Complex rotational motion of CH3 in solid CO as found by EPR

High-resolution CW EPR spectra of CH3 radicals trapped in solid CO matrix were obtained in a temperature range 7-32 K. The radicals were stabilized by simultaneous condensation onto a low temperature substrate of two gaseous flows: products of the discharge in a CH4-He mixture and pure CO. Both the A-symmetry quartet and E-symmetry doublet which are mainly from J=0 and J=1 rotational states, respectively, were found to contribute to the experimental EPR spectrum in the whole temperature region. Anisotropic splitting of the E-symmetry hyperfine lines was observed for the first time. Using simulation procedure, axially symmetrical g- and A-tensors were measured for both the quartet and doublet. Thorough analysis of the EPR spectrum revealed different coupling of the A- and E-symmetry rotational states of the CH3 radical to the matrix surrounding. The temperature dependent linewidth tensor was found to be axially anisotropic for the E-line doublet while isotropic for the A-line quartet. A complex model of the rotational motion of the CH3 radical trapped in gas matrices of linear molecules was suggested.