THE COLLISIONAL DEACTIVATION OF HIGHLY VIBRATIONALLY EXCITED PYRAZINEBY A BATH OF CARBON-DIOXIDE - EXCITATION OF THE INFRARED INACTIVE (10(0)0)-BATH, (02(0)0)-BATH, AND (02(2)0)-BATH VIBRATIONAL-MODES

The collisional quenching of highly vibrationally excited pyrazine, C4 H4N2, by CO2 has been investigated using high resolution infrared tran sient absorption spectroscopy at a series of cell temperatures. Attent ion is focused on collisions which result in excitation of the Fermi-m ixed bath vibrational states (10(0)0) and (02(0)0), along with the unm ixed overtone bend state (02(2)0). The vibrationally hot (E(vib)approx imate to 5 eV) pyrazine molecules are formed by 248 nm excimer laser p umping, followed by rapid radiationless decay to the ground electronic state. The nascent rotational and translational product state distrib utions of the CO2 molecules in each vibrationally excited state are pr obed at short times following the excitation of pyrazine. The temperat ure dependence of this process, along with the CO2 product state distr ibutions and velocity recoils, strongly suggest that the vibrational e xcitation of CO2 is dominated by a long-range electrostatic interactio n despite the fact that the dipole transition matrix elements connecti ng the CO2 ground state to the excited states vanish for the isolated molecule. The vibrational energy transfer is accompanied by very littl e rotational and translational excitation and displays the characteris tic strong, inverse temperature dependence (probability of transfer in creases with decreasing temperature) expected of energy transfer media ted by a long range attractive interaction. A number of possible expla nations for this apparent anomaly are considered, of which energy tran sfer mediated by dipole/quadrupole forces appears to be the most consi stent with the data. (C) 1998 American Institute of Physics.