Mechanisms of Strong-Field Double Ionization of Xe

We perform a fully differential measurement on strong-field double ionization of Xe by 25 fs, 790 nm laser pulses in intensity region $(0.4–3)\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$. We observe that the two-dimensional correlation momentum spectra along the laser polarization direction show a nonstructured distribution for double ionization of Xe when decreasing the laser intensity from $3\times {10}^{14}$ to $4\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$. The electron correlation behavior is remarkably different with the low-$Z$ rare gases, i.e., He, Ne, and Ar. We find that the electron energy cutoffs increase from $2.9U_p$ to $7.8U_p$ when decreasing the laser intensities from the sequential double ionization to the nonsequential double ionization regime. The experimental observation indicates that multiple rescatterings play an important role for the generation of high energy photoelectrons. We have further studied the shielding effect on the strong-field double ionization of high-$Z$ atoms.

Figure 1

The longitudinal momentum distributions of (a) ${{}^{40}\mathrm{Ar}}^{2+}$ and (b) ${{}^{134}\mathrm{Xe}}^{2+}$ at intensities of $(0.4–3)\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ (25 fs, 790 nm). (c) The ratio of double to single ionization counts as a function of laser intensity for Xe. The dotted curve is taken from Ref. [17], where the isotopes are not resolved. The data marked with open circles are measured with ${{}^{134}\mathrm{Xe}}^{2+}/{{}^{134}\mathrm{Xe}}^{+}$ in this experiment. The solid line is the ratio of the ${\mathrm{Xe}}^{2+}/{\mathrm{Xe}}^{+}$ ionization rate that is calculated by the ADK model. (d) The width (FWHM) of the longitudinal momentum distribution of ${\mathrm{Xe}}^{2+}$ with respect to the laser intensity.

Figure 2

The measured correlation momentum spectra of double ionization Xe at intensities of (a) $3\times {10}^{14}$, (b) $1.6\times {10}^{14}$, (c) $7\times {10}^{13}$, and (d) $4\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$ (25 fs, 790 nm).

Figure 3

Intensity dependence of electron energy cutoffs for double ionization Xe in the intensity region of $(0.4–3)\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ (25 fs, 790 nm). The solid lines indicate the electron temperature fitted by the Boltzmann distribution (see the text).

Figure 4

The calculated correlation momentum spectra of double ionization Xe at intensities of $0.6\times {10}^{14}$ (a) and $1.1\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ (b) with the Coulomb potential. The calculated correlation momentum spectra of double ionization Xe at intensity of $0.6\times {10}^{14}$ (c) and $1.1\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ (d) using the shielding potential.