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Photon emission by He+ in intense ultrashort laser pulses

An analysis of photon emission by He+ interacting with intense, short laser pulses is presented. Employing one- and two-dimensional model atomic systems, the time-dependent Schrödinger equation is solved numerically for few-cycle laser pulses with wavelength 800 nm and a peak intensity of 5.6×1015 W cm-2. The low ionization probability of positive ions leads to photon emission spectra that display prominent plateaus whose height and cut-off energy depend strongly on the phase and duration of the laser pulse. These features can be explained using the semiclassical three-step model of harmonic generation. Ions also allow the investigation of effects due to the magnetic field component of the laser pulse. For the laser parameters used, we find that photon emission polarized along the propagation direction is three to four orders of magnitude weaker than photon emission polarized parallel to the polarization direction of the incident wave.