Observation of an energy threshold for large ΔE collisional relaxation of highly vibrationally excited pyrazine (Evib = 31000-41000 cm-1) by CO2

Energy dependent studies of the collisional relaxation of highly vibrationally excited pyrazine through collisions with CO2 were performed for initial pyrazine energies E-vib = 31 000-35 000 cm(-1). These studies are presented along with earlier results for pyrazine with E-vib = 36 000-41 000 cm(-1). High-resolution transient IR laser absorption of individual CO2 (00(0)0) rotational states (J = 56-80) was used to investigate the magnitude and partitioning of energy gain into CO2 rotation and translation, which comprises the high energy tail of the energy transfer distribution function. Highly vibrationally excited pyrazine was prepared by absorption of pulsed UV light at seven wavelengths in the range lambda = 281-324 nm, followed by radiationless decay to pyrazine's ground electronic state. Nascent CO2 (00(0)0) rotational populations were measured for each UV excitation wavelength and distributions of nascent recoil velocities for individual rotational states of CO2 (00(0)0) were obtained from Doppler-broadened transient linewidth measurements. Measurements of energy transfer rate constants at each UV wavelength yield energy-dependent probabilities for collisions involving large DE values. These results reveal that the magnitude of large DE collisional energy gain in CO2 (00(0)0) is fairly insensitive to the amount of vibrational energy in pyrazine for E-vib = 31 000-35 000 cm(-1). A comparison with earlier studies on pyrazine with E-vib = 36 000-41 000 cm(-1) indicates that the V --> RT energy transfer increases both in magnitude and probability for E-vib>36 000 cm(-1). Implications of incomplete intramolecular vibrational relaxation, electronic state coupling, and isomerization barriers are discussed in light of these results. (C) 1999 American Institute of Physics. [S0021-9606(99)00612-1].