Brandon Jordon-Thaden

Imaging the Absolute Configuration of a Chiral Epoxide in the Gas Phase

Foil-Forged Images X-ray diffraction is widely used to determine molecular geometries and can often distinguish mirror image isomers (enantiomers), which generally requires well-ordered crystals. Herwig et al. (p. 1084) report an imaging technique to characterize enantiomers in the gas phase. A succession of ionization events were induced by passage through a carbon foil that culminated in a Coulomb explosion of mutually repelling nuclei. The trajectories of these nuclei precisely reflected the original molecular structure. Ultrafast electron stripping by a carbon foil enables precise elucidation of molecular geometries as the nuclei fly apart. In chemistry and biology, chirality, or handedness, refers to molecules that exist in two spatial configurations that are incongruent mirror images of one another. Almost all biologically active molecules are chiral, and the correct determination of their absolute configuration is essential for the understanding and the development of processes involving chiral molecules. Anomalous x-ray diffraction and vibrational optical activity measurements are broadly used to determine absolute configurations of solid or liquid samples. Determining absolute configurations of chiral molecules in the gas phase is still a formidable challenge. Here we demonstrate the determination of the absolute configuration of isotopically labeled (R,R)-2,3-dideuterooxirane by foil-induced Coulomb explosion imaging of individual molecules. Our technique provides unambiguous and direct access to the absolute configuration of small gas-phase species, including ions and molecular fragments.

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.

Coulomb Explosion Imaged Cryptochiral (R,R)-2,3-Dideuterooxirane: Unambiguous Access to the Absolute Configuration of (+)-Glyceraldehyde

The absolute configuration of (R,R)-2,3-dideuterooxirane, which has been independently determined using Coulomb explosion imaging, has been unambiguously chemically correlated with the stereochemical key reference (+)-glyceraldehyde. This puts the absolute configuration of D(+)-glyceraldehyde on firm experimental grounds.

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.

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.

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.

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.

Metastable states of diatomic hydrogen anions

Transient states of the negative hydrogen molecule H−2 have drawn attention as intermediate reaction complexes in important molecular reactions like associative detachment (AD: H− + H → H−2→ H−2 + e−) and dissociative attachment (DA: e− + H2 → H−2 → H + H−). Recently it has been shown that metastable states of hydrogen molecular anions exist that defy both auto detachment and spontaneous dissociation for several microseconds (in the case of H−2) and even milliseconds (in the case of D−2). Here we present Coulomb explosion measurements for H−2 and D−2 that provide detailed information on the shape of the molecular wave function. We compare these experimental results with calculated wave functions obtained using a nonlocal resonance model. Our measurements confirm the predicted stabilization of H−2 and D−2 in states of high angular momentum.

XUV photofragmentation of small water cluster cations at FLASH

Photofragmentation of small water cluster ions H+(H2O)n (n = 1..4) has been investigated at 35eV using fast beam momentum imaging in a crossed beams setup at the free electron laser FLASH at DESY in Hamburg, revealing the dominant fragmentation pathways of these systems under ionizing radiation.