High-order harmonic generation of para-helium and ortho-helium

Abstract

We theoretically study the high-order harmonic generation (HHG) of para-helium and ortho-helium with different symmetric spatial configurations. Our time-dependent Schrödinger equation simulations clearly show that the HHG spectra of para-helium and ortho-helium have a distinct difference in the harmonic amplitude. By performing a detailed time-frequency wavelet analysis of the HHG spectra, the distinction between the HHG spectra of para-helium and ortho-helium is ascribed to the different excitation cross-section of the cation of helium. Furthermore, in terms of a simple recollision model, this dependence of the excitation cross-section of the cation on the symmetry of the two-electron wavefunction of helium is revealed.