Ab initio calculation of harmonic generation spectra of helium using a time-dependent non-Hermitian formalism

The high harmonic spectra of helium atoms which are exposed to external monochromatic linearly polarized laser fields are calculated by solving the time-dependent non-Hermitian Schrodinger equation. The entire electronic correlation effects with and without the presence of the field are included in our calculations. The full high-harmonic generation spectra (HGS) were calculated (and not only the intensities of the integer harmonics frequencies as calculated before by non-Hermitian quantum mechanics). The HGS were calculated when the helium atoms are initially in their ground state, or in the 1s2p excited state or in a superposition of the two field free states. In the first two cases only odd-order harmonics are obtained. However, in the latter, in addition to the odd-order harmonics also pronounced even-order harmonics are obtained. The even order peaks are much broader than the peaks of the odd-order high harmonics. The association of the widths of the peaks in the HGS with the lifetime of the photoinduced resonances which control the dynamics is discussed.