The spin density matrix for slowly reorienting molecules in zero field

The time evolution of the spin density matrix of a spin embedded in a slowly moving molecule is investigated using Gordon's extended diffusion model for molecular reorientation and allowing the spin to follow the molecular motion. The results for this model are identical with those previously obtained using a jump diffusional model and do not give the adiabatic limit in slow motion. This model is valid provided the molecular angular velocity correlation time is short compared to the spin precession period. The perturbed angular correlation of rays or the spin resonance line shape can be calculated for any value of the rotational diffusion constant by diagonalizing a finite matrix. The necessary matrix elements are given for the 247 keV state of ¹¹¹Cd (I = 5/2).