Above-Threshold Ionization at the Few-Cycle Limit

Photoelectron spectra measured for rare-gas atoms ionized with intense few-cycle laser pulses are presented. Several aspects of the few-cycle regime are discussed. In particular, the persistence of the plateaulike structure of spectra for high electron energies is shown. In contrast, a resonancelike feature at similar electron energies is suppressed as compared with longer laser pulses. Differences in the behavior of different species and implications for the electron-ion scattering cross section are pointed out.

Figure 1

Photoelectron spectra of xenon ionized with few-cycle laser pulses ($\approx 8\mathrm{f}\mathrm{s}$) of different intensities. The intensity was calibrated with the spectrum shown in the inset and the $10U_{\mathrm{P}}$-cutoff law.

Figure 2

(a) ATI spectra of argon at different intensities. Arrows indicate the energy position $2U_{\mathrm{P}}$. (b) ATI spectra of argon for different pulse durations. The spectrum corresponding to 40 fs was taken from 10. The clear enhancement of ATI peaks between 15 and 30 eV disappears as the pulse duration is decreased. The laser intensity for all spectra is approximately the same within 20% and equals $0.8\times {10}^{14}{\mathrm{W}\mathrm{/}\mathrm{c}\mathrm{m}}^2$. (The curves in both panels are separated in the vertical direction for visual convenience; the ratio of the curves thus has no meaning.)

Figure 3

Comparison between xenon (left panel) and krypton (right panel). The relative height of different spectra is not of significance. For comparison, spectra for circular polarization (dotted curves) are shown. They correspond to intensities of $\approx 9\times {10}^{13}{\mathrm{W}\mathrm{/}\mathrm{c}\mathrm{m}}^2$ (Xe) and $7\times {10}^{13}{\mathrm{W}\mathrm{/}\mathrm{c}\mathrm{m}}^2$ (Kr).