Our previous model of two-frequency above-threshold ionization1,2 is extended to include dipole coupling between an infinite number of continua. The continuum-continuum (C-C) matrix elements are chosen from hydrogenic atom results, and the model is analytically soluble without recourse to the rotating-wave approximation.
We solve a model with one bound state and two continua.1 Two lasers are used to study the above threshold ionization process. The first laser couples the electron in a bound state to a continuum state. The photons are assumed to be energetic enough to promote the electron directly into the continuum far from the threshold; this avoids complications due to the Rydberg levels and the ponderomotive shifts of the threshold. The second laser mixes the electron wavefunction between two continuum states (C—C transitions); since we avoid the rotating wave approximation, multiple transitions between these continua are allowed. We find that the ionization rate is unaffected by the C—C transitions. Our model of hydrogenic atoms allows redistribution among the continua only during the ionization process and the photoelectron spectra exhibits peak suppression; i.e., the population is not the strongest at the energy of the ionization laser.
We present a discussion of the two-color above-threshold ionization process, performed with the aim of studying strong-field continuum–continuum transitions. We extend the approach of Rzażewski [ J. Opt. Soc. Am. B7, 481 ( 1990)] to obtain analytically solvable equations for the multiple continuum model analyzed by Deng Eberly [ J. Opt. Soc. Am. B2, 486 ( 1985)]. Some comments on the effect of continuum–continuum transitions on the line shape of a laser-induced continuum structure are also made.
We compute numerically the scattered light spectra accompanying MPI of a very simple atomic model. The model atom is similar to the hydrogen negative ion and has a single bound state and N + 1 continua (corresponding to angular momenta L = 0,…,N). The model is designed to focus attention on two parameters, a single representative dipole matrix element between the ground state and continuum states and a single dipole matrix element between continuum states. Each continua is modeled as a flat quasicontinuum. The bound state is coupled to any level in the L = 1 continuum with the same rate. Any level in the Lth continuum is coupled to any other level in the (L–1) st and (L+1)st continua with equal rates. We have calculated numerically the response of this atom to a laser pulse without introducing the rotating wave approximation.
A model of atomic ionization by smooth pulses is solved with bound-continuum and energy-dependent continuum-continuum (C-C) dipole coupling. The rotating-wave approximation for the C-C transitions is avoided. In contrast with the flat continuum result, our ionization rate is independent of the C-C coupling. We also derive the photoelectron energy spectra and show that counterrotating terms make important contributions. Relation to above-threshold ionization experiments with two frequencies is discussed.