Stochastic Liouville Equation for Weakly Driven System. II ---Application to a Quantal Spin---

The time-convolutionless stochastic equation derived in a previouspaper is applied to a quantal spin interacting weakly with itssurroundings under decoupled initial condition, up to second order inpowers of random perturbations. The transverse and longitudinalsusceptibilities are derived from the stationary solutions. Thetransverse and longitudinal spectra are displayed and are comparedwith the ones derived from the time-convolution equation in theconventional Markoffian approximation, and with the ones by the methodof second-order spin relaxation theory and Kubo's theory. It is shownthat the line shape of transverse spectrum is enhanced over theLorentzian at the resonance region, due to the effects of interferenceof the external driving field and the random interaction.