Photo-double-ionization of water at 20 eV above threshold

The photodouble ionization of the water molecule is studied at 20 eV excess energy in a combined experimental and theoretical investigation. In the experiments, two photoelectrons of equal kinetic energy are detected in coincidence after energy and angular selection. On the theoretical side, a generalized Sturmian function approach is implemented to describe accurately the correlated two-electron continuum, while separable products of Moccia orbitals [J. Chem. Phys. 40, 2164 (1964)] are used for the initial electronic state of the water molecule. The theoretical triple-differential cross sections (TDCSs) are averaged over all possible molecular orientations in order to be compared with the experiments. The measured TDCSs display rich angular distributions that are in large part well reproduced by the adopted first-order treatment of the interaction with a two-active-electron target.

Figure 1

Radial variation of the angular averaged effective charge [potential $r{\mathcal{U}}_{i,\mathrm{mol}}\left(r\right)$] obtained by removing electrons from four different pairs of molecular orbitals.

Figure 2

Theoretical TDCS of water for two electrons ejected with equal energy ($E_1=E_2=10\mathrm{eV}$) and for three fixed ${\theta }_1$ angles, calculated in both length (left) and velocity (right) gauges for those initial states which contribute to the PDI at 63 eV photon energy. The incident radiation is linearly polarized along the $0^{\circ }$ direction.

Figure 3

Same as in Fig. 2 but for those initial states which contribute to the PDI at 65 eV photon energy.

Figure 4

The TDCS of water measured at 63 eV photon energy in the condition of equal energy sharing ($E_1=E_2=10\mathrm{eV}$) for three fixed directions ${\theta }_1$ (red arrows) of one photoelectron compared with the theoretical predictions in the length (dashed red curves) and velocity (solid blue curves) gauges. The incident radiation is fully linearly polarized along the $0^{\circ }\text{−}{180}^{\circ }$ direction. In the bottom-right histogram the contributions of the different dication states are reported (blue bars for velocity gauge, red dashed bars for length gauge).

Figure 5

Same as in Fig. 4 but at 65 eV photon energy.

Figure 6

The TDCS of water measured at 63 eV photon energy for fixed direction ${\theta }_1=0^{\circ }$ (red arrow) of one photoelectron compared with the theoretical predictions in the length (dashed red curve) and velocity (solid blue curve) gauges. The incident radiation is fully linearly polarized along the $0^{\circ }\text{–}{180}^{\circ }$ direction. In the histogram the contributions of the different dication states are reported (blue bars for velocity gauge, red dashed bars for length gauge).