Optical Spin Polarisation OEP

Optical electron spin polarisation (OEP) is the term used to describe a non-Boltzmann distribution of the populations of the three zero-field or Zeeman components of an optically-excited triplet state. This non-thermal equilibrium can be a stationary or a non-stationary state. The optical excitation, that is e.g. the UV excitation, must be neither narrow-band nor polarised, and at low temperatures, OEP is the normal case for most triplet states in organic tt-electron systems. The OEP is 

Ty -o- Tz is rapidly saturated by a short resonant microwave pulse (1000.5 MHz) after the end of the UV excitation, thus when the radiative component Tz) has already mostly decayed, then the radiative component Tz) will again be populated from the non-radiative component Ty), thus inducing renewed phosphorescence. With a still longer delay of the microwave pulse, the increase in the phosphorescence intensity is smaller. From this, the lifetime of Tf) can be derived. With this method, the decay constants of all three zero-field components can be determined. 

For pure hydrocarbons, e.g. for naphthalene, there exist several pairs of singlet and triplet states with non-vanishing spin-orbit coupling, but here also, one triplet component ( T )) is predominant in the phosphorescence. Both in the pure hydrocarbons as well as in the hetero-aromatics, the main contributions to the spin-orbit coupling result from states which contain a a orbital. Contributions from states which contain only it orbitals are small enough to be neglected. 

An example of a quantitatively-analysed experimental result for these constants is shown in Fig. 7.29 in mixed crystals of naphthalene-dg 0.1% quinoxaline, the ESR transition T. To for the field direction Bo Xquinoxaiine and at a temperature T = 1.8 K is an absorption signal in the stationary state (Fig. 7.29a), while the transition I To) T-) in the stationary state exhibits stimulated emission of microwaves (Fig. 7.29b). After the end of the UV excitation at t = 0, the absorption line temporarily becomes an emission tine and vice versa. The interpretation of these results is simple (Fig. 7.29d) due to the negligible spin-lattice relaxation at T= 1.8 K, the three Zeeman components decay after the end of the U V excitation independently of one another, each with its own lifetime tj = into the So ground state. Since the difference of the populations of the three states is directly proportional to the intensity of the ESR signals, their time dependence can be used to determine the individual lifetimes of the Zeeman components involved. In the case of the particular orientation Boll, the state is To) = IT ), and one obtains directly from the measurements, e.g. the decay constant feo = kx and thus the lifetime of the zero-field constant Tx) of quinoxaline. 

Even at T = 4.2 K, the spin-lattice relaxation is so effective that the three triplet components are approximately in thermal equilibrium. Their populations then correspond roughly to the Boltzmann distribution and the two ESR lines thus decay with the same average lifetime of the states (Fig. 7.29c). 

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