Theory and computation of the rate of multiphoton two-electron ionization via the direct mechanism

This paper discusses aspects of the physics and the computation of rates of multiphoton two-electron ionization of polyelectronic atoms within a nonperturbative, time-independent framework. A fundamental characteristic of the theory is that the physically significant features of the spectrum, of electronic structure and of free-electron channels enter systematically in an N-electron field-dressed resonance trial wavefunction. This many-electron, many-photon theory produces the rate of a particular field-induced process as the imaginary part of a frequency- and intensity-dependent complex eigenvalue obtained from the solution of a suitably constructed non-Hermitian Hamiltonian matrix. The notion of direct two-electron ionization is expressed in terms of a specific form of the trial wavefunction, which consists of configurations with real and complex square-integrable functions, subject to orthogonality constraints so as to exclude the participation of single-ionization channels, assumed to contribute mainly to the sequential path. The applications were done to the two-electron ejection from He by the direct absorption of two linearly polarized photons (photon energy in the range 35.0-55.0 eV) and to H- from. the direct and the sequential absorption of four, three, two and one photons (photon energy in the range 4.08-15.00 eV). The comparison between the rates of the two paths in H-, for photon energies 7.18-10.5 eV, shows that the direct rate dominates. We also show that in the orbital Hartree-Fock approximation to the initial state, the frequency-dependent rates at the intensity of 1 x 10(13) W cm(-2) differ from those obtained with a correlated wavefunction by about two orders of magnitude.