INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS B: ATOMIC, MOLECULAR AND OPTICAL PHYSICS J. Phys. B: At. Mol. Opt. Phys. 34 (2001) 857–867 www.iop.org/Journals/jb PII: S0953-4075(01)19630-6 Ionization of H - by a strong ultrashort laser pulse G Lagmago Kamta 1 , T Grosges 2 , B Piraux 2 , R Hasbani 3 , E Cormier 3 and H Bachau 3 1 Department of Physics and Astronomy, The University of Nebraska, 116 Brace Laboratory, Lincoln, NE 68588-0111, USA 2 Laboratoire de Physique Atomique et Mol´ eculaire, Universit´ e Catholique de Louvain, 2, Chemin du Cyclotron B-1348 Louvain-la-Neuve, Belgium 3 Centre Lasers Intenses et Applications (UMR5107 du CNRS), Universit´ e de Bordeaux I, 351 Cours de la Lib´ eration, F-33405 Talence Cedex, France Received 30 November 2000 Abstract We compare the outcome of two different numerical methods aimed at solving the time-dependent Schr ¨ odinger equation associated with the interaction of H - with an ultrashort laser pulse. These methods of spectral and configuration interaction type are based on an expansion of the total wavefunction on eigenstates of H - built as products of either B -spline or complex Sturmian functions. A careful analysis of our results together with a comparison with other existing theoretical data sheds some light on subtle aspects of the theoretical treatments of H - in a strong laser field. A particular emphasis is put on the crucial role played by the density of states in the continua. 1. Introduction The behaviour of a two-active electron system exposed to a strong and ultrashort laser pulse remains a challenging theoretical problem which requires non-perturbative solution of the time- dependent Schr ¨ odinger equation (TDSE). The essential motivation is to understand the role of the electron–electron correlation in genuine two-electron processes like double excitation or (non-sequential) double ionization. Unless simple models are considered, non-perturbative solutions of the TDSE usually require heavy numerics. So far, few approaches have been developed which explicitly take into account the pulse envelope (see Lambropoulos et al [1] for a recent review). One, which is of spectral type, consists in developing the time-dependent solution of the Schr¨ odinger equation on a truncated basis of two-electron eigenfunctions. These latter states are obtained by diagonalizing the atomic Hamiltonian in the basis of two-electron configurations constructed as an antisymmetrized product of one-electron hydrogenic orbitals [2, 3]. With this method, based on configuration interaction (CI), the calculation of the observables of interest (ionization yield, above-threshold ionization spectra, . . . ) is simple and valuable information has been obtained on various multiphoton processes involving different species. However, the basis truncation leads to a poor description of highly correlated states (such as the ground state of 0953-4075/01/050857+11$30.00 © 2001 IOP Publishing Ltd Printed in the UK 857