Photolysis of water-radical ions H${}_2$O${}^{+}$ in the xuv: Fragmentation through dicationic states

The photofragmentation of the water cation H${}_2$O${}^{+}$ through dicationic states has been studied at $35.0\pm 0.2$ nm ($35.4\pm 0.3$ eV) and $21.8\pm 0.2$ nm ($56.8\pm 0.5$ eV) with a crossed ion-photon beams experiment at the free electron laser FLASH. The dissociation of the dications is found to be similar at the two wavelengths and to proceed into ${\mathrm{O}}^0+2{\mathrm{H}}^{+}$, ${\mathrm{OH}}^{+}+{\mathrm{H}}^{+}$, and ${\mathrm{O}}^{+}+{\mathrm{H}}_2{}^{+}$, with determined ratios ${\sigma }_{{\mathrm{OH}}^{+}+{\mathrm{H}}^{+}}/{\sigma }_{{\mathrm{O}}^{+}+{\mathrm{H}}_2{}^{+}}=4.2\pm 0.3$ and ${\sigma }_{{\mathrm{OH}}^{+}+{\mathrm{H}}^{+}}/{\sigma }_{{\mathrm{O}}^0+2{\mathrm{H}}^{+}}>0.7$. The measured kinetic-energy releases for these processes are consistent with three-body breakup (${\mathrm{O}}^0+2{\mathrm{H}}^{+}$) occurring mainly through the $2{}^3{A}^{\prime \prime }$ and $2{}^1{A}^{\prime \prime }$ states of ${\mathrm{H}}_2{\mathrm{O}}^{2+}$ and two-body breakup (${\mathrm{OH}}^{+}+{\mathrm{H}}^{+}$) occurring through $X{}^3{A}^{\prime \prime }$, $1{}^3{A}^{\prime }$, and $1{}^1{A}^{\prime \prime }$ states of H${}_2$O${}^{2+}$, as predicted in a recent theoretical study [Gervais et al., J. Chem. Phys. 131, 024302 (2009)]. In addition to the kinetic-energy releases, we also report on fragment correlation in the three-body channel where the two protons carry the major part of the released momentum.

Figure 1

Schematic illustration of the transitions from the water monocation (${\mathrm{H}}_2{\mathrm{O}}^{+}$) to the dissociating dicationic states (${\mathrm{H}}_2{\mathrm{O}}^{2+}$) under xuv irradiation based on the calculations by Gervais et al. [44]. The electron configurations and vertical ionizations energies are reproduced from Table I of Ref. [44]; in particular, the energetic positions are given as $T_{\tilde{X}}$ and $T_{\tilde{A}}$; that is, the zero-point energies [${\Omega }_0\left(X\right)=0.48$ eV for $\tilde{X}{}^2{B}_1$ and ${\Omega }_0\left(A\right)=0.50$ eV for ${\tilde{A}}^2{A}_1$) have not been subtracted. The red-colored levels ($X^3{A}^{\prime \prime }$, $1^1{A}^{\prime }$, and $1^1{A}^{\prime \prime }$) were predicted to yield two-body fragmentation into ${\mathrm{OH}}^{+}+{\mathrm{H}}^{+}$ and the blue-colored levels ($2^3{A}^{\prime \prime }$ and $2^1{A}^{\prime \prime }$) to yield three-body fragmentation into ${\mathrm{O}}^0+2{\mathrm{H}}^{+}$[44]. The gray levels were considered to be unimportant for photoabsorption from the monocation [44].

Figure 2

The experimental setup around the interaction region of the TIFF experiment [46] at FLASH [17, 18]. For the measurement at 35.0 nm, DET 3, the predeflector and the electrostatic mirror were not installed. The dashed arrows indicate the distances from the interaction point the to DET 1 ($L_1=0.273$ m) and to DET 2 ($L_2=0.872$ m during measurement at 35.0 nm and $L_2=0.967$ m at 21.8 nm), as well as the horizontal distance from the interaction point to the farthest surface of the electrostatic mirror and upwards 150${}^{\circ }$ to the plane defined by the surface of DET 3 ($L_3=L_{3a}+L_{3b}=0.975$ m). The solid lines show simulated trajectories for the parent ${\mathrm{H}}_2{\mathrm{O}}^{+}$ ion (black line) and some of the heavy photofragments observed in the present experiment. The photoelectron detection capability was not exploited in the this study.

