Computation of multiphoton IR spectra of small molecules: Application to CO

A non-perturbative theoretical method has been generalised to compute rovibrational multiphoton excitation probabilities in small molecules. The practicability of the method is illustrated by direct computation for CO. Novel non-linear features emerge in the absorption spectrum when the power broadening of individual rovibrational lines becomes comparable to the rotational constant of the molecule. The results also establish that over-simplified N-level models should be treated with caution.