Electron spin relaxation of triarylmethyl radicals in fluid solution.

Electron spin relaxation times of a Nycomed triarylmethyl radical (sym-trityl) in water, 1:1 water:glycerol, and 1:9 water:glycerol were measured at L-band, S-band, and X-band by pulsed EPR methods. In H(2)O solution, T(1) is 17+/-1 micros at X-band at ambient temperature, is nearly independent of microwave frequency, and exhibits little dependence on viscosity. The temperature dependence of T(1) in 1:1 water:glycerol is characteristic of domination by a Raman process between 20 and 80 K. The increased spin-lattice relaxation rates at higher temperatures, including room temperature, are attributed to a local vibrational mode that modulates spin-orbit coupling. In H(2)O solution, T(2) is 11+/-1 micros at X-band, increasing to 13+/-1 micros at L-band. For more viscous solvent mixtures, T(2) is much shorter than T(1) and weakly frequency dependent, which indicates that incomplete motional averaging of hyperfine anisotropy makes a significant contribution to T(2). In water and 1:1 water:glycerol solutions continuous wave EPR linewidths are not relaxation determined, but become relaxation determined in the higher viscosity 1:9 water:glycerol solutions. The Lorentzian component of the 250-MHz linewidths as a function of viscosity is in good agreement with T(2)-determined contributions to the linewidths at higher frequencies.

An L-band crossed-loop (Bimodal) EPR resonator.

Our crossed-loop resonator design has been enhanced to increase the filling factor and has been extended from S-band to L-band. High isolation between the two modes results in shorter dead time in pulsed EPR experiments than would occur with a reflection resonator of the same Q.

Absolute EPR spin echo and noise intensities.

EPR signal and noise, calculated from first principles, are compared with measured values of signal and noise on an S-band (ca. 2.7 GHz) EPR spectrometer for which all relevant gains and losses have been measured. Agreement is within the uncertainty of the calculations and the measurements. The calculational model that provided the good agreement is used to suggest approaches to optimizing spectrometer design.

Frequency dependence of EPR signal-to-noise.

Direct measurements of electron spin-echo signal and noise in well-characterized X-band and S-band spectrometers agree with predictions of frequency dependence based on first principles. For the particular spectrometers compared, the echo at 9.52 GHz was 9.5 times larger than the echo at 2.68 GHz, after scaling for differences in spectrometer gain. The calculated ratio was 7.6. This result contrasts with prior predictions that the frequency dependence would be much greater.

Multifrequency electron paramagnetic resonance of irradiated L-alanine.

The radical generated by gamma-irradiation of crystalline L-alanine was examined by continuous wave (CW) and pulsed electron paramagnetic resonance (EPR) at 1.8, 3.2, 4.9, 9.1 and 19.4 GHz. The spin-flip satellite lines that make a prominent contribution to the saturated spectra at 9.1 GHz are less conspicuous at lower frequencies because of overlap with the allowed transitions. The spin-lattice relaxation times measured by long-pulse saturation recovery and phase memory times measured by electron spin echo increase with increasing microwave frequency.

An infrared system for determining ocular position.

Bioengineering research in the Electronics Division of the Denver Research Institute has centered around the development of an ocular controlled communications device for the severely handicapped. The means for accurate determination of eye position for use in communication/control applications have been investigated by various groups for at least ten years. A highly satisfactory device for eye tracking has been developed in our laboratories and is currently undergoing clinical evaluation as part of a communications system. An infrared LED is mounted on the nose pad of an ordinary pair of eyeglass frames. This LED floods the cornea with light after reflection from the inside surface of an unground eyeglass lens in the frames. The cornea acts as a convex mirror and reflects the light (via the inside surface of the eyeglass (lens) into an image transducer mounted on the bow of the frames. The image transducer is a 32 x 32 cell (1k) dynamic RAM. Utilizing TTL scanning circuitry, the position of the eye can be determined from the address of the illuminated cell of the RAM.