Theory of linear response of a system in a strongly driven state with application to double resonance

A quantum-statistical theory for the nonequilibrium state of a system subjected to a strong external perturbation is derived with the method of the time-dependent generalized canonical statistical operator. Expressions for the response of the nonequilibrium system to a second perturbation are then given. The theory is applied to the problem of double resonance in a system with two different spin species.