Dennis Bing

Chemical probing spectroscopy of H3+ above the barrier to linearity

We have performed chemical probing spectroscopy of H(3) (+) ions trapped in a cryogenic 22-pole ion trap. The ions were buffer gas cooled to approximately 55 K by collisions with helium and argon. Excitation to states above the barrier to linearity was achieved by a Ti:sapphire laser operated between 11 300 and 13 300 cm(-1). Subsequent collisions of the excited H(3) (+) ions with argon lead to the formation of ArH(+) ions that were detected by a quadrupole mass spectrometer with high sensitivity. We report the observation of 17 previously unobserved transitions to states above the barrier to linearity. Comparison to theoretical calculations suggests that the transition strengths of some of these lines are more than five orders of magnitude smaller than those of the fundamental band, which renders them-to the best of our knowledge-the weakest H(3) (+) transitions observed to date.

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.

Comment on: “Lorentz violation in high-energy ions” by Santosh Devasia

In an article ”Missing Transverse-Doppler Effect in Time-Dilation Experiments with High-Speed Ions” by S. Devasia [arXiv:1003.2970v1], our recent Doppler shift experiments on fast ion beams are reanalyzed. Contrary to our analysis, Devasia concludes that our results provide an ”indication of Lorentz violation”. We argue that this conclusion is based on a fundamental misunderstanding of our experimental scheme and reiterate that our results are in excellent agreement with Special Relativity. We have performed experiments of the Ives-Stilwell (IS) type [1] that test time dilation of Special Relativity (SR) via the relativistic Doppler shift [2,3,4,5]. A beam of ions, which exhibit an optical transition with a frequency ν0 in their rest frame, is stored at velocity β = v/c in a storage ring. To resonantly excite these ions by a laser at rest in the laboratory frame, the frequency ν of the laser needs to be Doppler shifted according to ν = ν0/γ(1 − β cos θ), where θ is the angle between the laser and the ion beam, measured in the laboratory frame, and γ governs time dilation. For a parallel (θp = 0) or an antiparallel (θa = π) laser beam the frequencies required are νp,a = ν0/γ(1 ∓ β), respectively. Multiplying these two frequencies and using γ = (1 − β) as predicted by SR results in νpνa/ν 2 0 = 1, (1) i.e. the geometric mean of the Doppler shifted frequencies equals the rest frame frequency for all velocities β. In one of our implementations of the IS experiment saturation spectroscopy is used by overlapping simultaneously a parallel and antiparallel laser beam with the ion beam to select a narrow velocity class β0 within the ions’ velocity distribution. The parallel laser is held fixed at the laser frequency νp = ν0/γ(1 − β0) and is resonant with ions at β0, while the other laser is scanned over the velocity distribution. The fluorescence yield, measured with a photomultiplier (PMT) located around 90 degree with respect to the ion beam, will exhibit a minimum (a Lamb dip) when the antiparallel laser talks to the same velocity class β0, i.e. when its frequency is at νa = ν0/γ(1 + β0). SR thus predicts the Lamb dip to occur when Eq. 1 is fulfilled, which is shown to be confirmed by our experiments to an accuracy of < 2 × 10 on Li ions at β0 = 0.03 and β0 = 0.06 [3]. S. Devasia [6] claims that the Doppler shift of the emitted light has to be taken into account and replaces ν0 in Eq. 1 by γν0, i.e. by the frequency of the light detected exactly at θ = π/2. This is a misconception of our experimental measurement scheme. While it is true that the detected light is Doppler-shifted, this Doppler shift is irrelevant for the analysis. Neither do we measure the frequency ν0 β 0 PMT ν a =ν 0 /γ(1+β 0 ) IF ν p =ν 0 /γ(1−β 0 ) of the emitted light nor do we intend to observe at exactly right angle. We only record the number of re-emitted photons as a function of the scanning laser frequency to monitor the Lamb dip caused by the simultaneous resonance of both lasers with the same ions. Thus the angle of detection is irrelevant but θ ≈ π/2 helps to separate fluorescence from laser stray light. In fact, stray light suppression is the only reason for using an interference filter (IF) in front of the PMT; its transmission width of 10 nm corresponds to 10 THz, about 10 times broader than the width of the Lamb dip, and a factor of 10 larger than the transverse Doppler shift (at β = 0.064). None of the filters employed in our experiments [2,3,4,5] to improve the signal-to-noise ratio in the fluorescence light detection are affecting the shape and position of the signal indicating the resonance of the parallel and antiparallel laser with the same velocity class β0. The frequency ν0 occurring in Eq. 1 has nothing to do with the frequency of the emitted light in our experiment, but is the rest frame frequency ν0 deduced from experiments at smaller ion velocities [3,7]. In conclusion, SR predicts Eq. 1 as the outcome of our experiments, which is confirmed with high accuracy.

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.

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.

Preparatory measurements for a test of time dilation in the ESR

We present preparatory measurements for an improved test of time dilation at the experimental storage ring (ESR) at GSI in Darmstadt. A unique combination of particle accelerator experiments and laser spectroscopy is used to perform this test with the highest precision. 7Li+ ions are accelerated to 34% of the speed of light at the GSI Helmholtzzentrum fur Schwerionenforschung and stored in the experimental storage ring. The forward and backward Doppler shifts of an electric dipole transition of these ions are measured with laser spectroscopy techniques. From these Doppler shifts, both the ion velocity β = ν/c and the time dilation factor γ=γSR(1+αˆβ2) can be derived for testing Special Relativity. Two laser systems have been developed to drive the 3S1→3P2 transition in 7Li+. Moreover, a detector system composed of photomultipliers, both to monitor the exact laser ion beam overlap as well as to optimize fluorescence detection, has been set up and tested. We investigate optical-optical double-resonance spec...

Dissociative recombination of CF+: Experiment and theory

We present results from our recent studies of the dissociative recombination of the CF+ cation. On one hand, dissociative recombination was measured with 3 MeV CF+ ions in the heavy-ion Test Storage Ring in Heidelberg, using the twin electron beam configuration with an electron cooler and a separated electron target for collision measurements. In this experiment, the low temperatures of the electron beam provided by a photocathode (temperature in co-moving frame below 1 meV) account for a fast kinetic cooling of the heavy-ion beam and a high resolution in the measured rate coefficients. Fragment imaging measurements show a complete switching of the dissociation route by only a small change of the collision energy and the disappearance of neutral Rydberg product states on crossing the DE threshold. On the other hand, extensive calculations of energy positions and autoionization widths for the doubly excited states of CF between the first and second ionization thresholds have been obtained from electron scattering calculations using the complex Kohn variational method, followed by calculations of the dissociative recombination process with the multichannel quantum defect theory. In preliminary computations, only the first dissociative state in each molecular symmetry, which lies closest in energy to the ion potential at its equilibrium internuclear separation, and thus is dominant for the low-energy dissociative recombination, was included. Although only the direct mechanism of dissociative recombination reaction has been considered in this step, the size and the shape of the DR rate coefficient are already well reproduced.