Structure of the high harmonic spectra from ions

The structure of the spectra of harmonics generated from ions in the hydrog en isosequence interacting with a strong laser field is studied by solving the time-dependent Schrodinger equation with a 1-D soft-core model potentia l. The peculiarities of the harmonic structure from the ions under consider ation compared to that from atoms are mainly due to the existence of deeply bound states. Since the probability that a deeply bound electron tunnels t hrough the. potential barrier formed by the laser with a moderate intensity is very small, the part of the plateau at low orders disappears and makes a 'well' structure in the region of lower-order harmonies, which is not com mon in the ordinary high harmonic spectra. In fact, the ions in hydrogen is oelectronic sequence have deep Coulomb potentials that support several low- lying bound states which can contribute to the generation of harmonic spect rum unless the laser intensity is either too high or too low. Accordingly, we find protruding peaks around or at the energy difference between two bou nd states. In some cases. where an excited state is prepared as the initial state, we occasionally see two plateaus, which is also rare in ordinary hi gh harmonic spectra. From the structure of those plateaus and peaks, we und erstand how bound states are involved in high harmonic generation processes .