Abstract

By measuring the spectrum of the probe pulse, rather than just its energy, Shank and coworkers have demonstrated how to take advantage of the large spectral width of the fastest femtosecond pulses to obtain complete time-resolved absorption spectra of coherently evolving molecules on a 6 fs timescale.¹ To provide a quantitative analysis of these experiments, we have formulated the perturbative density matrix theory for the third order susceptibility of a multi-level system in terms of four-time correlation functions which can be interpreted as the time-dependent overlap of bra and ket vibrational wavepackets propagating independently on the ground and excited state electronic potential surfaces.² When vibrational motion during the excitation is minimal, or when the pump pulse is shorter than the optical T₂, the system will be prepared in an essentially pure state. The time-resolved differential absorption spectrum can then be treated as the first-order spectroscopy of the non-stationary state created by the pump pulse.

© 1990 Optical Society of America