Abstract

The dynamic aspects of electronic relaxation of molecules in solution are central to the understanding of a wide range of condensed phase photophysical and photochemical processes. While the population relaxation rate of electronically excited species can be measured by a number of spectroscopic methods, the evolution of the phase coherence in liquids has been difficult to characterize due to its ultrafast nature. With advances in the technology of generating ultrashort laser pulses, the relaxation of electronic coherence can finally be studied in real time. In the past decade, femtosecond photon echo measurements have provided information on the electronic dephasing dynamics of molecules in room-temperature solution [1-3]. A prerequisite for using this method to address such dynamics is that the laser pulses must be much shorter than the time scale of the dephasing process. It has been demonstrated by Shank and co-workers [2] and Wiersma and co-workers [3] that ~ 10 fs pulses are needed to resolve the electronic dephasing dynamics of dye molecules in room-temperature solution. In this paper, a new approach to study the ultrafast dephasing dynamics in liquids is presented: degenerate femtosecond pump-probe spectroscopy that utilizes variable laser pulse-widths and tunable wavelengths [4]. In particular, the dephasing dynamics of an infrared dye, HITCI (absorption and emission spectra are shown in Figure 1), is examined in detail. A comprehensive theoretical treatment based on a multimode Brownian oscillator model of solvation [5] is developed and quantitative agreement between theory and experiment is obtained.

© 1994 Optical Society of America