Few-Photon Multiple Ionization of Ne and Ar by Strong Free-Electron-Laser Pulses

Few-photon multiple ionization of Ne and Ar atoms by strong vacuum ultraviolet laser pulses from the free-electron laser at Hamburg was investigated differentially with the Heidelberg reaction microscope. The light-intensity dependence of ${\mathrm{Ne}}^{2+}$ production reveals the dominance of nonsequential two-photon double ionization at intensities of $I<6\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$ and significant contributions of three-photon ionization as $I$ increases. ${\mathrm{Ne}}^{2+}$ recoil-ion-momentum distributions suggest that two electrons absorbing “instantaneously” two photons are ejected most likely into opposite hemispheres with similar energies.

Figure 1

Time-of-flight mass spectra of ${\mathrm{Ne}}^{(1-2)+}$ and ${\mathrm{Ar}}^{(1-3)+}$ ions (in the inset) at $I\approx {10}^{12}–{10}^{13}\mathrm{W}/{\mathrm{cm}}^2$.

Figure 2

Ion yield of ${\mathrm{Ne}}^{2+}$ and ${\mathrm{H}}_2^{+}$ (inset) as a function of $I$. Solid black and green points are the present data for two different sizes of the light collimation exit slit. Solid blue and red lines are fits by $\log Y=n\log I+\log {\sigma }_n$ to two sets of data yielding $n=2.2\pm 0.2$ and $n=2.6\pm 0.2$. Single ionization of ${\mathrm{H}}_2$ residual gas leading to the slope $n=0.97\pm 0.05$ provides further evidence for the linearity of our intensity scale.

Figure 3

Yield of ${\mathrm{Ar}}^{q+}$ ions as a function of $I$, (a) $q=2$ and (b) $q=3$; for details, see the caption of Fig. 2.

Figure 4

Density plot of recoil-ion-momentum distributions for (a) ${\mathrm{Ne}}^{+}$ and (b),(c) ${\mathrm{Ne}}^{2+}$. The solid curves indicate the momentum positions for one- and two-photon absorption in (a) as well as for NSI and three-photon processes in (b) and (c) for various emission mechanisms; see text. ${\mathrm{Ne}}^{+}$ momenta are integrated over events with $|P_z|\le 0.1\mathrm{a}.\mathrm{u}.$ at low intensities corresponding to the green points in Fig. 2. ${\mathrm{Ne}}^{2+}$ distributions integrate all events along the $z$ direction at (b) $I<3\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$ and (c) $I>2\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$. The polarization of the light is along the $y$ axis.