Kinetic theory of gas phase microwave experiments. I. A two level approach

The response of a polyatomic gas to microwave radiation—including both steady state (pressure broadening) and time dependent (coherence transients) effects—is described theoretically. The treatment is based on solutions of a quantum Boltzmann equation and employs kinetic theory methods which have previously been used in the explanation of the field dependence of transport phenomena (Senftleben–Beenakker effects). Such methods allow for a proper description of the rotational aspects of a real molecular system, in contrast to standard approaches which effectively model the molecular system as having only two nondegenerate energy states. In the present work the magnetic quantum number degeneracies of two rotational levels connected by microwave radiation are specifically considered, and the resulting vector (and tensor) nature of the motions is emphasized throughout. The relaxation processes involved are related to matrix elements of the (rotationally invariant) collision superoperator occuring in the quantum Boltzmann equation.