STATE-TO-STATE VIBRATIONAL-RELAXATION FROM LEVELS AT STATE DENSITIES UP TO 2.3 STATES PER CM(-1) IN P-DIFLUOROBENZENE

Collision-induced vibrational energy transfer has been studied from th ree vibrational levels at intermediate state density in S-1 p-difluoro benzene in a supersonic foe jet expansion. Transfer was studied from t he 5(1) (E-vib = 818 cm(-1); rho(vib) = 0.6 per cm(-1)), 29(2) (E-vib = 876 cm(-1); rho(vib) = 0.6 per cm(-1)), and 5(1)8(2) (E-vib = 1179 c m(-1); rho(vib) = 2.3 per cm(-1)) levels. The collision partners inclu de a range of monatomics, diatomics, and polyatomics for 5(1) and 29(2 ). Hydrogen was the collision partner for 5(1)8(2). For 29(2), transfe rs involving multiple changes in vibrational quanta are important, and generally such transfers dominate. This behavior is different from th at observed at low state densities but is analogous to what has been o bserved previously at intermediate state densities in;p-difluorobenzen e [Mudjijono and W. D. Lawrance, J. Chem. Phys. 108, 4877 (1998)]. The re is a suggestion in the data for c-propane and ethane that transfer to vibrational modes of these collision partners is occurring. 5(1) sh ows very inefficient relaxation. With the exception of N-2, there is n o evidence in the spectra for significant transfer via channels involv ing multiple changes in vibrational quanta. The state-to-state branchi ng ratios for transfer from 5(1)8(2) were essentially in quantitative agreement with those expected based on transfer from 8(2). It appears that the in-plane mode v(5), and combinations involving low frequency modes with v(5), behave qualitatively differently to the lower frequen cy, out-of-plane modes. The lower frequency, out-of-plane modes change their state-to-state relaxation preferences with increasing vibration al state density, with multiple quantum changes becoming preferred, wh ile the higher frequency in-plane v(5) retains the state-to-state pref erences seen at low state densities. (C) 1998 American Institute of Ph ysics. [S0021-9606(98)01839-X].