L. Lammich

Dissociative recombination and low-energy inelastic electron collisions of the helium dimer ion

The dissociative recombination (DR) of He-3 He-4(+) has been investigated at the heavy-ion Test Storage Ring (TSR) in Heidelberg by observing neutral products from electron-ion collisions in a merged beams configuration at relative energies from near-zero (thermal electron energy about 10 meV) up to 40 eV. After storage and electron cooling for 35 s, an effective DR rate coefficient at near-zero energy of 3 x 10(-9) cm(3)s(-1) is found. The temporal evolution of the neutral product rates and fragment imaging spectra reveals that the populations of vibrational levels in the stored ion beam are nonthermal with fractions of similar to 0.1-1% in excited levels up to at least v=4, having a significant effect on the observed DR signals. With a pump-probe-type technique using DR fragment imaging while switching the properties of the electron beam, the vibrational excitation of the ions is found to originate mostly from ion collisions with the residual gas. Also, the temporal evolution of the DR signals suggests that a strong electron induced rotational cooling occurs in the vibrational ground state, reaching a rotational temperature near or below 300 K. From the absolute rate coefficient and the shape of the fragment imaging spectrum observed under stationary conditions, the DR rate coefficient from the vibrational ground state is determined; converted to a thermal electron gas at 300 K it amounts to (3.3 +/- 0.9) x 10(-10) cm(3)s(-1). The corresponding branching ratios from v=0 to the atomic final states are found to be (3.7 +/- 1.2) % for 1s2s S-3, (37.4 +/- 4.0) % for 1s2s S-1, (58.6 +/- 5.2) % for 1s2p P-3, and (2.9 +/- 3.0) % for 1s2p P-1. A DR rate coefficient in the range of 2 x 10(-7) cm(3)s(-1) or above is inferred for vibrational levels v=3 and higher. As a function of the collision energy, the measured DR rate coefficient displays a structure around 0.2 eV. At higher energies, it has one smooth peak around 7.3 eV and a highly structured appearance at 15-40 eV. The small size of the observed effective DR rate coefficient at near-zero energy indicates that the electron induced rotational cooling is due to inelastic electron-ion collisions and not due to selective depletion of rotational levels by DR.

Assignment of resonances in dissociative recombination of HD + ions: high-resolution measurements compared with accurate computations

The collision-energy resolved rate coefficient for dissociative recombination of HD(+) ions in the vibrational ground state is measured using the photocathode electron target at the heavy-ion storage ring TSR. Rydberg resonances associated with rovibrational excitation of the HD(+) core are scanned as a function of the electron collision energy with an instrumental broadening below 1 meV in the low-energy limit. The measurement is compared to calculations using multichannel quantum defect theory, accounting for rotational structure and interactions and considering the six lowest rotational energy levels as initial ionic states. Using thermal-equilibrium-level populations at 300 K to approximate the experimental conditions, close correspondence between calculated and measured structures is found up to the first vibrational excitation threshold of the cations near 0.24 eV. Detailed assignments, including naturally broadened and overlapping Rydberg resonances, are performed for all structures up to 0.024 eV. Resonances from purely rotational excitation of the ion core are found to have similar strengths as those involving vibrational excitation. A dominant low-energy resonance is assigned to contributions from excited rotational states only. The results indicate strong modifications in the energy dependence of the dissociative recombination rate coefficient through the rotational excitation of the parent ions, and underline the need for studies with rotationally cold species to obtain results reflecting low-temperature ionized media.

Soft-x-ray fragmentation studies of molecular ions

Imaging of photofragments from molecular ions after irradiation by soft x-ray photons has been realized at the ion beam infrastructure TIFF set up at the FLASH facility. Photodissociation of the two-electron system HeH+ at 38.7 eV revealed the electronic excitations and the charge-state ratios for the products of this process, reflecting the non-adiabatic dissociation dynamics through multiple avoided crossings among the HeH+ Rydberg potential curves. Dissociative ionization of the protonated water molecules H3O+ and H5O+2 at 90 eV revealed the main fragmentation pathways after the production of valence vacancies in these ionic species, which include a strong three-body channel with a neutral fragment (OH + H+ + H+) in H3O+ photolysis and a significant two-body fragmentation channel (H3O++ H2O+) in H5O+2 photolysis. The measurements yield absolute cross sections and fragment angular distributions. Increased precision and sensitivity of the technique were realized in recent developments, creating a tool for exploring x-ray excited molecular states under highly controlled target conditions challenging detailed theoretical understanding.

