Previous investigations have shown that the spin-lattice relaxation time T.^ is eqfeal to the inverse line width 1 T 2 and is independent of temperature in the range 77°K to room temperature in a crystalline free radical with a highly exchange-narrowed line. 2 We have found departures from this behavior in T x for several free radicals between 1.5°K and room temperature using the saturation technique.The measurements were made at 10 kMc/sec and 24 kMc/sec with a conventional magic tee microwave bridge. The solid free radicals used were a,a-diphenyl 0-picryl hydrazyl (DPPH), a,y-bisdiphenylene /3-phenyl allyl (BDPA), Wurster's blue perchlorate, and some solid solutions of DPPH in polystyrene. In all of these free radicals the exchange interaction between neighboring spins narrows the resonance line 3 in accordance with the approximate expression 4 * 5 Aco^co^Vug, where Aco is the observed line width, u>p is the dipolar frequency, and OJ E is the exchange frequency. From our observed line widths and calculations of the dipole width based on Van Vleck's second moment expression, 3 we estimate the exchange frequency to lie between 10 and 40 kMc/sec. In addition, we have investigated solid solutions of DPPH in polystyrene, where the exchange frequency varied from effectively zero to about 10 kMc/sec. DPPH samples recrystallized from different solvents were found to have different line widths as reported by Lothe and Eia. 6 This is because the solvent molecules influence the free radical
Measurements of the hyperfine structure of alkali metals trapped in solid rare gases have been made. When the zero-field hyperfine splitting values obtained in the solid matrix are compared with the free atomic values, it is found that they fall into two groups. In one group the hyperfine values are within a few percent of the free atomic value. The other group has hyperfine values smaller than the free atomic value by 10 to 30%. It has been found that the latter trapping sites occur only when the sample is exposed to room temperture infrared radiation during deposition on the liquid helium cooled finger. The effects of annealing and concentration have also been studied. The annealing experiments show that once the shifted sites are formed they are very stable.
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