Mechanism of the electron spin polarization of excited triplet states caused by the mutual annihilation of triplet states

Abstract

A new mechanism of the triplet spin polarization is considered qualitatively and quantitatively. The spin polarization arises due to the spin-selective triplet-triplet annihilation and the subsequent spin dynamics in the spin-correlated pairs of t¡ With the time-dependent permrbation theory the simple rules are found which specify the polarization pattern of triplets. lntroductionNowadays the nonequilibrium electron spin pola¡ is intensively studied. These investigations are motivated by several reasons. Indeed, the spin polarization increases the sensitivity of the electron paramagnetic resonance (EPR) detection of paramagnetic particles. Of particular interest is that the nonequilibrium spin polarization induced in the course of photochemical and photophysical processes allows one to investigate intermediate short-lived paramagnetic species [1,2]. The electron spin polarization pattern reflects details of the photochemical and photophysical processes responsible for the polarization. Therefore, spectroscopic manifestations of the spin polarization provide powerful methods to study elementary photoinduced processes. The polarized electron spins are of p¡ interest due to their applicati0ns in spintronics, etc.Various mechanisms of electron spin polarization have been studied. One of the most interesting among them is the so-called chemically induced dynamic electron polarization (CIDEP) effect [1,2]. The CIDEP formation proceeds in two stages. The first stage is a spin-selective molecular process. This might be, e.g., the spin-selective recombination of free radicals (the radical pair mechanism [RPM] of the CIDEP formation [3]) or the spin-selective quenching of triplet states by free radicals (the radical-triplet pair mechanism [RTPM] of the CIDEP formation [4]). In this stage the spin-correlated pairs of two paramagnetic spe-