Quasi-classical description of nonlinear interaction of negative alkali ions with a light field

The two-electron wavefunction for negative alkali ions is analysed. It follows that the states of weakly bound and external optical electrons are not equivalent. For a description of the nonlinear interaction of negative alkali ions with a light field this fact makes it possible to apply a quasi-classical approximation, from which the analytic expressions for the dynamic polarisability of the negative ion and for the multiphoton photodetachment cross sections are obtained.