On the effects of absolute laser phase on the interaction of a pulsed laser with polar versus non-polar molecules

Abstract

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 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 e⁰. When the laser-molecule coupling parameters b = m₁₂ e⁰ /E₂₁ > 0.2, where m ₁₂ 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 mlecular parameters associated with a S₀ to S₁ electronic transition in a dipolar molecule. However, they are presented in reduced form and therefore can be scaled to 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 ofa 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.

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