Effect of spin-orbit coupling on high-order harmonic generation in atoms.

We present an ab initio investigation of the influence of spin-orbit coupling (SOC) on high-order harmonic generation (HHG) in hydrogen atoms by numerically solving the time-dependent Schrödinger equation including the SOC term (TDSE-SOC). We find that the SOC does not affect the HHG spectra produced by the initial ground state. However, for the excited state 2P, and its spin-split 2P3/2 and 2P1/2 states which are superpositions of ml = 0 and ml = 1 wave functions, the harmonic yields generated from the above three states vary significantly. We developed a modified strong-field approximation theory incorporating the spin-split states to analyze the SOC effect on HHG spectra. Considering the depletion of bound states, the simulation results are qualitatively consistent with the TDSE calculations. Analysis indicates that the ml = 0 component in the spin-split 2P states predominantly contributes to harmonic yields, and the ml = 1 component has a non-negligible influence on lower-energy harmonics in a linearly polarized laser field.