DEGENERATE FOUR-WAVE MIXING SPECTROSCOPY OF A VAN DER WAALS COMPLEX. Paul C. DeRose, Po-Yuan Cheng, Hai-Lung Dai. Univ. of Penn., Philadelphia, PA First author contact: NIST, Bldg. 220, Rm. A320, (301)975-4572, paul.derose@nist.gov

Degenerate four-wave mixing (DFWM) spectroscopy is demonstrated to be an effective method for the detection and characterization of glyoxal dimer. DFWM is a coherent non-linear optical process through which three laser beams of the same optical frequency are coupled through the third order susceptibility of the medium to generate a fourth coherent beam at the same frequency. When the frequency of the input beams is resonant with an optical transition of the medium, the generation of the fourth signal beam is greatly enhanced. Glyoxal dimer is produced by expanding a glyoxal/He mixture through a pulsed valve nozzle into a vacuum chamber. The dimer is detected by taking DFWM spectra near the glyoxal S1-S0 excitation band region around 21975 cm-1. The detection limit for the dimer is ~1011/cm3. The DFWM spectrum is simulated using a standard rotational absorption band calculation where the N B dependence of linear absorption is replaced with a N2Bn dependence, where N is the ground state population of the dimer, B is the absorption cross section of glyoxal and n has a value between 1 and 4 depending on laser intensity. Rotational constants and geometries for both the ground and excited electronic states of the dimer are determined from these simulations.