			15min:

STATE-TO-FIELD VIBRATIONAL ENERGY TRANSFER FROM S₁ PARA -DIFLUOROBENZENE WITH HIGH VIBRATIONAL EXCITATION. ABSOLUTE COLLISION CROSS SECTIONS AND QUANTAL EFFECTS..

TODD A. STONE AND CHARLES S. PARMENTER, Indiana University Department of Chemistry, Bloomington, Indiana 47405.

State-to-field vibrational energy transfer (VET) from several high lying levels (E_vib \beginmath > \endmath 2800 cm^-1) within the S₁ (¹B_2u) manifold of para -difluorobenzene ( p DFB) vapor at 300 K in single collisions with Ar and He is probed. A laser pump-dispersed fluorescence probe approach provides absolute collision cross sections for this large molecule (30 modes) in a region where the vibrational state density approaches 10⁴ per cm^-1. In this region, p DFB is beginning to acquire the characteristics typical of high energy thermal unimolecular reactions, namely an enormous state density and highly mixed vibrational identities. Additionally, we have developed a technique based on electronic state quenching using molecular oxygen (chemical timing) by which we may investigate the behavior of the cross section as the mixed character of the pumped level is deliberately tuned. Tuning the vibrational character of the pumped level for a series of VET interactions provides the unique view of how (or whether) quantal effects influence VET in regions of large state density.