P. Hvelplund

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.

The combination of an electrospray ion source and an electrostatic storage ring for lifetime and spectroscopy experiments on biomolecules

An electrospray ion source has been coupled to an accelerator that injects ions into an electrostatic heavy-ion storage ring. Since the dc ion current produced by electrospray ionization is low (∼106 ions/s), ions are accumulated in a cylindrical ion trap filled with a helium buffer gas. The ions are collisionally damped in the buffer gas and confined to the central trap region by a rf field. Extraction from the trap occurs within a few microseconds and after acceleration through 22 kV, the ions of interest are selected by a magnet according to their mass to charge ratio. The ion bunch is subsequently injected into the ring. Both positive and negative ions have been stored, with masses ranging over 3 orders of magnitude (∼102–104 Da). From a pickup signal in the ring, the number of ions in a bunch is estimated to be of the order of 103–104 when the accumulation time is 0.1 s. Our first measurements show that we can store a sufficient number of ions to study the decay of metastable ions and to determine re...

Hidden Histidine Radical Rearrangements upon Electron Transfer to Gas-Phase Peptide Ions. Experimental Evidence and Theoretical Analysis

Protonated peptides containing histidine or arginine residues and a free carboxyl group (His-Ala-Ile, His-Ala-Leu, Ala-His-Leu, Ala-Ala-His-Ala-Leu, His-Ala-Ala-Ala-Leu, and Arg-Ala-Ile) form stable anions upon collisional double electron transfer from Cs atoms at 50 keV kinetic energies. This unusual behavior is explained by hidden rearrangements occurring in peptide radical intermediates formed by transfer of the first electron. The rearrangements occur on a approximately 120 ns time scale determined by the radical flight time. Analysis of the conformational space for (His-Ala-Ile + H)(+) precursor cations identified two major conformer groups, 1a(+)-1m(+) and 5a(+)-5h(+) , that differed in their H-bonding patterns and were calculated to collectively account for 39% and 60%, respectively, of the gas-phase ions. One-electron reduction in 1a(+) and 5a(+) triggers exothermic hydrogen atom migration from the terminal COOH group onto the His imidazole ring, forming imidazoline radical intermediates. The intermediate from 5a is characterized by its charge and spin distribution as a novel cation radical-COO(-) salt bridge. The intermediate from 1a undergoes spontaneous isomerization by imidazoline N-H migration, re-forming the COOH group and accomplishing exothermic isomerization of the initial (3H)-imidazole radical to a (2H)-imidazole radical. An analogous unimolecular isomerization in simple imidazole and histidine radicals requires activation energies of 150 kJ mol(-1), and its occurrence in 1a and 5a is due to the promoting effect of the proximate COOH group. The rearrangement is substantially reduced in Ala-Leu-His due to an unfavorable spatial orientation of the imidazole and COOH groups and precluded in the absence of a free carboxyl group in His-Ala-Leu amide. In contrast to His-Ala-Ile and Arg-Ala-Ile, protonated Lys-Ala-Ile does not produce stable anions upon double electron transfer. The radical trapping properties of histidine residues 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.

Absolute cross sections for multi-electron processes in low energy Arq+−Ar collisions: Comparison with theory

Abstract Absolute cross sections have been measured for a variety of multi-electron processes in low-energy collisions of multiply charged argon recoil ions with neutral argon. The cross sections are compared with theoretical estimates based on an extension of the classical barrier model. Comparison is also made with the statistical theory of Muller et al.

Electron capture and energy loss in ∼100 keV collisions of atomic and molecular ions on C60

Abstract We have determined absolute attenuation and electron capture cross sections in ∼100 keV collisions between atomic ions of moderate charge states (He + , He 2+ , Ar 2+ , Ar 3+ , Ar 4+ , Xe 2+ , Xe 3+ , Xe 4+ ) and C 60 molecules in the gas phase. The measured cross-sections support previous findings of the charge dynamics within C 60 from electron capture experiments with higher ionic charges. For a non-negligible fraction of the capture events we observed substantial ionic energy loss (≤2 keV), which is ascribed to collisions where the ion passes through the C 60 molecule. Furthermore, we found that a molecular ion (100 keV CO 2 2+ ) that captures an electron from C 60 can lose similar amounts of energy without dissociating. It is concluded that the predominant contribution to the ionic energy loss comes from electron excitation and ionization.

Equilibrium charge distributions of ion beams in carbon

Abstract The equilibrium charge fractions have been measured for beams of N, O, F, Ne, Na, Mg, Al, and Ar in the energy range from 100 to 500 keV, and for beams of Tl in the energy range from 200 to 1100 keV. The ions under investigation were sent through thin (5–10 μg/cm 2 ) carbon foils, and the emerging beam was separated electrostatically and detected by an open position- sensitive electron multiplier. Corrections were made for energy loss in the carbon foils by means of measured electronic and theoretical nuclear-stopping cross sections for the forward-directed beam. The beam was accepted by a small (0.3-mm diameter) aperture in front of the deflection plates. For Tl, the corrections for energy loss were calculated from the theoretical value of the electronic stopping cross section and the nuclear stopping cross section for the forward-directed beam. The main purpose of the present work has been to investigate the variation in charge-state population for atoms around Ne because shell effects are expected to drastically influence the charge distributions. (For example are the neutral fractions at 10 keV/amu 37% for Ne, 10% for Na, and 3% for Mg.) Preliminary measurements of charge-state fractions for Tl are reported.

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.

Ionization of helium and molecular hydrogen by slow antiprotons

Measurements of the cross sections for single and double ionization of helium as well as for the creation of H2+ and H+ ions from H2 for impact of antiprotons in the energy range 13-500 keV are presented. The results are compared with our earlier, less accurate data and with data for equivelocity proton impact. The single ionization cross section of helium agrees remarkably well with a continuum distorted wave (CDW-EIS) calculation. The ratio between the double and the single ionization cross sections of helium increases dramatically with decreasing projectile energy. New theoretical calculations are called for.

Lifetimes of C602− and C702− dianions in a storage ring

C60(2-) and C70(2-) dianions have been produced by electrospray of the monoanions and subsequent electron pickup in a Na vapor cell. The dianions were stored in an electrostatic ring and their decay by electron emission was measured up to 1 s after injection. While C70(2-) ions are stable on this time scale, except for a small fraction of the ions which have been excited by gas collisions, most of the C60(2-) ions decay on a millisecond time scale, with a lifetime depending strongly on their internal temperature. The results can be modeled as decay by electron tunneling through a Coulomb barrier, mainly from thermally populated triplet states about 120 meV above a singlet ground state. At times longer than about 100 ms, the absorption of blackbody radiation plays an important role for the decay of initially cold ions. The tunneling rates obtained from the modeling, combined with WKB estimates of the barrier penetration, give a ground-state energy 200+/-30 meV above the energy of the monoanion plus a free electron and a ground-state lifetime of the order of 20 s.