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

Abstract

We have applied the state-specific expansion approach (SSEA) to the solution of the time-dependent Schr\"odinger equation describing the interaction of He with laser pulses of $\ensuremath{\Elzxh}\ensuremath{\omega}=5\mathrm{eV},$ which have a shape with a sharp rise and peak intensities in the range $5\ifmmode\times\else\texttimes\fi{}{10}^{14}--3.46\ifmmode\times\else\texttimes\fi{}{10}^{15}{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}.$ 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 ${\mathrm{He}}^{+}$ 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.