Relaxation processes in singlet O/sub 2/ analyzed by laser-induced gratings

Summary form only given. The technique of laser-induced grating spectroscopy (LIGS) uses crossed beams of a pulsed laser, which generate in their overlap volume a spatially periodic and transient electromagnetic field. A population grating is generated by tuning the laser frequency to an absorption line of the medium in the beam overlap volume. By radiationless collision-induced molecular transitions, heat is released and also a thermal grating is generated. The time-resolved grating diffraction efficiency is measured by diffracting a beam of a CW-laser at the laser-induced grating and measuring the power of the diffracted laser light. Time-resolved LIGS is appropriate to study heat releases on time scales up to microseconds. The upper limit of a few microseconds is determined by the rate of the molecular diffusion by which the population grating decays. In addition to the thermal and population grating, the spatially periodic field intensity generates in nonabsorptive as well as absorptive media also an electrostrictive grating. It arises from the density change in the medium caused by the electrostatic force of a spatially inhomogeneous electric field. We present a theoretical analysis of the combined population, thermal, and electrostrictive grating formation, which is relevant to the excitation of O/sub 2/ molecules to the metastable b/sup 1//spl Sigma//sub g//sup +/(/spl nu/´=O) state.