Temperature dependence of collisional energy transfer in highly excited aromatics studied by classical trajectory calculations

The temperature dependence of the gas-phase collisional relaxation of highl y vibrationally excited aromatic molecules has been studied using large sca le classical trajectory calculations. The investigations have focused on az ulene collisions with different colliders (He, Ar and N-2) as well as pyraz ine self-collisions providing the moments of energy transfer (Delta E) and (Delta E-2) in the temperature range 50-1500 K. The interaction well depth epsilon(eff)/k(B) is found to be the key factor controlling the observed T dependence of collisional energy transfer. Systems with a relatively deep i nteraction well (pyrazine + pyrazine, azulene + Ar, azulene + N-2) show a p ronounced negative dependence of - (Delta E) when T < epsilon(eff)/k(B) (in the systems studied here roughly at T < 300-400 K). The increased efficien cy of collisional energy transfer at low T is due to additional contributio ns from collisions at large impact parameters. In systems with a very shall ow well (azulene + He), however, a positive T dependence is found in the lo w temperature regime (<300 K) due to the dominant contributions from impuls ive, adiabatic collisions at short impact parameters. At higher temperature s (T > 300-400 K) - when the temperature is well above epsilon(eff)/k(B) - all systems behave qualitatively similar, showing only a very weak, slightl y negative T dependence, as long as one is still far away from thermal equi librium.