ENDOR optically detected

The new techniques of phosphorescence-microwave multiplet resonance spectroscopy with optical detection have been reviewed by El-Sayed and Kwiram Such exciting experiments as the optical detection on electron-nuclear double resonance (ENDOR) and of electron-electron double resonance (EEDOR) in zero magnetic field have been achieved, and it is certain that much detailed knowledge concerning the phosphorescent states will evolve from this field. 

EPR but provides greatly enhanced resolution. Double resonance techniques (e.g. electron nuclear double resonance (ENDOR) and Overhauser shift measurements) combine the sensitivity of EPR with the resolution of NMR. Many such measurements on thin films are performed by combining optical detection with ENDOR, greatly enhancing the resolution of ODMR and taking advantage of its superior sensitivity. 

Electron nuclear double resonance (ENDOR) experiments have been performed by Koschnick and co-workers [59] and by Glaser and coworkers [60,61], with both groups employing optical detection of the EPR (i.e. ODENDOR). 

FIGURE 4 The optically detected Ga and71 Ga ENDOR on the EM donor line of a GaN film. The quadrupolar splitting of the 69Ga line is indicated. The microwave frequency was 24 GHz and the temperature was 1.6 K. The PL used in the measurements was dominated by the 2.2 eV band. 

Optical Detection of Electron-Nuclear Double Resonance (ENDOR) Transitions in Zero-Field... 

It is thus obvious that the energies required to change the direction of the nuclear spin in the field of the electron spin can be determined optically in zero-field. This is an optically detected n.m.r.-type experiment whereby the laboratory field is replaced by the field of the two unpaired spins (zf) of the triplet state in the molecular framework. The ENDOR frequencies are important in determining hyperfine and quadrupole parameters in the excited triplet state (23,24,53,57,58). 

There are a variety of techniques for the determination of the various parameters of the spin-Hamiltonian. Often applied are Electron Paramagnetic or Spin Resonance (EPR, ESR), Electron Nuclear Double Resonance (ENDOR), Electron Electron Double Resonance (ELDOR), Nuclear Magnetic Resonance (NMR), occassionally utilizing effects of Chemically Induced Dynamic Nuclear Polarization (CIDNP), Optical Detection of Magnetic Resonance (ODMR), Atomic Beam Spectroscopy and Optical Spectroscopy. The extraction of the magnetic parameters from the spectra obtained by application of these and related techniques follows procedures which may in detail depend on the technique, the state of the sample (gaseous, liquid, unordered solid, ordered solid) and on spectral resolution. For particulars, the reader is referred to the general references (D). 

Knowledge of the magnetic (and optical) properties of triplet states has been greatly enhanced by the development of zero-field (zf) resonance techniques, especially those employing optical detection. In what follows, we review the selection rules which govern the transitions in the zf experiment. We then present recent results from this laboratory on the lowest (nTc ) states of 1-halonaphthalenes and discuss in some detail the analysis of these spectra and their significance with respect to the intramolecular heavy-atom effect on the properties of the parent molecule. Next, we survey some representative results from other laboratories, including zf EPR, ODMR, ENDOR, and ELDOR experiments, and close with a brief description of other zf applications. 

The double-resonance techniques of electron-nuclear double resonance (ENDOR) and electron-electron double resonance (ELDOR) have been used mainly for hf studies of organic triplet states, both via microwave and optical detection (see below). They have also been used for (optically detected) zf studies of triplet states, although to a more limited extent. A recent review of this latter work is available (Harris and Buckley, 1976). 

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. 

Figure 12 shows optically detected ENDOR spectra of the lowest triplet states of C-benzophenone-dio (A), C-benzophenone-dio (B)> and C-benzophenone-/i,o (C) in 4,4 -dibromodiphenylether, which were obtained by monitoring the low-field Awj = 1 transition with Hllz using a... 

It is apparent from these spectra that optically detected ENDOR spectroscopy in hf is both feasible and practical for H, C, and other nuclei. 

Fig. 13. Angular dependence of the measured C optically detected ENDOR frequency of - C-benzophenone-dio in the ah plane of 4,4 -dibromodiphenylether. The data shown are for the low-field Anis = 1 transition and refer to molecule 1. See text for explanation of data in insets (Erode and Pratt, 1977).

Optical pumping in solids also provides new possibilities in material research. Relaxation process can be studied and optical detection of NMR and ENDOR signals can be obtained [7.66]. 

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