Time-resolved ODMR

The time-resolved ODMR (TRODMR) measurements on a-Si H have been carried out by Morigaki et al. (1978b), Depinna and Cavenett (1982), 

Street (1982), and Boulitrop (1982). These measurements have previously been reviewed (Morigaki, 1983). 

(a) TRODMR spectra observed at 9.08 GHz and 2 ° K in a-Si H No. 12 for various gate delays and (b) steady state ODMR. [From Morigaki et al. (1978b).] 

1982) and for donor - acceptor recombination in crystalline semiconductors (Block and Cox, 1981). 

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One might argue that the results least subject to ambiguity are those with the shortest delay between the generation of the radical cation and its observation. In this respect, the time-resolved ODMR results of Trifunac and Qin (Fig. 24) [368] and time resolved CIDNP results observed in the author s laboratory (Fig. 25) [380], may provide the least distorted view of the species in question. Of course, neither of these experiments qualifies as the coveted direct observation. Thus, the direct observation of the elusive hexamethyl-Dewar benzene radical cation must await further scrutiny. 

This mechanism leads to a highly spin-polarized triplet state with a characteristic intensity pattern in the EPR spectrum, which is observed by time-resolved techniques (either transient or pulse EPR). The zero field splitting (ZFS) of the triplet state, which dominates the EPR spectrum, is an important additional spectroscopic probe. It can also be determined by optical detection of magnetic resonance (ODMR), for a review of the techniques involved and applications see reference 15. These methods also yield information about dynamical aspects related to the formation, selective population and decay of the triplet states. The application of EPR and related techniques to triplet states in photosynthesis have been reviewed by several authors in the past15 22-100 102. The field was also thoroughly reviewed by Mobius103 and Weber45 in this series. 

The results discussed above have shown that time-resolved emission spectroscopy can provide detailed insight into vibronic deactivation paths of triplet substates, even when the zero-field splitting is one order of magnitude smaller than the obtainable spectral resolution (= 2 cm ). This is possible at low temperature (1.3 K), because the triplet sublevels emit independently. They are not in a thermal equilibrium due to the very small rates of spin-lattice relaxation between these substates. In the next section, we return to this interesting property by applying the complementary methods of ODMR and PMDR spectroscopy to the same set of triplet substates. 

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