Two-electron correlated spectra of non-sequential double ionization below laser-intensity threshold are known to exhibit back-to-back scattering of the electrons, viz., the anticorrelation of the electrons. Currently, the widely accepted interpretation of the anticorrelation is recollision-induced excitation of the ion plus subsequent field ionization of the second electron. We argue that another mechanism, namely simultaneous electron emission, when the time of return of the rescattered electron is equal to the time of liberation of the bounded electron (the ion has no time for excitation), can also explain the anticorrelation of the electrons in the deep below laser-intensity threshold regime. Our conclusion is based on the results of the numerical solution of the time-dependent Schrödinger equation for a model system of two one-dimensional electrons as well as an adiabatic analytic model that allows for a closed-form solution.

We study the effects on the electronic stopping power of large clusters in solids, resulting from multiple elastic scattering of each particle in the cluster by the target atoms, in the small-angle approximation. Evaluation of the cluster vicinage self-energy at the entrance of the target shows that large clusters may stabilize against the Coulomb explosion for a range of cluster sizes and speeds, so that the change of the cluster structure, due to multiple scattering, may have a noticeable influence on the vicinage, or interference, energy losses of clusters due to collective electronic excitations in the target. Stopping power calculations are presented for C 60 and (H 2 ) 50 clusters in an Al target, together with the results on stabilization conditions against the Coulomb explosion for (H 2 ) N m , based on self-energy considerations. ͓S1050-2947͑98͒08209-2͔

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