Translational and rotational excitation of the CO2(0000) vibrationless state in the collisional quenching of highly vibrationally excited 2-methylpyrazine:: Kinetics and dynamics of large energy transfers

The relaxation of highly vibrationally excited methylpyrazine (C5N2H6) 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 methylpyrazine (E'=40 987 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=58-80) of the 00(0)0 ground state of CO2 resulting from collisions with hot methylpyrazine 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 population distributions, distributions of recoil velocities, and quenching rates for production of CO2 high J states (J=58-80) 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 precollision thermal rotational distribution. A gap law model is used to estimate the average initial rotational state and velocity of the CO2 bath, which allows for the calculation of the energy transfer magnitudes, Delta E. The measured energy transfer probabilities which are indexed by final bath state are resorted as a function of Delta E to create the energy transfer distribution function, P(E,E') from E'-E similar to 1500-6000 cm(-1). P(E,E') is fit to both single exponential and biexponential functions to extract a value for the average energy transferred in a single collision of methylpyrazine and CO2. This average energy transfer value is compared to donor loss energy transfer studies as well as previous bath energy gain studies on the pyrazine/CO2 and C6F6/CO2 systems. On average, methylpyrazine donates more energy per collision to CO2 than pyrazine but not as much as C6F6; however, methylpyrazine has the lowest probability for single collision energy transfers larger than 2000 cm(-1) of the three molecules studied using this technique. (C) 2000 American Institute of Physics. [S0021-9606(00)00136-7].