On the competition between permanent dipole and virtual state two-photon excitation mechanisms, and two-photon optical excitation pathways, in molecular excitation

The control of an excited state molecular population, using two lasers and competitions involving different laser excitation pathways and different excitation mechanisms of the same photonicity, is discussed. One of the mechanisms requires a non-zero permanent dipole difference between the initial and final states, the other requires a virtual state. Analytical and numerical results for the orientationally averaged population of the final excited state, for a three-level model molecule, are used to help discuss the control of the final state population via changes in the relative phase, or in the angle between the polarization vectors, of the two lasers.