Ionization and double excitation of helium by short intense laser pulses

We present an ab initio calculation of single ionization, harmonic generation, and double excitation of helium by short (3.8--15.2 fs) moderately strong (peak intensity 1.49×10¹⁴W/cm²) laser pulses with wavelengths between 40 and 152 nm. We find excitation to be dominated by resonances with bound and doubly excited states. The processes are nonperturbative. The finite pulse duration leads to resonance broadening and far-off-resonant transitions.