Calculation of molecular absorption spectra using a density matrix propagation scheme: an extension of Hellerʹs formula Original Research Article

For the calculation of stationary molecular spectra is a well established technique to propagate wave packets on related potential energy surfaces. We show that the time-dependent formulation of the absorption coefficient can be generalized to a case where the molecular system interacts with a dissipative environment. Therefore, we utilize the concept of the time-dependent statistical operator reduced to some selected molecular degrees of freedom involved in the optical transition. The related quantum master equation defines the dissipative time propagation which is used, after an appropriate Fourier transformation, to derive the absorption coefficient. The environmental degrees of freedom enter the absorption via the various elements of the so-called Redfield tensor. The method is used to calculate the infrared absorption within a single potential energy surface of the O-H stretching vibration of the water molecule. As a second example, the UV-absorption is discussed resulting from an electronic transition into the coupled valence and Rydberg states of matrix isolated NO.