Tracing direct and sequential two-photon double ionization of ${\mathrm{D}}_2$ in femtosecond extreme-ultraviolet laser pulses

Two-photon double ionization (TPDI) of ${\mathrm{D}}_2$ is studied for $38$-eV photons at the Free Electron Laser in Hamburg (FLASH). Based on model calculations, instantaneous and sequential absorption pathways are identified as separated peaks in the measured ${\mathrm{D}}^{+}+{\mathrm{D}}^{+}$ fragment kinetic energy release (KER) spectra. The instantaneous process appears at high KER, corresponding to ionization at the molecule’s equilibrium distance, in contrast to sequential ionization mainly leading to low-KER contributions. Measured fragment angular distributions are in good agreement with theory.

Figure 1

Illustration of the dominant dissociative pathways for single and double ionization of ${\mathrm{D}}_2$ and experimental KER spectrum for coincident ${\mathrm{D}}^{+}+$D${}^{+}$ fragments created by TPDI via direct and sequential pathways. Depending on the time (horizontal arrow) when the second photon is absorbed after single ionization, the repulsive $1/R$ curve is populated at different R, leading to time-dependent KERs and allowing for the separation of SI and NSI pathways. The gray band indicates doubly excited states $Q$${}_2$ in $\Sigma$ and $\Pi$ symmetries. The Franck-Condon regime is indicated as a vertical shaded band.

Figure 2

KER spectrum of noncoincident ${\mathrm{D}}^{+}$ fragments taken under an emission angle of 90${}^{°}\pm 5{}^{°}$ with respect to the light polarization axis (i.e., for final $\Pi$ symmetry).

Figure 3

FIAD and KER spectra for coincident ${\mathrm{D}}^{+}+$D${}^{+}$ fragments. (a) Angular distribution of the fragments [cos($\theta$)] as a function of the KER; (b) experimental KER spectrum integrated over $|\cos (\theta )|<0.8$ compared to calculations in the non-FCA and FCA; (c) experimental and theoretical ${\beta }_2$ values as a function of the KER.