Strong Field Atomic Ionization: Origin of High-Energy Structures in Photoelectron Spectra

Two distinct interpretations have been proposed to account for conspicuous enhancements of the ionization peaks in the high energy part of above-threshold ionization spectra. One of them ascribes the enhancement to a multiphoton resonance involving an excited state, while other analysis performed for zero-range model potential link it to “channel closings, ” i.e., to the change in the number of photons needed to ionize the atom when the laser intensity increases. We report the results of model calculations that confirm the existence of a resonant process in atoms and shed light on why short-range potential models can mimic the experimental observations.

Figure 1

Photoelectron spectra for the long-range potential Eq. (1) and for $\omega =0.0577\mathrm{a}\mathrm{.}\mathrm{u}\mathrm{.}$ Open circles, diamonds, and crosses correspond to intensities of $1.145\times {10}^{14}\mathrm{W}{\mathrm{c}\mathrm{m}}^{-2}$, $1.02\times {10}^{14}\mathrm{W}{\mathrm{c}\mathrm{m}}^{-2}$, and $1.22\times {10}^{14}\mathrm{W}{\mathrm{c}\mathrm{m}}^{-2}$, respectively. The pulse duration is $18T_L$. Full circles are the same as open circles for a pulse duration of $34T_L$.

Figure 2

(a) Real part of the Floquet quasienergies as a function of the intensity. (b) Variations of one ATI peak at intensities around the $n=6$ state resonance. Thin line: $I=1.140\times {10}^{14}\mathrm{W}{\mathrm{c}\mathrm{m}}^{-2}$; dotted line: $I=1.143\times {10}^{14}\mathrm{W}{\mathrm{c}\mathrm{m}}^{-2}$; thick line: $1.145\times {10}^{14}\mathrm{W}{\mathrm{c}\mathrm{m}}^{-2}$; dashed line: $I=1.150\times {10}^{14}\mathrm{W}{\mathrm{c}\mathrm{m}}^{-2}$; dash-dotted line: $I=1.155\times {10}^{14}\mathrm{W}{\mathrm{c}\mathrm{m}}^{-2}$.

Figure 3

(a) Photoelectron spectra (linear scale), as computed for the short-range potential Eq. (3), for different intensities in the energy range where the enhancement takes place. Open circles: $9.1035\times {10}^{13}\mathrm{W}{\mathrm{c}\mathrm{m}}^{-2}$; solid line: $9.464\times {10}^{13}\mathrm{W}{\mathrm{c}\mathrm{m}}^{-2}$; crosses: $1.0206\times {10}^{14}\mathrm{W}{\mathrm{c}\mathrm{m}}^{-2}$; dotted lines: $E=n\omega$. (b) Relevant part of the Floquet spectrum.