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

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.