaction energetics and dynamic (friction) effects, are properly incorporated.