Variation of harmonic generation from He interacting with short laser pulses of 5 eV as a function of pulse rise time and intensity

We have applied the state-specific expansion approach (SSEA) to the solution of the time-dependent Schrodinger equation describing the interaction of He with laser pulses of (h) over bar omega = 5 eV, which have a shape with a sharp rise and peak intensities in the range 5 x 10(14)-3.46 x 10(15) W/cm(2). The calculations include the effects of the state-specific self-consistent field, the electron correlation, and the n = 2 intrashell doubly excited states. The results show that, provided the pulses have fast rise times, neutral He can generate harmonics of higher order than previously thought before the He+ contribution takes over. The computed spectra agree with the observations of Sarukura et al. [Phys. Rev. A 43, 1669 (1991)]. In particular, the SSEA results reveal "anomalously" high peaks for the 13th and 21st harmonics, in agreement with experiment.