We have measured dissociative recombination of HCl + with electrons using a merged beams configuration at the heavy-ion storage ring TSR located at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. We present the measured absolute merged beams recombination rate coefficient for collision energies from 0 to 4.5 eV. We have also developed a new method for deriving the cross section from the measurements. Our approach does not suffer from approximations made by previously used methods. The cross section was transformed to a plasma rate coefficient for the electron temperature range from T = 10 to 5000 K. We show that the previously used HCl + DR data underestimate the plasma rate coefficient by a factor of 1.5 at T = 10 K and overestimate it by a factor of 3.0 at T = 300 K. We also find that the new data may partly explain existing discrepancies between observed abundances of chlorine-bearing molecules and their astrochemical models.

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.

The dissociation pathways of HeH + have been investigated below the first ionization continuum by photoabsorption at 32 nm, using fragment momentum imaging in a crossed-beams experiment at the free-electron laser in Hamburg (FLASH). Investigations were done both for ions with several vibrational levels excited in the ion source and for ions vibrationally cooled in an electrostatic ion trap prior to the irradiation. The product channels He + (1s) + H(nl) and He(1snl) + H + were separated and the He(1snl) + H + channel was particularly studied by coincidence detection of the He and H + fragments on two separate fragment detectors. At 32 nm excitation, the branching ratio between the product channels was found to be σ He + +H /σ He+H + = 0.96 ± 0.11 for vibrationally hot and 1.70 ± 0.48 for vibrationally cold ions. The spectra of kinetic energy releases for both channels revealed that photodissociation at 32 nm leads to high Rydberg states (n 3-4) of the emerging atomic fragments irrespective of the initial vibrational excitation of HeH +. The fragment angular distributions showed that dissociation into the He + H + channel mostly (∼70%) proceeds through 1 states, while for the He + + H channel 1 and 1 states are of about equal importance.

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.

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.