A possible test of the photoexcited triplet mechanism of chemically induced electron spin polarization: Dependence of the spin polarization on polarized light orientation

Abstract

Therè have been a number of observations of electron spin polarization in radicals produced by hydrogen atom abstraction or electron transfer reactions of photoexcited triplet molecules. Two mechanisms have been offered to explain the observed polarization: (1) The radical pair mechanism (RPM) in which the polarization results from mixing of the singlet and triplet states of the radical pair by the magnetic interactions within the radicals; (2) the triplet mechanism (TM) in which the polarization originates in the triplet as the result of spin selective intersystem crossing from the photoexcited singlet state. In the TM, the initial spin polarization is with respect to the molecular spin states of the triplet and evolves with time into a polarization with respect to the electron spin Zeeman levels in the external magnetic field. This process depends on the initial orientation of the triplet molecule with respect to the external field; thus, if a polarized photoexcitation source is used so that the initial distribution of molecular orientations is anisotropic, then the electron spin polarization of the radicals will depend on the orientation of the electric vector of the polarized light with respect to the magnetic field. This effect is both a possible test of the triplet mechanism, and a means of obtaining information about the excited states involved in the photochemical reaction.