Effect of electron correlation on the two-particle dynamics of a helium atom in a strong laser pulse

This review discusses the complicated two-electron dynamics of a helium atom in an intense, short laser pulse. A helium gas in femtosecond laser pulses at long wave lengths (lambda similar to 700 nm) and high intensities (I similar to 10(15) W/cm(2)) produces surprisingly high numbers of He2+ ions. These laser fields cause large and fast electron oscillations, which makes a solution of the time-dependent Schrodinger equation numerically demanding. The system can be studied using a one-dimensional model atom, which has many of the same properties as the He atom. Using the one-dimensional model, the importance of including electron correlation in a simplified description of the two-electron dynamics is demonstrated. It is shown that electron correlation becomes much less important if the laser field has a short wave length, in which case the electron oscillations are smaller and slower. The problem of including electron correlation in the calculations is discussed in terms of approaches such as time-dependent Hartree-Fock, time-dependent density functional theory and time-dependent extended Hartree-Fock. Some of the commonly used semi-classical models for describing the double-ionization process are presented.