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The harmonic two-colour excitation of a two-level molecule, where the frequencies of the two continuous wave lasers are multiples of a certain frequency, is studied in the rotating wave approximation (RWA) and by using exact methods. Included are the effects of a non-zero different *d* between the permanent dipole moments of the two states involved in the transition. For two independent frequencies, the two-colour RWA yields analytical results only when one two-colour resonance dominates the transition, while for the harmonic analogue, analytic solutions, exhibiting the effects of both laser and molecular parameters, are available for problems involving competing two-colour resonances. Analytical solutions for both the time-dependent populations of the excited state, and for the associated resonance profiles, are derived, applied to a model two-level molecule, and tested by comparison with exact results obtained using Floquet techniques. The results are used to discuss the phase control of molecular excitation through the interplay of competing resonance involving the effects of *d* ≠ 0; the competition vanishes if *d* = 0. Both fixed molecule-laser configurations, and the effects of orientational averaging, are considered.

The effects of permanent dipole moments and those due to the randomness of molecular orientation in the phase control of molecular excitation are discussed for the simultaneous one- and three-photon excitation of a two-level model molecule. In this transition scheme both transitions can occur with or without the presence of permanent dipoles and the results are contrasted to those corresponding to the one- and two-photon excitation of a two-level molecule, which requires the presence of permanent dipoles. The dependence of the temporal evolution of the excited state and the associated resonance profiles on the relative phase of the lasers is used to monitor the control of the excitation process. Analytical perturbation theory, the rotating-wave approximation, and exact Floquet results for those observables are used for this purpose. Both fixed molecule-laser configurations and situations where the absorbing molecules assume random orientations with respect to the laser beams are considered.

The absolute laser phase dependence of the time-dependent populations of the molecular states, including the steady-state (long time) populations of the states, associated with the interaction of a molecule with a pulsed laser is investigated using illustrative two-level examples. One-photon transitions, including the effects of permanent dipoles, are discussed as a function of the pulse duration, intensity, and (absolute) laser phase, for selected laser frequencies. The effects of laser phase can be large, depending on the values of the pulse duration for a given frequency and intensity. The effects of permanent dipoles, relative to no permanent dipoles, are significant for large laser field strengths ε0. When the laser-molecule coupling parameter b=μ12ε0/E21⩾0.2, where μ12 and E21 are the transition dipole and energy difference between the ground and excited states, respectively, the dynamics of the pulse-molecule interaction are (strongly) phase dependent, independent of pulse duration, whereas the corresponding steady-state populations of the molecular states may or may not be phase-dependent depending on the pulse duration. Analytical rotating wave approximations for pulsed laser-molecule interactions are useful for interpreting the dynamics and the steady-state results as a function of field strength and pulse duration, including the effects of permanent dipole moments. The results reported in this paper are based on molecular parameters associated with an S0→S1 electronic transition in a dipolar molecule. However, they are presented in reduced form and therefore can be scaled to other regions of the electromagnetic spectrum. Short, intense pulses at or beyond the limits of current laser technology will often be required for the types of absolute laser phase effects of this paper to be appreciable for electronic excitations. The discussion, in the UV-VIS, also suffers from the use of a two-level model and from the requirement of field intensities that can be beyond the Keldysh limit. For other spectral regions, these absolute laser phase effects will be much more readily applicable.

Adiabatic potential energy surfaces of the three lowest lying singlet states, X̃ 1A‘ 2 1A‘, and 11A‘ ‘, of N2O have been computed as a function of the *R*N2-O bond distance and the Jacobi angle. The calculations are performed using the complete-active-space self-consistent field (CASSCF) and the multireference configuration interaction (MRCI) electronic structure methods. It is shown that there is a wide avoided crossing between the ground, X̃ 1A‘, and lowest excited, 2 1A‘, electronic state. This avoided crossing is thought to give rise to a seam of conical intersection at other N−N separations. Both excited state surfaces display important conical intersections at linear geometries. The transition dipole moment surfaces for the two excitation processes (2 1A‘ ← X̃ 1A‘ and 1 1A‘ ‘ ← X̃ 1A‘) are also presented. These calculations provide the basic data needed to compute the dynamics of the N2O + *h*ν → N2 + O(1D) photodissociation process for photon frequencies in the range 5.2 eV (240 nm) to 7.3 eV (170 nm).