COLLISIONAL ENHANCEMENT OF RYDBERG LIFETIMES OBSERVED IN VIBRATIONAL WAVE-PACKET EXPERIMENTS

Evidence for lifetime enhancement of Rydberg states by interactions with surrounding ions is obtained from femtosecond pump-probe experiments on the vibrational wave packet dynamics of the I-2(B)-state, in which either detection of I-2(+) ions or zero-kinetic energy (ZEKE) electrons was used. The use of the ZEKE technique leads to the appearance of new frequency components in the Fourier transform power spectra of pump-probe time delay scans, as well as the observation of enhanced peak-to-valley ratios in these scans. These observed frequency components do not correspond to any energy level spacings in the molecule; they correspond to a sum of two energy level spacings, suggesting a nonlinear detection mechanism in the ZEKE technique (i.e., interaction with ions). Additional evidence for our interpretation is presented through experiments in which additional ions were formed through nonresonant multiphoton ionization of I-2 with a picosecond 266 nm laser, as well as experiments in which both the decay time of the ZEKE signal and the appearance of the pump-probe time delay scans as a function of the pulsed field time delay were studied. Theoretical wave packet calculations which support the conclusions are presented. (C) 1995 American Institute of Physics.