Mechanism of the Photodiss ociation Channels of Alkyl Iodides at 248nm

Experimental results of the laser photofragmentation for CH₃I, C₂H₅I, n-C₃H₇I, i-C₃H₇I, n-C₄H₉I, t-C₄H₉I. and CH₂C1I showed that the photodissociation channel ratio I^* /I was mainly controlled by the curve crossing between electronic excited states ³Q_o and ¹Q₁ during the dissociative dynamic processes. The effects of the n→σ^* transition on the dissociation dynamics, due to the substitution of alkyl groups with different Taft polar factor, were found to be important too. Using Landau-Zener formula and Hammett equation, the relation of the photodissociation channel ratio p^* with the energy partitioning of the photofragments and the Taft factor σ^* of alkyl substitutes were deduced as follows: \operatornameIn\left(\fracv \ln P^*v_0 \ln P_0\right)=-\sigma^* \rho where v is velocity passing through curve crossing region and is closely related to the energy partitioning of the fragments. The observed results can be well described by this deduced formula.