Investigating two-photon double ionization of ${\mathrm{D}}_2$ by XUV-pump–XUV-probe experiments

We used a split-mirror setup attached to a reaction microscope at the free-electron laser in Hamburg (FLASH) to perform an XUV-pump–XUV-probe experiment by tracing the ultrafast nuclear wave-packet motion in the ${\mathrm{D}}_2$${}^{+}$$(1s{\sigma }_g$) with $<$$10$ fs time resolution. Comparison with time-dependent calculations shows excellent agreement with the measured vibrational period of $22\pm 4$ fs in ${\mathrm{D}}_2$${}^{+}$, points to the importance of accurately knowing the internuclear distance-dependent ionization probability, and paves the way to control sequential and nonsequential two-photon double-ionization contributions.

Figure 1

(a) Illustration of the dominant dissociative pathways for direct and sequential TPDI of ${\mathrm{D}}_2$. (b) Density plot for the experimental KER spectrum of coincident $D^{+}+{\mathrm{D}}^{+}$ fragments as a function of delay time up to 80 fs. (c) The same as (b), but for theoretical results.

Figure 2

Pump-probe delay-time dependence of ion yields in different KER regimes as indicated in the figure, after proper background subtraction, as described in the text. Dashed lines: theoretical ionization probabilities (right ordinate) for the corresponding KER ranges.

Figure 3

KER-dependent ${\mathrm{D}}_2$${}^{+}$($1s{\sigma }_g$) ionization probability obtained by integrating the KER spectrum of Fig. 1b, after background subtraction, in the time-delay interval 0–80 fs. Lines: theoretical results (see text).