Optische Elektronenspin-Polarisation (OEP) in Triplett-Zuständen organischer Moleküle

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Abstract
Stimulated microwave emission is observed in the ESR spectrum of 2-methyl-naphthalene and anthracene molecules in their triplet states at 4,2 °K. This is the consequence of a strong deviation from thermal equilibrium distribution within the three magnetic sublevels, a phenomenon called optical electron spin polarization (OEP). The OEP is due to spin selection rules either during the population or during the depopulation processes of the triplet states. In most cases both processes occour predominantly radiationless and are termed intersystem-crossing (ISC). In the present paper we report in detail about the kinetics of population, depopulation and spin-lattice relaxation of the phosphorescent triplet states of 2-methyl-naphthalene and anthra-cene. In addition the kinetical constants, the fine structure constants and the orientation in the host crystal of eleven different metastable triplet state molecules are presented in a table. As opposed to former investigations (quinoxaline, naphthalene, pyrazine) spin polarization in 2-methyl-naphthalene and anthracene is due to the selectivity of the depopulation mechanism. This is demonstrated by the time dependence of the ESR-signals after switching on and off ex-citation light. The evaluation of the experiments yields a common rule concerning the intrinsic zero-field decay rate constants of naphthalene, 2-methyl-naphthalene and anthracene: kx : ky : kz ≈ 2 : 1 : 0,2 The selectivity of the population decreases in the sequence naphthalene, 2-methyl-naphthalene, anthracene.

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