ABSTRACT
Conventional spectroscopic measurements are concerned with the intensity of absorption, emission, or scattering as a function of the frequency of radiation. In such steady-state, frequency-domain approaches, the emphasis is on the eigenstates and their energies, and the spectral response conveys structural information. Complementary to the behavior of the system as a function of frequency is the time-dependent response of matter. The focus of this chapter will be on the intimate relationship between the frequency and time domains. The formalism presented here will give rise to the point of view that sufficiently weak electromagnetic fields can be exploited to probe the molecular dynamics that take place in the unperturbed system. The fluctuation-dissipation theorem, to be discussed in Section 5.4, states that the energy imparted to the system by the probing field is dissipated by fluctuations characteristic of the system at equilibrium. The mathematical essence of this theory is that equilibrium time-correlation functions (TCFs) can be obtained by Fourier transformation of spectral intensity data.
