Vibrational relaxation studied with light

In this lecture I will deal with optical studies of vibrational dynamics. I will address the question of what (minimal number of) parameters characterize vibrational motion and under what conditions the relaxation-time approximation is valid. Within this assumption the vibrational dynamics are represented by, besides the eigenfrequency, the decay constants T2 and T1. More of these constants will be needed when different oscillators are coupled. Following we will consider the extraction of these quantities from optical experiments. Relevant optical techniques are linear and nonlinear absorption spectroscopy, spontaneous Raman scattering and forced or “coherent” Raman scattering. The complications of strong coupling of the oscillators to the light field, in absorption spectroscopy referred to as polariton formation, will be dealt with. It will be demonstrated that the conventional response theory that is employed to calculate line shapes has serious shortcomings. I refer to these “Kubo-type” response principles as to “one-way response theory” in contrast to, what I have coined, “two-ways response theory”.