Classical Aspects in Above-Threshold Ionization with a Midinfrared Strong Laser Field

We present high resolution photoelectron energy spectra of noble gas atoms from high intensity above-threshold ionization (ATI) at midinfrared wavelengths. An unexpected structure at the very low-energy portion of the spectra, in striking contrast to the prediction of the simple-man theory, has been revealed. A semiclassical model calculation is able to reproduce the experimental feature and suggests the prominent role of the Coulomb interaction of the outgoing electron with the parent ion in producing the peculiar structure in long wavelength ATI spectra.

Figure 1

Experimental [(a), (c), and (e)] and calculated [(b),(d)] photoelectron spectra of xenon. (a) $I=8.0\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$, $\lambda =800$, 1250, 1500, and 2000 nm from bottom to top, respectively. The complete spectra are shown in the inset. The laser pulse durations are $\sim 40\mathrm{fs}$ at 800 nm, $\sim 30\mathrm{fs}$ at 1250 and 1500 nm, while $\sim 90\mathrm{fs}$ at 2000 nm. (b) $I=8.0\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$ and $\lambda =800$, 1250, 1500, and 2000 nm, with Coulomb potential for the curves from bottom to top, respectively. While the uppermost curve is for $I=8.0\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$ and $\lambda =2000\mathrm{nm}$ without Coulomb potential. (c),(d) $\lambda =2000\mathrm{nm}$, $I=3.2$, $6.4\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$ for the lower and upper curves, respectively. (e) $\lambda =1500\mathrm{nm}$, $I=4.0$, $10.0\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$ for the lower and upper curves, respectively. In (c) and (e), the boundaries of the second hump are indicated by the dashed lines for higher intensities. The curves are shifted in the vertical direction for visual convenience.

Figure 2

(a)–(d) Calculated electron momentum distributions ($z$ is the laser polarization direction). $I=8\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$ and $\lambda =2000\mathrm{nm}$. See the text for the details. (e),(f) Electron yields as a function of the laser phase at the moment of tunnel ionization, without and with the Coulomb potential considered, respectively. Here the statistics are only collected for the electron trajectories that contribute to the energy range of the second hump structure (i.e., $1.0<E_{\mathrm{kin}}<3.7\mathrm{eV}$, where $E_{\mathrm{kin}}$ is the electron energy) in Fig. 1b at $I=8\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$.