High Harmonic Generation Spectra of Neutral Helium by the Complex-Scaled (t,t′) Method: Role of Dynamical Electron Correlation

Using highly correlated Hylleraas-type wave functions of ${}_{}{}^1{}_{}{}^{}S$ and ${}_{}{}^1{}_{}{}^{}P$ symmetry, we show how the harmonic generation spectrum of $1{}_{}{}^1{}_{}{}^{}S$ He (including short wavelength members previously attributed to ${\mathrm{He}}^{+}$) arise from a radial-angular “descreening” mechanism. This provides the first indication that harmonic generation spectrum can arise from strong dynamical electron correlations rather than purely as a “one-electron phenomenon.”