Femtosecond spectroscopy of stimulated emission from highly excited dye molecules

Abstract

Intense femtosecond pulses in ultaviolet and visible wavelengths are generated in an excimer-laser-pumped amplifier system with input pulses originating from a colliding-pulse mode-locked dye laser. The relaxation of high-lying singlet states in laser dyes is studied after ultraviolet excitation. Two time scales of relaxation are observed: (i) The intramolecular redistribution of energy occurs in rhodamine dyes with short time constants of ~200 fsec. (ii) The subsequent cooling of the vibrationally hot molecules on the picosecond time scale is monitored through the temperature-dependent broadening of the emission spectrum. Theoretical calculations of the emission spectrum of the hot molecules account well for the experimental results.