The photofragmentation of the water cation H 2 O + through dicationic states has been studied at 35.0 ± 0.2 nm (35.4 ± 0.3 eV) and 21.8 ± 0.2 nm (56.8 ± 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 O 0 + 2H + , OH + + H + , and O + + H 2 + , with determined ratios σ OH + +H + /σ O + +H 2 + = 4.2 ± 0.3 and σ OH + +H + /σ O 0 +2H + > 0.7. The measured kinetic-energy releases for these processes are consistent with three-body breakup (O 0 + 2H +) occurring mainly through the 2 3 A and 2 1 A states of H 2 O 2+ and two-body breakup (OH + + H +) occurring through X 3 A , 1 3 A , and 1 1 A 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.

We describe the design of and the first commissioning experiments with a newly constructed electrostatic storage ring named SAPHIRA (Storage Ring in Aarhus for PHoton-Ion Reaction Analysis). With an intense beam of Cu(-) at 4 keV, the storage ring is characterized in terms of the stored ion beam decay rate, the longitudinal spreading of an injected ion bunch, as well as the direct measurements of the transverse spatial distributions under different conditions of storage. The ion storage stability in SAPHIRA was investigated systematically in a selected region of its electrical configuration space.

The decay of metastable states of the water radical cation H 2 O + has been observed in an experiment that combines photofragment momentum imaging and electrostatic ion-beam trapping in a crossed-beam geometry. Photoabsorption of 532-nm laser light from a fast beam of H 2 O + is observed to yield fragmentation into both OH 0 + H + and OH + + H 0 . Using coincident photofragment momentum imaging, the initial state of the observed photofragmentation is associated with low vibrational levels of the second excitedB 2 B 2 state of H 2 O + , dissociating via absorption onto a repulsive part of theà 2 A 1 state. Electrostatic ion trapping in the laser interaction region is used to follow the photofragment intensity as a function of time and to determine the lifetime of the metastable states to be τB 2 B 2 = 198 ± 11 μs.

Single and double photodetachment of the oxygen anion O − have been investigated at 41.7 nm (29.8 eV) in a crossed beams experiment using intense photon pulses from a free-electron laser. The ratio of single (O 0 + e −) and double (O + + 2e −) detachment was determined to be σ O 0 /σ O + = 4.12± 0.17 as identified directly from the yield of O 0 and O + fragments after irradiation. The absolute cross section for the dominating single detachment channel was measured to σ O 0 = (2.1 ± 0.6) × 10 −19 cm 2. Analysis of photoelectrons detected in coincidence with neutral fragments (O 0) suggests that single photodetachment primarily happens via the ground (3 P) or lowest excited (1 D) state of oxygen. The results demonstrate the feasibility and advantage of crossed beams experiments for complete studies of photodetachment reactions.

The photolysis of the hydronium cation H 3 O + has been studied at the extreme ultraviolet wavelengths of 35.56 ± 0.24 nm (34.87 ± 0.24 eV) and 21.85 ± 0.17 nm (56.74 ± 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 ± 4%), OH 0 + 2H + (18 ± 6%), and OH + + H + + H 0 (10 ± 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 linearà 2 A 1 state of H 2 O + and the 2 E state decays into the strongly bentB 2 B 2 state. Finally, from the 2 E state of H 3 O 2+ , dissociation into OH 0 (X 2 ) + 2H + was identified to occur with a near linear dissociation geometry.