Rotating-wave approximation for the interaction of a pulsed laser with a two-level system possessing permanent dipole moments

A rotating-wave approximation (RWA) is developed to describe the evolution of a two-level system, which has permanent dipole moments, interacting with a pulsed laser. Comparisons with exact calculations for one- and two-photon excitations involving the two lowest vibrational states of the ground electronic state of HeH+ are given to illustrate the validity of the RWA formula.