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

The infrared multiple photon trans → cis isomerization of crotonitrile was studied over the pressurerange 0.02 torr to 10 torr at fluences of approximately 4J/cm 2 using collimated TEA CO 2 laser radiation at942 cm -1 . Only minor quantifies of fragmentation products were found at these conditions. At pressuresof 0.1 torr or less, the trans form was completely converted to cis form by about 5000 laser pulses. Onincreasing the pressure a collisional regime was entered in which photostationary states were reached withfewer pulses and having smaller conversions than at low pressure. Both the number of pulses and theconversion decreased monotonically with increasing pressure. The tactic of driving the reaction in theexothermic direction gave evidence that the reaction does not occur solely by thermal isomedzation evenin the highly eollisional 10 torr experiments, but that it still has a nonthermal component due to laserinduced isomerization.