PURE DEPHASING OF A NONLINEAR VIBRATIONAL-MODE - NON-MARKOVIAN LINE-SHAPE

The method of thermodynamic Green's functions is applied for calculations of spectral line shape of a vibrational transition in a molecule interacting with a thermal bath (liquid solvent or a solid state matrix) whose characteristic frequencies are much lower than the transition frequency. Line broadening arises due to cubic anharmonicity of the vibrational mode according to known mechanism of Oxtoby of dephasing in liquids. Our approach makes it possible to take into account interaction with the bath exactly, regardless of the strength of coupling, and to develop a convenient perturbation theory in nonlinearity of the mode. It is shown that at low temperatures and for strong coupling to the bath the line shape is strongly asymmetric and cannot be described in terms of a Markovian approximation usually assumed in previous studies. Actual line shape is related to the spectrum of excitations of the bath which can be studied using data on the line shape at low temperatures. Line shapes are studied both in quantum and classical case beyond the weak coupling limit and the Markovian approximation. © 1990 American Institute of Physics.