REGULAR AND CHAOTIC MULTIPHOTON DISSOCIATION

The rate of multiphoton dissociation of the BeH2+ molecule in its grou nd and first two excited vibrational levels has been computed via clas sical mechanics as a function of laser frequency. There is agreement w ith earlier quantum-mechanical results as regards the existence and ma gnitude of an optimal frequency, omega, for which the dissociation ra te is maximized. This fact has been analyzed and understood via the ap plication of the theory of chaotic scattering. Indeed, we find fractal singularities in the function T-d(x) Of the duration of photodissocia tion, and we compute their dimension to be equal to 1, in agreement wi th the conjecture of Lan, Finn, and Ott [Phys. Rev. Lett. 66, 978 (199 1)] that this must be a characteristic of systems exhibiting nonhyberb olic scattering. Turning to the problem of interpreting the appearance of an optimal omega, we propose the following two mechanisms for the reduction of the multiphoton dissociation rate. First is the increase of fractal singularities when the frequency omega attains values larg er than omega. Second is the gradually increasing overlap of the clas sical initial state with the region of Kolmogorov-Arnold-Moser tori wh en omega<omega. Finally, as the intensity is increased there is a tra nsition from chaotic to regular photodissociation, where the singulari ties in T-d(x) are finite. It is conjectured that this reflects the em ergence of the quantum-mechanical phenomenon of above-threshold dissoc iation.