L. H. Andersen

Dissociative Recombination of H3O+, HD2O+, and D3O+

We present an experimental study of the dissociative recombination (DR) of H3O+ and its isotopomers D3O+ and HD2O+ performed at the ASTRID storage ring. DR cross sections have been measured as a function of energy, while complete branching ratios have been measured at E = 0. The H3O+ data yield an accurate determination of the branching ratio for water formation (0.25 ± 0.01). The three molecular ions—H3O+, D3O+, and HD2O+—show a marked resemblance concerning cross sections and branching ratios. The only observed isotope effect is in the fragmentation pattern of HD2O+, where the release of a light fragment is favored over release of a heavier fragment. As a consequence, an enhanced production of deuterated molecules takes place as a result of the DR process.

Complete Branching Ratios for the Dissociative Recombination of H2O+, H3O+, and CH3+

Dissociative recombination of the polyatomic ions H2O+, H3O+, and CH+3 with electrons has been measured at the heavy-ion storage ring ASTRID. Complete branching ratios for all the possible product channels have been determined at zero relative energy using an energy-sensitive detector masked by grids with known transmissions. In the dissociative recombination of H3O+, water molecules are produced with a probability of 33%, whereas the production of atomic oxygen is negligible. Atomic carbon is, on the other hand, produced with a branching ratio of 30% in the dissociative recombination of CH+3. For all three molecular ions, the three-particle breakup is a major process. Relative cross sections for dissociative recombination of H3O+ and for dissociative excitation of H3O+ have been measured for relative electron energies up to 40 eV. Implications for the modeling of the chemistry of interstellar molecular clouds are discussed.

Laser photodetachment of C60− and C70− ions cooled in a storage ring

Abstract Results are reported of a new type of experiment in which a heavy ion storage ring is utilized to cool fullerene ions, C 60 − and C 70 − , and to carry out laser photodetachment experiments in order to measure the electron affinities of ions possessing the minimum amount of internal energy. Fullerene negative ions are stored in the ring for periods of up to 5 s, and the time evolution of the electron affinity is measured, yielding limiting values of 2.666 ± 0.001 and 2.676 ± 0.001 eV for cold C 60 − and C 70 − , respectively.

Physics with electrostatic rings and traps

The recent development of electrostatic devices which allow us to store keV ion beams has launched several new kinds of investigations. We review the basic ideas behind the development of the electrostatic ion storage ring and the electrostatic ion beam trap techniques. The various experiments performed with atomic and molecular ion beams, ranging from the measurement of lifetimes of metastable atomic states up to biological applications and single component plasma studies are discussed.

Long-lived, doubly charged diatomic and triatomic molecular ions

Lifetimes of doubly charged diatomic and triatomic molecules have been measured by monitoring the decay curves of such ions in a heavy-ion storage ring. CO2+, N22+, CO22+, CS22+ and SH2+ are all found to possess long-lived components which survive for time periods greater than a few seconds. All these dications are found to be essentially stable and their ultimate destruction is due to interactions with residual gases in the ring. CO2+ possesses many more lifetime components in the millisecond range than the isoelectronic N22+ ion. Translational energy spectrometry experiments on the latter species also fail to reveal any short-lived (microsecond) components. Ab initio configuration interaction calculations have been carried out and the potential energy curve for the lowest-energy metastable state of N22+ (1 Sigma g) has been determined, along with Franck-Condon factors for vertical transitions to different vibrational levels from the ground state of neutral N2; tunnelling times of each vibrational level have been computed.

Experimental studies of the photophysics of gas-phase fluorescent protein chromophores

