Relaxation Study in the Extremely Short Time Region by Stationary and Transient Nonlinear Optical Spectroscopy

An outline of new nonlinear spectroscopic methods which are developed for13 measuring ultrashort relaxation times of excited states in a range of 10−13– 10−14 s. They are all based on the third-order nonlinear optical processes in resonant media with two kinds of input light beams. Both the frequency-domain (stationary) and time-domain (transient) methods are developed with particular emphasis on the measurement of dephasing time T2. Resonant Rayleigh-type mixing and resonant Kerr effect are typical of the former, while transient spatial parametric effect is typical of the latter. A description of subpicosecond dye laser work necessary as a source for the time-domain measurement is also given. All the theories are based on the semiclassical description of the system with density matrix formalism incorporating phenomenological relaxation times. The S0 → Sl electronic transitions of dye molecules in both liquids and solids were selected as main objects of the present study.