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.