Disappearance and reappearance of above-threshold-ionization peaks

It is shown that above-threshold-ionization peaks disappear when the kinetic energy associated with the nondipole radiation-pressure-induced photoelectron momentum in the laser propagation direction becomes comparable to the photon energy, and how peaks can be made to reappear if knowledge of the length and direction of the photoelectron momentum is at hand and an emission-direction-dependent momentum shift is accounted for. In this sense appropriate analysis of the nondipole effects restores the energy-conserving signature of the above-threshold-ionization peaks. The reported findings should be observable with intense midinfrared laser pulses.

Figure 1

Disappearance of ATI peaks. ATI spectra from hydrogen for a ten cycle pulse with intensity ${10}^{14}$ W/${\mathrm{cm}}^2$ for wavelengths of (a) 1600 nm, (b) 2400 nm, (c) 3200 nm, and (d) 6400 nm. The full curves show results including nondipole effects. The dashed curves show the results within the electric dipole approximation.

Figure 2

Reappearance of ATI peaks. (a) ATI spectra from hydrogen for a ten cycle pulse with intensity ${10}^{14}$ W/${\mathrm{cm}}^2$ and $\lambda =3200$ nm. The full curves show results including nondipole effects. The dashed curves show results within the electric dipole approximation. (b) As (a), but with momenta measured with respect to the nondipole-induced shifted center of the energy conserving ATI rings according to Eq. (4).