Optically detected magnetic resonance spectra

Fig. 9. ODMR investigations at T = 1.4 K of Pd(2-thpy)2 dissolved in an n-octane Shpol skii matrix. Concentration = 10 mol/1 cw excitation Ag c = 330 nm (30.3 x 10 cm 0- Detection of the emission at 18418 cm (Tj —> Sq transition), (a) Zero-field ODMR (optically detected magnetic resonance) spectrum (b) Zero-field microwave recovery ODMR signal after pulsed microwave excitation with a microwave frequency of 2886 MHz. The best fit of the recovery signal is obtained with Eq. (4). (Compare Ref. [61])...

The luminescence properties of siloxene have now been studied in great detail. A typical siloxene PL spectrum is shown at the bottom of Figure 15.4 (b). It has a maximum in the yellow-green spectral range at around 2.4 eV. At low temperatures, a radiative lifetime of 10 ns and a polarization memory were observed. These properties and the small Stokes shift between the photoluminescence and its excitation spectra provide experimental evidence for the hypothesis that siloxene does indeed have a direct band gap. Details of the excited states in siloxene leading to the luminescence have also been obtained from measurements of optically detected magnetic resonance... 

Fig. 10. Luminescence spectrum and spectral dependence of (A///)esr at resonances of the Dj and A centers at 2 ° K in a-Si H,F No. 813 (7, — 250 C). Each value in the ordinate is plotted in arbitrary units. [Reprinted with permission from Solid State Communications, Vol. 43, K. Morigaki, Y. Sano, I. Hirabayashi, M. Konagai, and M. Suzuki, Level of dangling-bond centers and its broadening due to disorder in amorphous silicon as elucidated by optically detected magnetic resonance measurements. Copyright 1982, Pergamon Press, Ltd.]...

Double-resonance Spectroscopy.—A review has been given of double-resonance methods in spectroscopy.378 Attention will be focused here on optically (usually phosphorescence) detected magnetic resonance experiments (ODMR). Microwave-optical double-resonance experiments have been carried out on the spectrum of gaseous N02,379 permitting assignment of the rotational = 0—4 side-bands of the 493 nm band. 

Optical detection of magnetic resonance (ODMR) was attempted for measurements of the pH effects on the triplet state of purine to investigate the protonation site of purine at low temperatures (78JA7131). The ODMR spectrum did not show the presence of more than one triplet state at liquid helium temperatures. Since the protonated tautomers 1H,9H (3a) and H,1H (3b) have similar bond structures, their triplets should have similar zero-field parameters and are thus not easy to distinguish by ODMR. 

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. 

One other pyridine alkaloid has been detected in dendrobatid frogs. The structure of this minor alkaloid, noranabasamine (XIII), was established by proton and carbon-13 magnetic resonance spectroscopy (14). The ultraviolet spectrum was as follows X ,ax (CH3OH) 244 nm, e 11,000, 275 nm, e 10,000. The optical rotation, [a]o, was -14.4° (CH3OH). Anabasamine, a plant alkaloid, also is levorotatory, but it is unknown whether noranabasamine, now given a code number 239J, has the same 2S configuration. 

Slow-passage ODMR signals frequently are observed by the continuous wave method in which the optical effect is monitored using broadband detection. On the other hand, if the triplet state decay constants are sufficiently large, the microwave power may be amplitude modulated at an audio frequency which results in modulated phosphorescence when the microwave frequency is at resonance. The phosphorescence is then monitored with narrow-band phase-sensitive detection, for a great improvement in the signal/noise ratio. The latter detection method is frequently used to produce a magnetic resonance-induced phosphorescence spectrum by a technique referred to as phosphorescence-microwave double resonance (PMDR). The microwave frequency is fixed at resonance,... 

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