Strong-Field Tunneling without Ionization

In the tunneling regime of strong laser field ionization we measure a substantial fraction of neutral atoms surviving the laser pulse in excited states. The measured excited neutral atom yield extends over several orders of magnitude as a function of laser intensity. Our findings are compatible with the strong-field tunneling-plus-rescattering model, confirming the existence of a widely unexplored neutral exit channel (frustrated tunneling ionization). Strong experimental support for this mechanism as origin of excited neutral atoms stems from the dependence of the excited neutral yield on the laser ellipticity, which is as expected for a rescattering process. Theoretical support for the proposed mechanism comes from the agreement of the neutral excited state distribution centered at $n=6–10$ obtained from both, a full quantum mechanical and a semiclassical calculation, in agreement with the experimental results.

Figure 1

Experimental data for total ion yield (□) and for excited neutral atom yield with ($n<30$) (○). Equal MCP detection efficiency for both species has been assumed. Also shown are the total ion yield (■) and total yield of excited states (▴) with ($n<30$) from full two-electron CI calculations. The open triangles (△) denote the theoretical excited neutral atom yield corrected for the radiative decay as explained in the text. The experimental laser intensity was divided by a factor 1.8, see text. The red dots (●) indicate the captured tunneling electrons from classical calculations normalized on the total ion yield from the CI calculations. Shown also is an estimated representative error bar for the corrected theoretical curve, which is due to the uncertainty in the correction of the radiative decay process.

Figure 2

Dependence of the ${\mathrm{He}}^{+}$ (■) and ${\mathrm{He}}^{*}$ (○) yield on ellipticity at fixed pulse energy corresponding to a laser intensity of about ${10}^{15}\mathrm{W}/{\mathrm{cm}}^2$ for $ϵ=0$. For better comparison both measurements have been set equal at $ϵ=0$.

Figure 3

$n$ distribution of the population of excited states from a MC simulation (red ○), a quasi-one-electron (■) and a full two-electron quantum mechanical calculation (blue ▴) at a laser intensity ${10}^{15}\mathrm{W}/{\mathrm{cm}}^2$. The MC simulation has been normalized to the quasi-one-electron calculation at $n=10$.