Magnetic Resonance and Rapid Passage in Irradiated LiF

The experimental result of Portis that at room temperature the paramagnetic dispersion signal (dχₘ^'dH) of radiation-induced paramagnetic centers in LiF has the shape of an undifferentiated Gaussian curve and lags the modulation field by 90 degrees has been confirmed. His theory, explaining this result as a manifestation of rapid-passage behavior, has been tested over a range of experimental parameters and appears valid. In terms of this theory we found that the spin-lattice relaxation time T₁=1.5×10^-4 sec at room temperature.The prediction has been made by Portis that, if ωₘT₁1, where ωₘ is the modulation frequency, the rapid-passage signal should lag the modulation field by 180 degrees. A lag of 130 degrees has been found at 4^∘{}K in LiF, using a modulation frequency of 23 cps, and the phase appears to approach a constant value. The same phase shift (130^∘{}) can be found at room temperature by using 400-cps modulation and can be interpreted as indicating that T₁=2.6×10^-3 sec at 4^∘{}K. It seems more likely, however, that at 4^∘{}K the assumption made by Portis that T₂T₁ fails, and that spin-spin effects make the situation more complex.An argument is presented that the magnetic resonance arises from the F center. From its shape we have obtained a value for the hyperfine splitting: h_γ=32±1 gauss. This result is compared with those of other workers who have reported resolved structure in LiF, and the conclusion is reached that the resolved structure resonances probably do not arise from the F center.