Picosecond Resolution Studies of Ground State Quantum Beats and Rapid Collisional Relaxation Processes in Sodium Vapor

We report experimental studies of coherent transients and collisional relaxation processes in Na vapor, using degenerate four-wave mixing (DFWM) with picosecond pulses. These include investigation of the quantum beat modulation (due to the hyperfine splitting of the ground state) of the intensity of the DFWM signal as a function of excitation pulse separation. The picosecond resolution obtained by varying the pulse delay in our DFWM experiments has resulted in the first observation of harmonic structure in the quantum beat modulation, and in the observation of an anomalously rapid (e−1 decay time <50 psec) collisional relaxation process at increased temperatures.