Picosecond Transient Grating Experiments: Application to the Study of Gas Phase Dynamics and Spectroscopy

Abstract

Picosecond transient grating experiments in sodium and iodine vapors, involving the 3S → 3P and X → B transitions respectively, are discussed. Population gratings in sodium demonstrate that the technique can be used to measure velocity distributions in the gas phase. It is shown that the time dependent transient grating signal is related to the Fourier transform of the velocity distribution. Similar experiments on iodine illustrate the effect of state changing collisions on the grating signal. In addition, the sodium experiments are used to illustrate a new type of time domain high resolution spectroscopy. When the grating excitation pulses have perpendicular polarizations, a polarization grating, rather than the usual population grating, is formed. Diffraction from the sodium polarization grating shows large time dependent oscillations in the diffraction efficiency. These oscillations yield the ground state and excited state hyperfine frequencies (1.77 GHz and 189 MHz, respectively). The results suggest that polarization grating spectroscopy can have applications in other areas, such as molecular rotational dynamics.