Cluster ionization via two-plasmon excitation

We calculate the two-photon ionization of clusters for photon energies near the surface plasmon resonance. The plasmon is described in a schematic jellium-RPA model assuming a separable residual interaction between electrons determined so that the plasmon energy is reproduced. The ionization rate of a double plasmon excitation is calculated perturbatively. In Na-93(+) clusters we find an ionization rate of the order of at most 0.05-0.10 fs(-1). This rate is used to determine the ionization probability in an external field in terms of the number of absorbed photon pairs and the duration of the field. We discuss the dependence of the results on the choice of our empirical separable force. The number of emitted electrons per pair of absorbed photons is found to be small, in the range 10(-5) to 10(-3).