State-to-State Collisional Dynamics by Coherent Laser Pulse Phase, Shape, and Frequency Modification

Abstract

Conventional coherent pulse sequences such as photon echoes measure only highly averaged relaxation rates in complex multilevel systems, such as molecules undergoing state-changing collisions. Pulse frequency, phase, and shape control lets us generate sequences which give a more detailed understanding of the dynamics. Results of dual frequency, crafted shape sequences on I₂ are presented which show that the "coherence dephasing" time T₂ is primarily due to population redistribution (energy changing collisions) in the electronically excited state, and that the electronic ground state has a much smaller cross section for such collisions. Quantitative analysis is only possible with modified laser pulse shapes which excite a single velocity component, and requires pulse sequences which correct for the hyperfine dependence of predissociation. Since this correction is rarely made in previously reported coherent transient measurements, literature values of T₂ and T₁ may not be reliable in the zero pressure limit.