To better understand the photophysics of photoactive proteins, the absorption bands of several gas-phase biomolecules have been studied recently at the electrostatic heavy-ion storage ring ELISA by a photo-fragmentation technique. In the present paper we discuss the involved photophysics and photochemistry for protonated and deprotonated model chromophores of the Green Fluorescent Protein (GFP) and the Red Fluorescent Protein (RFP). We show specifically that the delayed dissociation after photoabsorption can be understood in terms of a thermally activated process of the Arrhenius type. The rate of dissociation as a function of time after laser excitation is modeled in a calculation which is based on calculated heat capacities of the chromophores. Absorption of only one photon is enough to dissociate the deprotonated GFP chromophore on a time scale of milliseconds whereas absorption of two to three photons occurs for other chromophore ions. The difference is attributed to different activation energies, pre-exponential factors and locations of the absorption bands. We obtain activation energies for the dissociation that are of the order of 1–3 eV. Collision-induced dissociation experiments were performed to help identifying the fragmentation channels. Loss of methyl is found to be the dominant fragmentation channel for the deprotonated GFP chromophore and is also likely to be important for the protonated GFP chromophore at high temperatures. Other channels are open for the RFP chromophores. For the deprotonated RFP chromophore there is evidence that dissociation occurs through a non-trivial dissociation with substantial rearrangement.

An experimental investigation of double ionisation of helium atoms in collisions with fast, fully stripped ions

Experimental cross sections for single and double ionisation of helium by 0.13-15 MeV AMU-1 ions of charge 1-8 are presented. Where necessary, coincidence techniques were applied to distinguish between pure ionisation and ionisation involving electron capture. For high ion velocities, double ionisation takes place either through a shake-off or through a two-step process. The perturbative shake-off mechanism dominates only for the lowest ion charges and projectile energies above 10 MeV AMU-1. A reliable semi-empirical expression which determines the ratio between the double and single ionisation cross sections over a wide region of ion charge and velocity is given. The difference between double ionisation by equi-velocity electrons and positive ions is discussed. It is concluded that no reliable theoretical description of double ionisation exists for combinations of ion charge and ion velocity which give cross sections close to the maximum values.

Gas Phase Absorption Studies of Photoactive Yellow Protein Chromophore Derivatives

Photoabsorption spectra of deprotonated trans p-coumaric acid and two of its methyl substituted derivatives have been studied in gas phase both experimentally and theoretically. We have focused on the spectroscopic effect of the location of the two possible deprotonation sites on the trans p-coumaric acid which originate to either a phenoxide or a carboxylate. Surprisingly, the three chromophores were found to have the same absorption maximum at 430 nm, in spite of having different deprotonation positions. However, the absorption of the chromophore in polar solution is substantially different for the distinct deprotonation locations. We also report on the time scales and pathways of relaxation after photoexcitation for the three photoactive yellow protein chromophore derivatives. As a result of these experiments, we could detect the phenoxide isomer within the deprotonated trans p-coumaric acid in gas phase; however, the occurrence of the carboxylate is uncertain. Several computational methods were used simultaneously to provide insights and assistance in the interpretation of our experimental results. The calculated excitation energies S(0)-S(1) are in good agreement with experiment for those systems having a negative charge on a phenoxide moiety. Although our augmented multiconfigurational quasidegenerate perturbation theory calculations agree with experiment in the description of the absorption spectrum of anions with a carboxylate functional group, there are some puzzling disagreements between experiment and some calculational methods in the description of these systems.

Dissociative recombination of NO + : calculations and comparison with experiment

Multichannel quantum defect calculations for NO + dissociative recombination (DR) for electron energies from threshold to 8 eV are presented. The calculations use electronic energies and autoionization widths of valence states obtained from ab initio R-matrix calculations with the corresponding potential curves calibrated using available spectroscopic data. Six valence states open to dissociation are included in the final calculations. Excellent agreement with the measured cross sections is obtained for the low-energy DR, up to 3 eV and, for the first time, the peak observed in the cross section at high energy is accounted for. The importance of the various dissociative states at different electron energies, as well as the direct and indirect processes, is discussed. Compared to previous theoretical studies, the inclusion of a third dissociative state of 2 � symmetry and the larger autoionization width of the B� 2� state are found to be particularly important for the agreement with experiment. (Some figures in this article are in colour only in the electronic version; see www.iop.org)

Antihydrogen Production by Positronium-Antiproton Collisions in an Ion Trap

A method for producing antihydrogen by the + Ps → + e- reaction is described. Included are the calculated capture cross sections, and short descriptions of the electrostatic slow positron beam, positronium-formation, and the antiproton ion trap to be used for antihydrogen production. With present available technology, collimated monoenergetic beams with an energy of a few keV and an intensity in the order of one per second can be produced by this method. Possible enhancements of this rate are discussed.