Role of the initial-state configuration in multiphoton ionisation of helium

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Abstract
The role of the initial electronic configuration in the multiphoton ionisation of two-electron atoms is studied by considering three different wavefunctions to describe the ground state of helium. The theoretical treatment of the multiphoton process is an extension to a two-electron problem of the non-perturbative Keldysh model. Though the initial electronic charge distributions predicted by the chosen wavefunctions are largely similar, the ionisation rates provided by them are found to differ by orders of magnitude when the number of photons required to ionise increases. Accordingly, an accurate description of the initial electronic configuration in a process which is highly non-linear is found necessary. Besides, as a process involving a large number of photons may occur only in the presence of a sufficiently strong Coulomb field, it is argued that a multiphoton process may be used as a probe of the electronic correlation in atoms.