High-field ODMR

Ayyjh = 28.0 MHz and including the high-field ODMR result that k /I I are compared with the experimental frequencies in Table 5. The agree-... 

Fig. 6. Angular dependence of the high-field ODMR spectrum of carbonyl- C-...

Fig. 9. High-field ODMR spectra using magnetic field modulation showing C hyperfine structure observed in canonical orientations of C-benzophenone-d,o in 4,4 -dibromo-diphenylether (Brode and Pratt, 1977).

Fig. 10. High-field ODMR spectra using magnetic field modulation or (im ) 4,4 -diiluoro-benzophenone in 4,4 -dibromodiphenylether showing F hyperfine structure observed with H x and r. Asterisks denote forbidden transitions of the Am, 0 type (Mucha and Pratt,...

Fig. 11. High-field ODMR spectrum using magnetic field modulation of (mt ) 4,4 -di-bromobenzophenone in 4,4 -dibromodiphenylether showing bromine hyperfine and quadrupole fine structure with // z. The dashed lines indicate the centers of the Br and Br quadrupole satellites, which are separated from the center of the main line by 97 and 82 G, respectively (Mucha and Pratt, 1977a).

Fig. 20. High-field ODMR spectra using amplitude modulation of (nrc ) C-benzophe-none in 4,4 -dibromodiphenylether in the vicinity of HI, at different microwave frequencies (GHz). A and A denote allowed (Ants = 1) transitions, whereas F and F denote forbidden (Anis = +2) transitions (Mucha and Pratt, 1975).

Figure 7.44. Full-field ODMR of DHO-PPV fabricated by Yoshino and co-workers, as described by Murase al. [51], and highly ordered, pristine unsubstituted PPV, fabricated by Holmes and co-workers [80,84], Note the absence of a triplet exciton ODMR pattern [60],...

Fig. 1. Zero field ODMR spectra of ICN at 1.4 K. (A) Allowed T - T,. transition at low power, (B) allowed T <- T, transition at low power, (C) 7, -> Ty transition showing forbidden quadrupole satellites observed at high power, (D) T, Tj transition under the same conditions (Kothandaraman et ai, 1979).

The introduction of a laboratory magnetic field gradually decouples the triplet electron spins from the molecular framework. The energies of the spin sublevels then become dependent on the strength of the field as well as its orientation with respect to the fine-structure (and other magnetic) axes. This introduces new features into the interpretation of the data, which were first explored in detail by Hutchison and Mangum (1961) for the case of molecular triplet states. We wish to illustrate some of these effects by reference to recently published high-field (hf) ODMR experiments on the lowest (nn )... 

Fig. 7. Angular dependence in the ab plane of the average fields for resonance in the high-held ODMR spectrum of C-benzophenone in 4,4 -dibromodiphenylether showing that the principal transverse axes of g are parallel to the corresponding axes of D and that g,, - < g,y (Mucha, 1974).

Previous experiments in this laboratory (Chen et ai, 1973) have shown that it is possible to resolve hyperfine structure in a hf ODMR spectrum providing the spectrometer is operated at low power in order to avoid the combined effects of microwave saturation and forbidden electron-nuclear transitions (as in the zf experiment). To illustrate this, we show in Fig. 9 first-derivative hf spectra of C-benzophenone-dio with 1 mW power. The lines shown are the Antg = 1 transitions with H x (low-field), H y (low-field), and H z (high-field). Each is split into a doublet by the first-order hyperfine interaction. Similar splittings are observed in the... 

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. 

Magnetic field effects on the luminescence of a liquid polyphenoxy polymer excited in the VUV region show that highly excited singlet states of aromatic chromophores can relax to form two triplets. A spectroscopic and ODMR study has been reported on the triplet state of N-methyl-p-nitroaniline ° . 

The very rich ODMR observed for Ti in SiC led to another experiment called cross relaxation [11]. A change in emission occurs when the Zeeman splitting of two Ti-exciton states equals the splitting for a nearby, coupled defect. No microwaves are necessary but the sensitivity and resolution are very high when magnetic-field modulation is employed. In as-grown materials, cross relaxation reveals the N-donor in 4H and 6H SiC and the Al acceptor in 4H SiC (see TABLES 1 and 2). The linewidths are smaller than those observed by the ODMR of donor-acceptor pairs. 

Photoexcitation at /.e = 488 and 457.9 tim was accomplished by an Ar" " laser. An Oriel high-intensity high-pressure broadband Hg emission lamp provided the UV source. Nearly monochromatic 25 mW excitation at 353, 308, and 248 ntn was provided by 6 nm-bandwidth mirrors blazing at these wavelengths 5 mW excitation at 430, 406, and 380 nm was obtained by appropriate bandpass filters. As the ODMR signal-to-noise ratio was much poorer at these latter the microwave power was increased to up to 1400 mW. The resulting microwave field, however, was then sufficiently intense to broaden the narrow polaron resonance. Detailed descriptions of the visible- and UV-ODMR systems are given elsewhere [59,60]. 

In addition to the ODMR investigations of Rh +(4d )-chelates, recently similar studies have been performed for the Pd +(4d )-complexes, Pd(thpy)2 and Pd(qol)2 (with qol" = 8-hydroxyquinolinate) [80, 81]. Optical investigations of Pd(thpy)2 (with (thpy) 2,2 -thienylpyridinate, see Fig. 1) doped into an n-oc-tane Shpol skii matrix revealed highly resolved emission spectra and showed that the phosphorescent triplet state decays with three hfetime components of T] = 1200 ps, T]] = 235 ps, and rm = 130 ps characteristic of the triplet state sub-levels [82 - 84]. The emission data of Pd(qol)2 in an n-octane ShpoTskii matrix have been reported recently [81,85]. Two distinct emissive sites in the matrix were found,with electronic origins at 16,090 cm (77%) and 16,167 cm (23%), respectively. From the Zeeman splittings of the optical hne transitions in magnetic fields up to 12 T, the emission for the two sites was assigned as Tj Sg. 

Electron nuclear double resonance is a powerful tool for the study of the electronic structure of triplet states because of its high precision. ENDOR linewidths can be as narrow as 10 kHz, which represents an increase in resolution of better than six orders of magnitude over that which can be obtained optically. The technique is particularly useful when combined with hf methods owing to the first-order nature of the hyperfine interaction in the presence of a field. Although such experiments are difficult, the information obtained is unique. Accordingly, the hf EPR (or ODMR) spectrometer has been modified for ENDOR operation in several laboratories. In order to illustrate the power of the method, we discuss here some recent optically detected hf ENDOR experiments on (njr ) benzophenone and its iso-topically labeled derivatives (Brode and Pratt, 1977, 1978a,b). The results, although incomplete, show considerable promise for the ultimate determination of the complete spin distribution in this prototype triplet state. 

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