Rotating-frame transformations: A new approximation for multiphoton absorption and dissociation in laser fields

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Abstract
The time-dependent Schrödinger equation for the coherent interaction of a multilevel molecular system with an intense electromagnetic field is transformed with a new rotating-frame transformation into a frequency-dependent representation in which much of the information relevant to multiphoton excitation is contained in a time-independent interaction matrix, the solution of which follows by standard eigenvalue techniques. The flexibility of this transformation and simplicity of the resulting approximation (the general rotating-frame approximation) make it a powerful tool for solving the multiphoton dynamics of many-level systems. Extension to interactions with several laser fields is straightforward. The theory is illustrated by application to the model of Schek, Jortner, and Sage [Chem. Phys. 59, 11 (1981)] for the multiphoton dissociation of a diatomic molecule. The results from the general rotating-frame approximation are in good agreement with the exact results obtained by numerical integration of the full time-dependent Schrödinger equation. The implications for treatment of multiphoton absorption and dissociation of polyatomic molecules are discussed.