INTERROGATING THE VIBRATIONAL-RELAXATION OF HIGHLY EXCITED POLYATOMICS WITH TIME-RESOLVED DIODE-LASER SPECTROSCOPY - C6H6, C6D6, AND C6F6+CO2

The vibrational relaxation of highly excited ground state benzene d6, and hexafluorobenzene by CO2 has been investigated with high resolution diode laser spectroscopy. The vibrationally hot polyatomics are formed by single photon 248 nm excitation to the S1 state followed by rapid radiationless transitions. It has been found that in all cases less than 1% of the energy initially present in the polyatomics is deposited into the high frequency mode of CO2 (v3). An investigation of the CO2(00(0)1) nascent rotational distribution under single collision conditions reveals that very little rotational excitation accompanies vibrational energy transfer to the v3 mode. The CO2(v3) rotational states can be described by temperatures, T(rot), as follows: C6H6, T(rot) = 360 +/- 30 K; C6D6, T(rot) = 350 +/- 35 K and C6F6, T(rot) = 340 +/- 23 K. An estimate of <DELTA-E>v3, the mean energy transferred to the CO2 v3 mode per collision, suggests that as the availability of low frequency modes in the excited molecule increases, less energy is deposited into the high frequency mode of CO2. Finally, evidence is presented suggesting that even at moderate laser fluences, the two-photon ionization of benzene can lead to substantial CO2 v3 excitation via electron + CO2 inelastic collisions.