Correlated Multielectron Dynamics in Ultrafast Laser Pulse Interactions with Atoms

We present the results of the detailed experimental study of multiple ionization of Ne and Ar by 25 and 7 fs laser pulses. Whereas in multiple ionization of Ar different mechanisms, involving field ionization steps and recollision-induced excitations, play a role, for Ne only one channel, where the highly correlated instantaneous emission of up to four electrons is triggered by a recollisional electron impact, is found to be important. Using few-cycle pulses we are able to suppress those processes that occur on time scales longer than one laser cycle.

Figure 1

Mechanisms of strong-field ionization. If the electric field changes with time as $E(t)=E_0\sin \omega t$, an electron which is set free at the time $t$ acquires a final drift momentum of $P_{\mathrm{d}\mathrm{r}\mathrm{i}\mathrm{f}\mathrm{t}}(t)=q/\omega E_0\cos (\omega t)$. Thus, an electron created near the maximum of the field (1) emerges with zero momentum after the laser pulse has gone. In sequential double ionization (4) tunneling happens twice leading to small momenta of the produced ion, which balances the electron sum momentum. If an electron tunnels out shortly after the field maximum it can recollide with its parent ion where it may either excite (2) or directly knock out (3) another atomic electron. In the latter case both electrons acquire a large net momentum of up to $2\sqrt{U_P}$ each. The corresponding final ion momentum displays clear maxima. In cases where an electronic excitation occurs during recollision only the slowed down projectile electron acquires a namable momentum from the laser field, while the excited electron, which is field ionized at a later time, appears as a slow electron. This pathway [recollision-induced excitation followed by subsequent field ionization (RESI)] gives rise to ion-momentum distributions slightly broader than for the sequential case.

Figure 2

Longitudinal momentum distributions of neon ions obtained with 25 fs 800 nm laser pulses. The laser peak intensity in $\mathrm{P}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$ as well as the ion charge states are indicated in the each panel. The arrows indicate momenta of $\pm 2n\sqrt{U_P}$ with $n=3$ for the upper two panels and $n=4$ for the lowest panel.

Figure 3

The same as Fig. 2 for argon. The arrows indicate momenta of $\pm 2n\sqrt{U_P}$ with $n=3$ for the upper two panels and $n=4$ for the lowest panel.

Figure 4

Longitudinal momentum distributions of recoil ions obtained with 25 fs (squares) and 7 fs (solid line), 800 nm laser pulses. (a) ${\mathrm{N}\mathrm{e}}^{2+}$, $0.8\mathrm{P}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$. (b) ${\mathrm{A}\mathrm{r}}^{2+}$, $0.5\mathrm{P}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$. (c) ${\mathrm{A}\mathrm{r}}^{3+}$, $0.8\mathrm{P}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$. (d) ${\mathrm{A}\mathrm{r}}^{4+}$, $1.0\mathrm{P}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$.