Electron Spin Resonance of Atomic and Molecular Free Radicals Trapped at Liquid Helium Temperature

Electron spin resonance spectra of H, D, N, and CH₃ trapped in solid matrices at liquid helium temperature have been observed and interpreted. The effect of the matrix field on the resonance properties of the radicals has been investigated by depositing the radicals in matrices with different binding energies. The effect of the matrix on the g factor is extremely small in all cases. The deviation of the hyperfine coupling constant from the free-state value increases in a systematic way with increase in binding energy of the matrix, the percentage deviations being small for H, D, and CH₃ but rather large for the case of N. The widths and shapes of the spectral lines are discussed in terms of dipolar broadening, spin-lattice relaxation, anisotropic broadening, rate of passage and the modulation parameters used for observation. Complex spectra, not adequately identified, have been observed from discharges in hydrogen and hydrogen-oxygen systems. Deductive evidence for an HO₂ resonance spectrum is presented. The stable molecular free radicals O₂, NO, and NO₂ have been studied. Only NO₂ yielded a positive result. Resonances for oxygen and chlorine atoms have been sought but not observed. It is suggested that radical species with orbital angular momenta may escape spin resonance observation because of matrix field anisotropy and that radical species with an even number of electrons may be unobservable because of crystalline field splitting resulting in a singlet ground level.