Thermalization in Infrared Multiple Photon Induced Reactions. The Pressure Dependence of the Trans → Cis Isomerization of Crotonitrile

Abstract

The infrared multiple photon trans → cis isomerization of crotonitrile was studied over the pressure range 0.02 torr to 10 torr at fluences of approximately 4J/cm² using collimated TEA CO₂ laser radiation at 942 cm^-1. Only minor quantifies of fragmentation products were found at these conditions. At pressures of 0.1 torr or less, the trans form was completely converted to cis form by about 5000 laser pulses. On increasing the pressure a collisional regime was entered in which photostationary states were reached with fewer pulses and having smaller conversions than at low pressure. Both the number of pulses and the conversion decreased monotonically with increasing pressure. The tactic of driving the reaction in the exothermic direction gave evidence that the reaction does not occur solely by thermal isomedzation even in the highly eollisional 10 torr experiments, but that it still has a nonthermal component due to laser induced isomerization.