Radiative distortion of the hydrogen atom in superintense, high-frequency fields of linear polarization

We study the structure of the hydrogen atom when placed in a high-frequency, superintense laser field, within the framework of a nonperturbative theory recently developed for this purpose. The theory predicts that in the high-frequency limit the atom is stable against decay by multiphoton ionization, and that its structure is determined by a time-independent Schrödinger equation containing a ``dressed'' Coulomb potential. The laser frequency ω and the intensity I enter only combined in the parameter α₀=I^1/2ω^-2 a.u. We first analyze the symmetry of the eigenvalue problem for the case of linear polarization under consideration and adopt an appropriate classification scheme for the levels. The small-α₀ limit of the levels is obtained analytically. In the large-α₀ limit scaling laws are derived for the α₀ dependence of the eigenvalues and eigenfunctions. At finite α₀ we have carried out a very accurate numerical computation over an extended range of α₀ values (0<=α₀<=200 a.u.) for a number of symmetry manifolds, by diagonalization of the Hamiltonian in a Gaussian basis. The correlation diagrams relating the small- and large-α₀ limits exhibit several avoided crossings. The binding energies show an overall decrease with α₀, in some cases preceded by an increase through a maximum. For the ground state this decrease is quite steep. The extreme distortion of the atomic structure accompanying it is studied. It is shown that, with increasing α₀, the (oscillating) electronic cloud undergoes radiative stretching, which eventually culminates at large α₀ in its splitting into two parts (dichotomy). The consequences of our findings for the experimental energy spectrum of the ejected electrons are considered.