The collisional deactivation of highly vibrationally excited pyrazine by a bath of carbon dioxide:: Excitation of the infrared inactive (1000), (0200), and (0220) bath vibrational modes

The collisional quenching of highly vibrationally excited pyrazine, C4H4N2, by CO2 has been investigated using high resolution infrared transient absorption spectroscopy at a series of cell temperatures. Attention is focused on collisions which result in excitation of the Fermi-mixed bath vibrational states (10(0)0) and (02(0)0), along with the unmixed overtone bend state (02(2)0). The vibrationally hot (E(vib)approximate to 5 eV) pyrazine molecules are formed by 248 nm excimer laser pumping, followed by rapid radiationless decay to the ground electronic state. The nascent rotational and translational product state distributions of the CO2 molecules in each vibrationally excited state are probed at short times following the excitation of pyrazine. The temperature dependence of this process, along with the CO2 product state distributions and velocity recoils, strongly suggest that the vibrational excitation of CO2 is dominated by a long-range electrostatic interaction despite the fact that the dipole transition matrix elements connecting the CO2 ground state to the excited states vanish for the isolated molecule. The vibrational energy transfer is accompanied by very little rotational and translational excitation and displays the characteristic strong, inverse temperature dependence (probability of transfer increases with decreasing temperature) expected of energy transfer mediated by a long range attractive interaction. A number of possible explanations for this apparent anomaly are considered, of which energy transfer mediated by dipole/quadrupole forces appears to be the most consistent with the data. (C) 1998 American Institute of Physics.