Photoemission and Ionization of He⁺ under Simultaneous Irradiation of Fundamental Laser and High-Order Harmonic Pulses

We theoretically study the response of He⁺ exposed simultaneously to an intense Ti:sapphire laser and its 27th or 13th harmonic pulses. High-order harmonic emission from He⁺ is enhanced by many orders of magnitude compared with the case of the fundamental pulse alone. Moreover, while an individual 10 fs laser (wavelength λ_F=800  nm) or its 27th harmonic pulse with a peak intensity of 3×10¹⁴ and 10¹³   W/cm², respectively, ionizes no more than 5×10^-6 of He⁺, their combined pulse leads to a surprisingly high He²⁺ yield of 17%.