Strong-field non-sequential double ionization: wavelength dependence of ion momentum distributions for neon and argon

Abstract

Strong-field non-sequential double ionization: wavelength dependence of ion momentum distributions for neon and argonAbstract Strong-field double ionization of atoms in a non-sequential regime produces longitudinal ion momentum distributions with a characteristic double-peak structure. At 800 nm laser wavelengthinNe 2+ the structure is very pronounced with a well-resolved dip at zero momentum, while for Ar 2+ the dip is very shallow, possibly indicating different mechanisms in the two atoms. We investigated the source of this difference by measuring longitudinal momentum distributions of Ne 2+ and Ar 2+ ions at different laser wavelengths (485, 800, 1313 and 2000 nm) and intensities. The shapes of the experimental momentum distributions for the two atoms exhibit strong dependence on laser wavelength: for both the dip becomes more pronounced at longer wavelengths. At 1300 nm the longitudinal momentum spectrum for Ar

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“…The dependence on the atomic structure can be illustrated by comparing a light atom like Ne to a heavier one like Ar (see for example [67,74,75]). For comparable intensities, the RESI channel is much more significant in Ar than in Ne, as shown in figure 2.7.…”

“…By comparing ion momentum spectra with similar ratios of recollision energy to the second ionisation potential 3.17U p /I p + , de Jesus et al [74] attribute this to the larger excitation cross-section of Ar. Alnaser et al [75] argue that the scaling of the above recollision energy ratios is limited by the atom's saturation intensity I sat . Therefore the difference in ionisation potentials cannot be excluded at all intensities in this way but the authors argue that it will always manifest itself in the Keldysh adiabaticity parameter γ = I p /U p .…”

“…Broad studies have been performed by [71,75] for example. Starting from a direct impact ionisation scenario, both decreasing and increasing the intensity will lead to a single peak ion momentum distribution [71].…”

“…The wavelength dependence is qualitatively similar to intensity, but quantitatively stronger due to its quadratic scaling in U p . Several studies [67,75] show that the impact ionisation channel becomes increasingly dominant for higher wavelengths. This is due to an increase in recollision energy and decrease in the Keldysh parameter and hence going deeper into the tunneling regime.…”

“…Starting from a low intensity, recollision excitation with subsequent field ionisation (RESI) [68] will dominate until the recollision energy exceeds the second ionisation potential I + p . Here, direct impact ionisation becomes possible [71,75] (see fig 6.1). For further increase of the intensity, sequential ionisation becomes more likely until the saturation regime is reached, where it dominates.…”