Efficient photoemission and ionization of He+ by a combined fundamental laser and high-order harmonic pulse

We theoretically study the behavior of He+ exposed simultaneously to an ultrashort intense Ti:sapphire laser and its 27th or 13th harmonic pulses, by solving the time-dependent Schrödinger equation using the alternating direction implicit method. This system is chosen as an interesting application of state-of-the-art high-order harmonic sources. Our results show that the simple hydrogenlike system exhibits surprising responses. The addition of the 27th harmonic enhances high-order harmonic photoemission from He+ even by 17 orders of magnitude compared with the case of the fundamental pulse alone, i.e., usual high-order harmonic generation, and the H13 addition also increases photoemission efficiency by more than ten orders of magnitude. Moreover, while an individual 10 fs laser (wavelength lF=800 nm) or its 27th harmonic pulse with a peak intensity of 331014 and 1013 W/cm2, respectively, ionizes no more than 5310−6 of He+, their combined pulse leads to a much higher He2+ yield of 17%. The photoemission efficiency only weakly depends on the fundamental wavelength, and the main plateau efficiency scales as the intensity of the 27th harmonic pulse. The He2+ yield is linear in the intensity of the 27th harmonic and quadratic in that of the 13th harmonic pulse. Its dependence on fundamental intensity is much more complex, even containing a range where the yield decreases with increasing laser intensity, due to the dynamic Stark effect. Detailed analyses reveal that the mechanism of the enhancement of harmonic photoemission as well as ionization is a combination of harmonic generation from a coherent superposition of states and two-color frequency mixing enhanced by the presence of a near-resonant intermediate level, and that their relative importance depends on fundamental wavelength.