PHASE-CONTROL OF ABSORPTION IN LARGE POLYATOMIC-MOLECULES

The phenomenon of interference of the amplitude for absorption of one photon of frequency 3 omega and the amplitude for absorption of three photons of frequency omega was theoretically predicted by Shapiro, Hep burn, and Brumer. The interference was demonstrated experimentally by varying the relative phase between the tripled frequency photon and th ree photons with the fundamental frequency by the groups of Elliott an d Gordon in atoms and small molecules. In order to see how general thi s phenomenon is, five compounds were studied, ammonia, trimethylamine, triethylamine, cyclooctatetraene, and 1,1-dimethylhydrazine. CH3I was used as the tripling gas for light in the range 604-600 nm, Interfere nce was observed in all cases. The last four compounds have low ioniza tion potentials and interference was observed between a 3+1 and a 1+1 ionization process with a maximum modulation of 22%. NH3 with a higher ionization potential requires absorption of 3+2 or 1+2 photons and ex hibits a maximum modulation of 33% We conclude that molecular size is no obstacle and that as long as a molecule has sufficiently strong abs orption at the tripled frequency, and sufficient vapor pressure, and t he laser fundamental beam is very strong, phase control of interferenc e is observable. (C) 1996 American Institute of Physics.