THERMAL CONTRIBUTIONS IN INFRARED-LASER INDUCED GRATING EXPERIMENTS AT HIGH-PRESSURE

Degenerate four wave mixing (DFWM) and laser induced thermal grating s pectroscopy (LITGS) have been used to detect ethylene and methanol id mixtures with nitrogen, argon or helium in the pressure range between 0.005 and 18 bar. The grating forming step for both techniques was acc omplished via absorption of radiation in the interference region from two beams from a single longitudinal mode CO2 laser. For DFWM the same CO2 laser and for LITGS a diode pumped Nd:YAG laser were employed to read out the transient grating. Using photo thermal deflection spectro scopy (PDS) as an independent spectroscopic method thermal effects wer e confirmed for total pressures above 0.1 bar. For both DFWM and LITGS signal intensities increased with buffer gas pressure due to collisio n induced transfer of optical excitation energy into translational deg rees of freedom. The temporal behaviour of the IR-grating generation a nd thermalization processes and the advantages of hydrocarbon trace sp ecies detection from thermal gratings at high pressures is discussed.