Time-dependent formation of the profile of the He 2s2p P-1(o) state excited by a short laser pulse

We produce quantitatively accurate data for the energy- and time-dependent formation of the profile of the differential ionization probability of the He 1s(2) S-1 ground state from the coherent excitation and decay of the doubly excited He 2s2p P-1(o) resonance state induced by a pulse of duration of 450 a.u. and field strength in the range F=0.4x10(-3) a.u. to F=0.4x10(-1) a.u. Two general methods were applied. One is analytic, using Fano's configuration interaction in the continuum in the framework of first order time-dependent perturbation theory. The other is numerical, using the state-specific expansion approach for the nonperturbative solution of the time-dependent Schrodinger equation. Electronic structures and electron correlation are incorporated via the use of state-specific wave functions for the initial state, the resonance state, and the continuum of scattering states. The results from the two methods are in perfect agreement, with a small discrepancy starting at F=0.4x10(-1) a.u. The weak field analytic formulas show explicitly the dependence of the profile formation on the pulse characteristics. In the limit of large times, the system becomes stationary and the computed resonance state profile yields the Fano asymmetry parameter of q=-2.8, with energy E-r=60.20 eV and width Gamma=0.038 eV. These values agree with previously published ones obtained from time-independent calculations and from photoabsorption measurements of the type initiated by Madden and Codling in 1963.