Photoselection with intense laser pulses

Abstract

The method of photoselection uses a polarized photon beam to create a preferentially oriented population of excited state molecules. This population is examined using polarized absorption or emission techniques in order to obtain information about the symmetries of transition moments. Prelaser photoselection was limited in application to exceptionally long-lived states (or stable photoproducts) in perfectly rigid matrices or to strongly fluorescent states in moderately viscous media. With lasers, extension is possible to almost any state with a lifetime of picoseconds or longer, making photoselection a very powerful, general technique. This paper rederives the expected experimental behavior and extends the calculations to include saturation effects, which are prominent in practical laser experiments but were not important in classical studies. Furthermore, it shows that in the nonlinear regime, information on transition moment symmetry is available even from a single beam experiment. One very simple experiment is reported which verifies that one can distinguish ’’planar’’ from ’’linear’’ symmetry in absorbers using absorption methods in very fluid media. These results are most timely for picosecond mode-locked laser experiments but apply also to nanosecond Q switched or even intense continuous wave laser studies.