TIME-DEPENDENT MULTIPHOTON IONIZATION FROM THE HE 1S2S S-1 METASTABLESTATE

Using the state-specific expansion approach to the solution of the tim e-dependent Schrodinger equation, we calculated a number of observable s from the response of the He 2(1)S metastable state to strong as well as to weak laser pulses. The photon energies (HBAR omega) were 1 eV, 2 eV, 2.47 eV and 3 eV, and the intensities (I) covered the range from 10(8) W cm(-2) to 10(14) W cm(-2). For the weak intensities, the resu lts are in full accord with the experimental findings of Haberland and co-workers. For the strong intensities, our predictions are of quanti tative as well as of qualitative value. They permit conclusions about the fingerprints left on observable spectra by electron correlation, b y Rydberg levels and by doubly excited states and about the extent to which high harmonics of short wavelength can be produced as a function of l and omega w when the initial state lies above the ground state. Finally, we argue that calculations of harmonic spectra based on the u se of the acceleration form of the dipole operator will produce, in ge neral, unreliable results even when some correlations are accounted fo r.