Translational and rotational excitation of the CO2(00(0)0) vibrationless state in the collisional quenching of highly vibrationally excited perfluorobenzene: Evidence for impulsive collisions accompanied by large energy transfers

The relaxation of highly vibrationally excited perfluorobenzene (C6F6) by collisions with CO2 molecules has been investigated over the temperature range 243-364 K using diode laser transient absorption spectroscopy. Particular focus is placed on understanding both the dynamical features and the kinetics of collisions which are accompanied by large energy transfers into the CO2 rotational and translational degrees of freedom. Vibrationally hot perfluorobenzene (E-vib=41 822 cm(-1)) was prepared by 248 nm excimer laser pumping, followed by rapid radiationless transitions to the ground electronic state. The nascent rotational population distributions (J=64-80) of the 00(0)0 ground state of CO2 resulting from collisions with hot perfluorobenzene were probed at short times following the excimer laser pulse. Doppler spectroscopy was used to measure the distributions of CO2 recoil velocities for individual rotational levels of the 00(0)0 state. In addition, the temperature dependence of the state resolved, absolute rate constants for collisions populating high J states of CO2 was determined. The rotational distributions, distributions of recoil velocities, and quenching rates for production of CO2 high J states (J=64-80) exhibit a very weak temperature dependence. The slight temperature dependence indicates that CO2 molecules which scatter into high J states of the ground vibrationless level originate from rotational levels near the mean of the pre-collision thermal rotational distribution. A gap law model is used to estimate the probability of collisions which are accompanied by large energy transfers yielding values less than 2X10(-5)/cm(-1) for Delta E >2000 cm(-1). (C) 1997 American Institute of Physics.