Analysis of Microwave Fast-Passage Effects in Optically Detected Magnetic Resonance Spectra
✍ Scribed by Jie Q. Wu; Andrzej Ozarowski; August H. Maki
- Publisher
- Elsevier Science
- Year
- 1996
- Tongue
- English
- Weight
- 169 KB
- Volume
- 119
- Category
- Article
- ISSN
- 1064-1858
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✦ Synopsis
Optically detected magnetic resonance (ODMR) transitions of lattice relaxation, SLR, is effectively quenched, and the popphoto-excited triplet states in zero magnetic field usually are not ulations of the magnetic sublevels are governed by the kinetmeasured under slow-passage conditions. At a typical temperature ics of optical pumping and decay to the electronic ground of about 1.2 K, spin-lattice relaxation is largely quenched; return state. The reestablishment of the initial stationary-state subto steady-state sublevel populations occurs primarily by electronic level populations after they have been modified by microdecay and optical pumping via the ground state. This process is wave saturation relies on these processes rather than on SLR often slower than the microwave passage time through the homoas it does in ordinary EPR. The correct ''slow-passage'' geneous line, leading to fast-passage effects observable in the inho-ODMR lineshape of a homogeneously broadened line is only mogeneously broadened ODMR band. These include a sweep-rateachieved in the limit of an infinitely slow microwave sweep dependent shift of the peak frequency and distortion of the band rate, but an acceptable approximation of the lineshape is shape. A mathematical model that predicts the spectrum of phosphorescence-detected ODMR as a function of microwave sweep nonetheless achieved provided that rate is presented. The independent variables are band center, band shape and width, sublevel decay constants, and radiative decaydn/(dn/dt) ӷ t i , [1] constant ratio. The model is applied to the ÉDÉ 0 ÉEÉ, and 2ÉEÉ transitions of tryptophan in lysozyme, and to the 4.36 GHz transition of 1-methyl-2-thiouracil. Nonlinear least-squares fitting of the where t i is the largest sublevel lifetime, i Å x, y, or z, dn spectra to the model produces consistent band center frequencies is the homogeneous linewidth, and dn/dt is the microwave and widths, independent of microwave sweep rate. The reliability sweep rate. of a decay constant increases with its radiative quantum yield.
A large number of ODMR studies have been carried out The peak frequency shift with sweep rate is very nonlinear; its on biological samples such as proteins, whose chromophores determination by extrapolation to zero rate is not reliable. ᭧ 1996 are subjected to inhomogeneous broadening of both the opti-Academic Press, Inc.
cal spectra and the ODMR spectra (3-6). These are the result of heterogeneity of the microenvironments that are frozen out at cryogenic temperatures. Microwave hole-burn-82