Mario B. Mendes

COLD ELECTRON REACTIONS PRODUCING THE ENERGETIC ISOMER OF HYDROGEN CYANIDE IN INTERSTELLAR CLOUDS

Using event-by-event fragment momentum spectroscopy in a storage-ring merged-beams experiment, we find laboratory evidence that in the dissociative recombination (DR) of HCNH{sup +} with cold electrons the energetic isomer HNC is produced with a high yield, similar to that of HCN. With a newly implemented mass-sensitive fragment imaging detector, we analyze the kinetic energy release of the triatomic fragments DCN/DNC from the DR reaction of the isotopologue DCND{sup +} with cold (near 10 K) electrons. The results show that the internal energy of these fragments is extremely high, far exceeding the isomerization barrier between DNC and DCN. From this laboratory characterization of the DR reaction we conclude that also the triatomic fragment HCN/HNC from the DR of HCNH{sup +} will carry a large amount of ro-vibrational excitation and show that this implies an isomeric production ratio in a narrow range near unity.

Fragmentation Channels in Dissociative Electron Recombination with Hydronium and Other Astrophysically Important Species

We report on our recent studies of dissociative recombination (DR) employing two different fragment imaging detection techniques at the TSR storage ring in Heidelberg, Germany. Principles of an upgraded 3D optical system and the new energy-sensitive multistrip detector (EMU) are explained together with possible applications in reaction dynamics studies. With the EMU imaging detector we succeeded to observe the branching ratios after DR of deuterated hydronium ions D(3)O(+) at energies of 0-0.5 and 4-21 eV. The branching ratios are almost constant at low energies while above 6 eV both oxygen-producing channels O + D + D + D and O + D(2) + D strongly increase and dominate by about 85% at 11 eV. To demonstrate further capabilities of our fragment imaging detectors, we also summarize some of our additional recent studies on DR of molecular ions important for astrophysics as well as for fundamental unimolecular dynamics.

Energy-sensitive imaging detector applied to the dissociative recombination of D2H+

We report on an energy-sensitive imaging detector for studying the fragmentation of polyatomic molecules in the dissociative recombination of fast molecular ions with electrons. The system is based on a large area (10x10 cm{sup 2}) position-sensitive, double-sided Si-strip detector with 128 horizontal and 128 vertical strips, whose pulse height information is read out individually. The setup allows us to uniquely identify fragment masses and is thus capable of measuring branching ratios between different fragmentation channels, kinetic energy releases, and breakup geometries as a function of the relative ion-electron energy. The properties of the detection system, which has been installed at the Test Storage Ring (TSR) facility of the Max-Planck Institute for Nuclear Physics in Heidelberg, is illustrated by an investigation of the dissociative recombination of the deuterated triatomic hydrogen cation D{sub 2}H{sup +}. A huge isotope effect is observed when comparing the relative branching ratio between the D{sub 2} + H and the HD + D channel; the ratio 2B(D{sub 2} + H)/B(HD + D), which is measured to be 1.27{+-}0.05 at relative electron-ion energies around 0 eV, is found to increase to 3.7{+-}0.5 at {approx}5 eV.

Electron collisions and rovibrational action spectroscopy of cold H3+ molecules

Electron recombination of H3+ has found a lot of attention due to its outstanding relevance for the chemistry of the interstellar medium (ISM) and its role as a benchmark for the treatment of dissociative recombination (DR) of polyatomic ions. We report DR measurements performed at the TSR storage ring utilizing a cryogenic ion trap injector. Furthermore, a chemical probing spectroscopy technique is described that allows for a very sensitive monitoring of the populated states inside the ion injector. Since H3+ exists in two different nuclear spin modifications, a controlled manipulation of the ortho/para fraction is needed in order to perform state-selective measurements.