Figure 3

Momentum imaging of photofragments from ${\mathrm{H}}_2{\mathrm{O}}^{+}$ under 35.0 nm irradiation using the TIFF experimental system (Fig. 2). (a) Momentum imaging of light fragments with DET 1. White dashed lines mark the limitations imposed by the geometrical arrangement of the experimental system: lower line, effect of the central hole in DET 1 [46]; upper line, effect of the finite size of the detector surface (40-mm outer radius); middle line, limitation from the extended electrode structure close to the interaction region (see Fig. 2). Events observed between the two upper lines result from slightly nonconcentric alignment of the beam in the interaction region. Gray half circle, correlation for two-body breakup with a kinetic-energy release $E_k{m}_F/(m_I-m_F)=71.4$ eV ($E_k=4.2$ eV for $F=H^{+}$). Dashed gray lines, fragment emission angles ${\theta }_F={60}^{\circ }$ (left) and ${\theta }_F={135}^{\circ }$ (right) between which fragmentation detection is strongly limited outside the gray circle. (b) Momentum imaging of heavy neutral fragments (here O${}^0$) with DET 2. (c) Momentum imaging of light fragments on DET 1 requiring additionally coincidence with hits on DET 2 (O${}^0$). White dashed lines as in (a).

Figure 4

Experimental identification of photofragmentation channels of ${\mathrm{H}}_2{\mathrm{O}}^{+}$ under 35.0- and 21.8-nm irradiation using TIFF (Fig. 2). (a) Relative longitudinal momentum [see Eq. (2)] for particles impacting on DET 1 after photoabsorption at 35.0 nm obtained with the interaction zone nonbiased ($V_c=0$) (solid black line) and biased to $V_c=50$ V (dashed blue line). Essentially all fragments detected on DET 1 can be identified as charged, that is, H${}^{+}$ or H${}_2{}^{+}$ (no H${}^0$). (b) Distributions of the ratio of fragment kinetic energy and mass [$E_F/m_F$; see Eq. (6)] impacting on DET 1 after photofragmentation at 35.0 nm (solid black line) and 21.8 nm (gray line). The dashed red line shows the distributions for the coincidence DET 1 and DET 2, that is, corresponding to the momentum image on Fig. 3c. (c) Relative longitudinal momentum for particles impacting on DET 3 after photoabsorption at 21.8 nm. The offset of the observed peaks from 0 reflects the deceleration and bending of charged particles in the electrostatic mirror.

Figure 5

Photofragmentation ${\mathrm{H}}_2{\mathrm{O}}^{+}$ at 35.0 nm leading to three-body breakup into ${\mathrm{O}}^0+2{\mathrm{H}}^{+}$ studied by coincidence momentum imaging of the H${}^{+}$ fragments detected with DET 1 and O${}^0$ fragments detected with DET 2 (see Fig. 2). (a) Observed distribution (solid black line) of total kinetic-energy release $E_T$ [Eq. (10)]. The gray dashed line shows the geometrical efficiency of the detection systems obtained from Monte Carlo simulations. The ladders above the experimental distribution show the expected kinetic-energy releases [44] (see Fig. 1) for vertical transitions from the vibronic ground state of ${\mathrm{H}}_2{\mathrm{O}}^{+}$ to five states of ${\mathrm{H}}_2{\mathrm{O}}^{2+}$ followed by dissociation into three possible final states of ${\mathrm{O}}^0+2{\mathrm{H}}^{+}$. (b) Dalitz plot representing the momentum sharing in the fragmentation process [Eqs. (11) and (12)].

Figure 6

Photofragmentation of ${\mathrm{H}}_2{\mathrm{O}}^{+}$ at 35.0 nm leading to two-body breakup into ${\mathrm{OH}}^{+}+{\mathrm{H}}^{+}$ and ${\mathrm{O}}^{+}+{\mathrm{H}}_2{}^{+}$ at 35.0 nm studied by momentum imaging of the ${\mathrm{H}}^{+}$ or ${\mathrm{H}}_2{}^{+}$ fragment detected with DET 1 (Fig. 2). The black line shows the observed distribution of two-body kinetic-energy release with angular sections of events ${\theta }_F\le {60}^{\circ }$ or ${\theta }_F\ge {135}^{\circ }$ [Eq. (5)]. The gray line shows the distribution including all fragment angles at low values of $E_k(m_I-m_F)/m_F$. The ladders above the experimental distribution show the expected kinetic-energy releases [44] (see Fig. 1) for vertical transitions from the vibronic ground state of ${\mathrm{H}}_2{\mathrm{O}}^{+}$ to five states of ${\mathrm{H}}_2{\mathrm{O}}^{2+}$ followed by dissociation into three possible final states of ${\mathrm{OH}}^{+}+{\mathrm{H}}^{+}$.