Channel closing and power-law scaling in multiphoton ionization

Abstract

We present numerical data giving the intensity dependence of the multiphoton ionization rate for two different model atoms. As the intensity increases, the pon-deromotive shift causes peaks in the electron energy spectrum to disappear, one by one, at the threshold. We carefully examine the ionization rate near to and between the intensities at which these ponderomotive channel closings occur. The data have been obtained from ab initio wave functions calculated for 1-D atoms in the presence of intense laser fields. In the first model the potential V ( x ) = − 1 / 1 + x 2 is used to simulate the one-electron binding potential of a typical atom, and in the second model the potential V ( x ) = − exp ( − | x | ) / x 2 + α 2 is used to simulate the binding potential of the extra electron in the H– negative ion. The first potential is quasicoulombic1 with long tails at large ±x and supports a Rydberg series of bound levels near the continuum limit, while the second potential is short range in nature and supports only a single bound state. Both potentials are enclosed in a box (terminated at x = ±L by rigid walls) that is large enough to support a relatively dense set of positive energy states.