Cooper minima in electron spectra after multiphoton above-threshold ionization

We have performed calculations of electron momentum and energy distributions after multiphoton above-threshold ionization (ATI) for several one-electron quantum systems (H, He$^{+}$, H$_{2}^{+}$, and HeH$^{2+}$) in intense laser fields. We use the carrier wavelengths in the near-infrared band (730 to 800 nm) and the peak intensities $5\times 10^{13}$ to $1\times 10^{14}$ W/cm$^{2}$. For some initial states of the systems under consideration, the spectra exhibit minima in the low-energy region (3 to 7 eV), which resemble the famous Cooper minima in one-photon ionization processes. The minima are well pronounced for the initial states with the electronic orbitals that have nodal surfaces, such as $2s$ state of He$^{+}$, $1\sigma_{u}$ state of H$_{2}^{+}$, and $2\sigma$ state of HeH$^{2+}$. Such minima are not observed for the initial ground electronic states, as well as for initial $2p$ state of He$^{+}$, which possess nodeless orbitals. The effect is essentially non-perturbative; the positions of the minima depend on the intensity and frequency of the laser field. Nonetheless, it seems the nodal structure of the initial electronic orbital plays a crucial role in shaping these minima in the ATI electron spectra.