Continuum limit theory of absorption in the presence of dissipation

A second-order cumulant expansion is used to derive continuum limit expressions for the electronic absorption spectrum of a polyatomic molecule interacting with a bath, within the Condon approximation and weak fields. The small expansion parameter is the difference between the vibrational Hamiltonians in the ground and excited electronic states. The second-order cumulant expansion is shown to be a good approximation for a reasonable model of a polyatomic molecule with 45 degrees of freedom. Friction tends to shift the maximum in the absorption peak to the blue. When the vibrational frequencies in the excited electronic state are lower than those in the ground electronic state, one finds a stochastic resonance feature. Friction first narrows the peak and then broadens it. This narrowing is absent when one shifts only the equilibrium positions in the excited state relative to the ground state.