Theoretical studies of high-harmonic generation based on the Keldysh-Faisal-Reiss theory

We use the Keldysh-Faisal-Reiss (KFR) approach to study the process of high-harmonic generation (HHG) for hydrogen and helium atoms and a hydrogen molecule where the helium atom and hydrogen molecule wavefunctions are described by a 1s single Slater determinant. In this model we directly use the photoionization amplitude calculated by the KFR approach and relate it to the HHG spectra. We plot HHG from hydrogen and helium atoms, and a hydrogen molecule in the presence of a linearly and circularly polarized laser field, and also discuss the nuclear motion corrections for the hydrogen molecule, which are calculated under the Frank-Condon (FC) approximation by using both displaced-distorted harmonic oscillators and Morse wavefunctions. Contributions from each vibrational state are also discussed