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U. Grigoleit, T. Lenzer, K. Luther

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 highly vibrationally excited aromatic molecules has been studied using large scale classical trajectory calculations. The investigations have focused on azulene collisions with different colliders (He, Ar and N₂) as well as pyrazine self-collisions providing the moments of energy transfer <ΔE> and <ΔE²>in the temperature range 50-1500 K. The interaction well depth ε_eff/k_B is found to be the key factor controlling the observed T dependence of collisional energy transfer. Systems with a relatively deep interaction well (pyrazine + pyrazine, azulene + Ar, azulene + N₂) show a pronounced negative dependence of -<ΔE> when T < ε_eff/k_B (in the systems studied here roughly at T < 300-400 K). The increased efficiency of collisional energy transfer at low T is due to additional contributions from collisions at large impact parameters. In systems with a very shallow well (azulene + He), however, a positive T dependence is found in the low temperature regime (< 300 K) due to the dominant contributions from impulsive, adiabatic collisions at short impact parameters. At higher temperatures (T > 300-400 K) - when the temperature is well above ε_eff/k_B - all systems behave qualitatively similar, showing only a very weak, slightly negative T dependence, as long as one is still far away from thermal equilibrium.

Zeitschrift für Physikalische Chemie, Oldenbourg Wissenschaftsverlag

Print ISSN: 0942-9352
Volume: 214, 08/2000
Pages: 1065

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