Photoionization and fragmentation of H${}_3$O${}^{+}$ under XUV irradiation

The photolysis of the hydronium cation H${}_3$O${}^{+}$ has been studied at the extreme ultraviolet wavelengths of $35.56\pm 0.24$ nm ($34.87\pm 0.24$ eV) and $21.85\pm 0.17$ nm ($56.74\pm 0.44$ eV) using a crossed ion-photon beam setup at the free-electron laser FLASH. Coincidence photoelectron and photofragment spectroscopy was performed at 21.85 nm, where both inner and outer valence ionization are allowed, and revealed that the XUV photolysis of H${}_3$O${}^{+}$ is by far dominated by ionization of outer valence electrons forming the ${}^1{A}_1$ and ${}^{2}E$ states of the dication H${}_3$O${}^{2+}$. The dications were found to dissociate into the channels H${}_2$O${}^{+}$+H${}^{+}$ ($72\pm 4\%$), OH${}^0$+2H${}^{+}$ ($18\pm 6\%$), and OH${}^{+}$+H${}^{+}$+H${}^0$ ($10\pm 1\%$). A kinematic analysis of the H${}_2$O${}^{+}$+H${}^{+}$ channel after photoabsorption at 35.56 nm (where only outer valence ionization is possible) showed dissociation into excited states of the water radical ion, where the ${}^1{A}_1$ state breaks up into the linear $\tilde{A}{}^2{A}_1$ state of H${}_2$O${}^{+}$ and the ${}^{2}E$ state decays into the strongly bent $\tilde{B}{}^2{B}_2$ state. Finally, from the ${}^{2}E$ state of H${}_3$O${}^{2+}$, dissociation into OH${}^0(X{}^2\Pi$)+2H${}^{+}$ was identified to occur with a near linear dissociation geometry.

Figure 1

Illustration of the fragmentation channels accessible after valence shell photoionization of the hydronium ion (H${}_3$O${}^{+}$) at the applied XUV energies ($E_1$ and $E_2$). Calculated potential energy surfaces for the dication (H${}_3$O${}^{2+}$) electronic states in the Franck-Condon region of the ground state of H${}_3$O${}^{+}$ are not available; the energetic position of these states is estimated from the calculated orbital energies of H${}_3$O${}^{+}$ [23] and Koopmans’ theorem [32]. The energies of the fragmentation limits are derived from data available at NIST [33]. The blue dot (at $B{}^2{B}_2$ and $X{}^2\Pi$) and the red dot (at $A{}^2{A}_1$) mark final states identified after ionization into the ${}^{2}E$ and ${}^2{A}_1$ states of H${}_3$O${}^{2+}$, respectively.

Figure 2

(a) Illustration of the interaction region at the TIFF experiment [25, 29] at the FLASH facility [26, 27]. For the experiment at 35.56 nm, DET 3, the predeflector and the electrostatic mirror were not installed. The dashed arrows indicate the distances from the center of the interaction region to eDET 1 ($L_e=0.142$ m), to DET 1 ($L_1=0.263$ m), to DET 2 ($L_2=0.872$ m during measurement at 35.56 nm and $L_2=0.967$ m at 21.85 nm), as well as the horizontal distance from the interaction point to the most distant surface of the electrostatic mirror ($L_{3a}$) and upwards 150${}^{\circ }$ to the plane defined by the surface of DET 3 ($L_{3b}$); this yields $L_3=L_{3a}+L_{3b}=0.975$ m. Solid lines show simulated trajectories for the parent H${}_3$O${}^{+}$ ion (black line) and heavy photofragments observed in the present experiment. The trajectory of a photoelectron extracted perpendicular to the ion and photon beams is also illustrated. (b) Potential landscape of the interaction zone when configured with an electric saddle point potential to extract photoelectrons to eDET 1–2. Here, the electrode were biased to $E_{1-4}=X_{1-4}=0$, $C_{1-2}=-144$ V, $U_0=112$ V, $U_1=U_{2-7}=-30$ V, and $U_{8-9}=0$. The fronts of eDET 1–2 were biased to 100 V. (c) Photoelectrons registered on eDET 1 in coincidence with H${}_2$O${}^{+}$ fragments registered on DET 3 from correlated and noncorrelated ion-photon crossings (real and random coincidences, respectively). The spectra were obtained at 21.85 nm with reduced laser intensity ($\sim$2.9 $\mu$J/pulse).

