Modification of the spectrum of high harmonics by a weak vacuum ultraviolet field

Within the framework of the time-dependent effective range theory, we have calculated the spectra of high harmonics in a field containing an intense low-frequency component and a weak (perturbative) high-frequency component with a photon energy exceeding the ionisation potential of the atomic target. We have presented a quantum-mechanical substantiation of the quasi-classical theory [A.V. Flegel et al. Quantum Electron., 47, 222 (2017)], which describes additional generation channels of high harmonics induced by a weak high-frequency field.