Electron Induced Vibrational Deexcitation of the Molecular Ions H2+ and D2+

The interaction of molecular ions with low kinetic energy electrons has been studied extensively at different heavy-ion storage rings during the last ten years. The usage of storage rings made it possible to store infrared-active molecular ion beams, such as HD+, for a time that is sufficiently long so that these ions are completely relaxed to their vibrational ground state by spontaneous emission of radiation. The stored ion beams are then merged with an intense and cold electron beam by using the electron cooling device present in the ion storage ring as a target for electron-ion collision studies. Here we report on the extension of such experiments to the homonuclear hydrogen ions which do not cool radiatively and for which we hence can study electron-impact vibrational de-excitation.

Physics with molecular ions in storage rings

We report on the possibility to produce homonuclear vibrationally cold molecular ion beams (using D as an example) via the interaction of cold electrons with a stored beam. We demonstrate that cooling of the vibrational degrees of freedom can be achieved within a reasonable time, which is comparable to the beam lifetime. We also present preliminary results on the electron impact vibrational excitation of HD+

DR rate coefficient measurements using stored beams of H3+ and its isotopomers

In studies of the rate coefficient of the dissociative recombination of H3+ and its isotopomers, the rovibrational excitation of the molecular ions was found to play an important role, in particular when employing the technique of heavy-ion storage rings. The dependence of the DR rate on rotational excitation was investigated in recent experiments at the Test Storage Ring TSR in Heidelberg through time-resolved measurements on D2H+ and H3+ over long storage times. For both molecules, an influence of rotational excitation on the DR rate was observed. The level of excitation in turn was found to be dominated by radiative coupling to the surrounding 300 K background for D2H+. In the case of H3+, a strong influence of electron collisions on the excitation level was found, whereas an additional influence of collisions with residual gas in the storage ring cannot be excluded.

Multiparticle Imaging of Fast Molecular Ion Beams

For many years, two-dimensional (2D) and three-dimensional (3D) fragment imaging techniques have been successfully used in the study of molecular structure [1] and for the study of the dynamics of various molecular dissociation processes, such as photodissociation [2] , dissociative recombination [3], atom—molecule collision-induced dissociation [4] , and dissociative charge exchange [5] . For fast molecular ion beams (in the present context, fast means kinetic energies in the range of keV to several MeV), the basic experimental scheme includes the induced dissociation of a single molecule from the beam, and the fully correlated measurement of the asymptotic velocity vectors of the outgoing atomic and molecular fragments. If the initial velocity of the molecule is large, then all the fragments will be projected into a cone defined by the ratio of their transverse velocities and the initial beam velocity. In such a case, the transverse velocities are deduced from the 2D position on the surface of a position—sensitive detector, while the longitudinal velocities can be derived from the (relative) time of arrival at the detector. The specific physical information provided by the images depends on the particular dissociation process. In general, one obtains information about the initial molecular quantum state prior to the dissociation, the final state of the fragments and about the dynamics of the reaction, such as angular dependence, kinetic energy release or potential curves.

Soft X-Ray Photolysis of the Hydronium Ion

Photolysis processes in H3O+ have been observed with fragment momentum imaging at the free electron laser FLASH. The observed fragmenation patterns and measured kinetic energy release distributions show that photolyis of H3O+ proceeds via valence ionization. The observed final dissociation products and fragment molecular states are strongly dependent on the ionized valence shell.

Electron collisions with 4He2+ at the TSR

The dissociative recombination (DR) of 4He2+ has been investigated at the heavy-ion storage ring TSR in Heidelberg. Rate coefficients were measured up to collision energies of 40 eV. Vibrational level populations were monitored using the Coulomb explosion imaging technique showing relaxation to the v = 0 level (>95%) through collisions with cold electrons within 50s. Low-energy DR rate coefficients are derived for v = 0, 1 and ≥2 which show a strong v-dependence. A low-energy super-elastic collision (SEC) rate coefficient of αv = 1→0SEC (Ed = 0)≈1.8×10−7cm3s−1 was found.

Electron-impact detachment from PO{sub n}{sup -} (n=0-3)

Electron detachment from