Multiphoton One-step Ionization of Helium at 1064 nm

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Abstract
The multiphoton ionization of helium by a strong radiation field having a wavelength of 1064 nm and a field intensity up to 5 × 1014 W cm −2 is considered within a non-perturbative treatment partially accounting for the electron-electron correlations in the initial bound state and approximately for the Coulomb interaction between the residual ion and the ejected electron in the final state. Only single-electron transitions are considered, the main emphasis being on the field polarization effects. The obtained results are compared with the few results available in the literature in the same regime as well as with very recent experimental findings. In particular, the calculated photoelectron energy spectra, angular distributions and ionization rates are found to reproduce satisfactorily all the main features observed in the experiments. The proposed formalism appears to be a particularly flexible method for studying field polarization effects; for the channel-by-channel analysis in the case of strong fields; and for multiphoton processes of very large order. Potential development and improvement of the technique is discussed.