Ionization of Atoms by the Spatial Gradient of the Pondermotive Potential in a Focused Laser Beam

Ionization of atoms by the spatial gradient of the pondermotive potential in a focused laser beam is investigated. Rydberg ions, formed during the interaction of noble gas atoms with an intense laser pulse, are used to probe the gradient field. Rydberg ion species with higher ionization potentials are produced at locations where the gradient field is largest. The measured Rydberg ion yields differ dramatically from estimates that ignore gradient-field ionization, but are in good agreement with predictions that include the effect.

Figure 1

$U_p$ (solid curve) and $\nabla U_p$ (dashed curve) in the focal plane of a Gaussian beam, plotted as a function of distance, $r$, from the beam axis for typical experimental conditions at a peak intensity of $5\times {10}^{15}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$. Spatial regions where different charge states of Kr are produced are noted. Above their saturation intensities, ions are produced in the wings of the focal volume where higher IP species experience greater gradient fields.

Figure 2

A typical time-of-flight spectrum for Kr. The inset shows a schematic of the TOF spectrometer.

Figure 3

Normalized Rydberg yields vs laser intensity for Xe, Kr, and Ar. Open (filled) circles and dashed (solid) lines correspond to the experimental and simulated $A^{+*}:A^{2+}$ ($A^{2+*}:A^{3+}$) ratios, respectively. The simulated yields at the extreme left of each plot correspond to the intensity-independent results ignoring gradient-field ionization. The calculated low-intensity yields are not identical for each species due to the different Rydberg ion detection efficiencies and the energy normalization factors from Eq. (1). The simulated yields decrease sharply at two different intensities, corresponding to the onset of gradient-field ionization in two distinct spatial regions near the sagittal and tangential foci of the astigmatic beam. Inset: $A^{+*}:A^{2+}$ ratios measured with two different focusing geometries for Xe (+), Kr (×), and Ar (◇). Data acquired with weaker focusing ($35\%$ larger $f$ number) are shown in green (gray) and those obtained with tighter focusing (also in main figure) are in black.