Spectroscopy and dissociative recombination of the lowest rotational states of H+3

The dissociative recombination of the lowest rotational states of H3+ has been investigated at the storage ring TSR using a cryogenic 22-pole radiofrequency ion trap as injector. The H3+ was cooled with buffer gas at ~15 K to the lowest rotational levels, (J, G)=(1,0) and (1,1), which belong to the ortho and para proton-spin symmetry, respectively. The rate coefficients and dissociation dynamics of H3+(J, G) populations produced with normal-and para-H2 were measured and compared to the rate and dynamics of a hot H3+ beam from a Penning source. The production of cold H3+ rotational populations was separately studied by rovibrational laser spectroscopy using chemical probing with argon around 55 K. First results indicate a ~20% relative increase of the para contribution when using para-H2 as parent gas. The H3+ rate coefficient observed for the para-H2 source gas, however, is quite similar to the H3+ rate for the normal-H2 source gas. The recombination dynamics confirm that for both source gases, only small populations of rotationally excited levels are present. The distribution of 3-body fragmentation geometries displays a broad part of various triangular shapes with an enhancement of ~12% for events with symmetric near-linear configurations. No large dependences on internal state or collision energy are found.

Ultraviolet and Visible Light Photodissociation of H3+ in an Ion Storage Ring

Ultraviolet and visible photodissociation of a vibrationally excited H(3)(+) ion beam, as produced by standard ion sources, was successfully implemented in an ion storage ring with the aim of investigating the decay of the excited molecular levels. A collinear beams configuration was used to measure the photodissociation of H(3)(+) into H(2)(+) + H fragments by transitions into the first excited singlet state with 266 and 532 nm laser beams. A clear signal could be observed up to 5 ms of storage, indicating that enough highly excited rovibrational states survive on the millisecond time scale of the experiment. The decay into H(2)(+) + H shows an effective time constant between about 1 and 1.5 ms. The initial photodissociating states are estimated to lie roughly 1 eV below the dissociation limit of 4.4 eV. The expected low population of these levels gives rise to an effective cross section of several 10(-20) cm(2) for ultraviolet and some 10(-21) cm(2) for visible light. For using multistep resonant dissociation schemes to monitor rotational populations of cold H(3)(+) in low-density environments, these measurements open promising perspectives.

Low-energy collisions with atomic and molecular ions in a photocathode electron target

Dielectronic recombination of highly charged atomic ions and dissociative recombination of molecular ions have been investigated using fast ion beams merged with cold intense electron beams (down to <1meV beam temperature) from a GaAs photocathode. For beryllium-like germanium ions (Ge28+) isolated fine structure Rydberg resonances (n = 9 and 14) were observed at <0.2eV revealing configuration interaction with triply excited resonances and appearing as candidates for deriving radiative shifts of the 2s2-2s2p(3P0) excitation energy. For hydrogen fluoride ions (HF+) dissociative recombination with the cold electrons, producing n = 2 hydrogen atoms, was seen to occur for specific excited initial rotational levels. These levels were identified by their fragment energies down to about 4meV and it is envisaged to measure these energies with sub-meV accuracy.

Branching ratios in dissociative recombination of formyl and isoformyl cations

The energy dependence of the branching ratios in dissociative recombination of DCO+ with a known small admixture of DOC+ has been measured for collision energies from 0 to 25eV using an energy- and position-sensitive surface barrier detector which is designed for the analysis of multi-fragment events occurring in a molecular fragmentation study. The measurements are compared with theoretical calculations on the direct mechanism of dissociative recombination of HCO+ including the contribution from HOC+ at the experimental abundance fraction. At low collision energies, dissociative recombination of HCO+ is dominated by dissociation into H + CO. For collision energies above 2eV there is a transition into dissociation to HC+O, which can be explained by electron capture into resonant states. Signatures of DOC+ dissociative recombination are found experimentally and confirmed by the calculations for HOC+. Three-body breakup becomes important for collision energies above 6eV.

Anisotropic fragmentation in low-energy dissociative recombination

On a dense energy grid reaching up to 75 meV electron collision energy the fragmentation angle and the kinetic energy release of neutral dissociative recombination fragments have been studied in a twin merged beam experiment. The anisotropy and the extracted rotational state contributions were found to sensitively depend on energy. Both show pronounced variations on a likewise narrow energy scale as the rotationally averaged rate coefficient. For the first time angular dependences described by Legendre polynomials higher than 2nd order could be deduced. Moreover, a slight anisotropy at zero collision energy was observed which is caused by the flattened velocity distribution of the electron beam.

Dissociative recombination of D2Cl+ and other astrophysically relevant polyatomic ions

We report on dissociative recombination measurement of D2Cl+ using low-energy-spread merged electron and ion beams.