Figure 3

Analysis of light photofragments (H${}^0$,H${}^{+}$) from H${}_3$O${}^{+}$ obtained at ${\lambda }_2=35.56$ nm using DET 1 (see Fig. 2) and with field-free conditions in the interaction region. (a) Momentum image using the coordinates defined in Eqs. (2) and (3). The lower dashed line indicates the acceptance limit imposed by the central hole of DET 1 while the upper dashed line indicates the limit imposed by the finite size of DET 1. The dotted lines mark the angular cuts (${\theta }_F\le {60}^{\circ }$ and ${\theta }_F\ge {130}^{\circ }$) applied in the kinetic energy analysis. (b) Ratio of fragment kinetic energy and fragment mass obtained from Eq. (6) using the angular cut indicated in (a). The full (blue) line shows the distribution obtained for all fragments observed on DET 1 while the dashed (red) line shows the distribution for particles detected in coincidence with a neutral fragment on DET 2. The latter has been multiplied by a factor of 3.3 to facilitate the comparison of the two distributions, compensating for the finite efficiency of DET 2.

Figure 4

Identification of fragmentation channels and analysis of channel branching ratios in the photolysis of H${}_3$O${}^{+}$ at ${\lambda }_1=21.85$ nm. Distributions of fragment longitudinal momentum release in normalized coordinates [Eq. (2)] are shown for the events registered on DET 1–3 (see Fig. 2) under field-free conditions in the interaction region. The diagonal panels show the total noncoincidence distributions, while the off-diagonal panels display the distributions obtained after coincidence analysis between events on the three detectors where also the background of random coincidences has been subtracted.

Figure 5

Distribution of photoelectrons emitted in the photolysis of H${}_3$O${}^{+}$ at ${\lambda }_1=21.85$ nm [upper full (black) curve], where the dimensionless coordinate $1-1/{\tau }_e$ is defined in Sec. 2c2. The electrons were detected on eDET 1 in coincidence with H${}_2$O${}^{+}$ fragments detected with DET 3 (see Fig. 2). In the lower part, simulated distributions of photoelectrons emitted isotropically ($\beta =0$) relative to the laser polarization and with different energies as indicated are shown.

Figure 6

Relative longitudinal fragment momenta [see Eq. (2)] in the three-body break-up of H${}_3$O${}^{2+}$ formed by ionization of H${}_3$O${}^{+}$ at 35.56 nm. The two-dimensional distribution of observed longitudinal momenta of fragments detected in coincidences with two hits on DET 1 and one hit on DET 2 (see Fig. 2) is shown. Sets of particles belonging to the fragmentation channel OH${}^0$+2H${}^{+}$ appear as a bright line corresponding to the condition for momentum conservation [Eq. (14)]. The white dotted lines indicate the range for which momentum conservation was considered fulfilled for the further analysis of kinetic energy release [Eq. (15)] and angular correlations [Eqs. (16) and (17)].

Figure 7

Photofragmentation of H${}_3$O${}^{+}$ at 35.56 nm leading to three-body break-up into OH${}^0$+2H${}^{+}$ obtained from coincidence detections of H${}^{+}$ fragments with DET 1 and OH${}^0$ fragments with DET 2 (see Fig. 2). (a) Total kinetic energy release (solid black line) $E_T$ [Eq. (15)]. The dashed line (blue) shows a Gaussian fit to the distribution. The ladders above the experimental distribution show the estimated expected kinetic energy releases (see Fig. 1) for vertical transitions from the rovibrational ground state of H${}_3$O${}^{+}$ to states of H${}_3$O${}^{2+}$ with subsequent dissociation into final states of OH${}^0$+2H${}^{+}$. (b) Dalitz plot representing the momentum sharing in the fragmentation process [Eqs. (16) and (17)].

Figure 8

Photofragmentation of H${}_3$O${}^{+}$ at 35.56 nm leading to H${}_2$O${}^{+}$+H${}^{+}$ studied by momentum imaging of the H${}^{+}$ fragment detected with DET 1 (Fig. 2). The black line shows the observed distribution of two-body kinetic energy releases for events with ${\theta }_F\le {60}^{\circ }$ and ${\theta }_F\ge {130}^{\circ }$ [Eq. (5)]. The thick dashed line (red) shows a fit to the distribution with two Gaussian distributions, while the thin dashed lines (red and blue) show the contributions from the individual Gaussians. The ladders above the experimental distribution show the expected kinetic energy releases (see Fig. 1) for vertical transitions from the rovibrational ground state of H${}_3$O${}^{+}$ into states of H${}_3$O${}^{2+}$ followed by dissociation into final states of the H${}_2$O${}^{+}$+H${}^{+}$ system.