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We present differential cross sections of electron capture in $7.5\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ and $12.5\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ proton-helium collisions. Complete experimental separations of the Thomas and the kinematic single electron capture processes in the two-dimensional ${\mathrm{He}}^{+}$ momentum distribution in the plane perpendicular to the fast ion beam have been achieved. We compare the resulting projectile angular differential cross section with the two most recent theoretical calculations and expose significant deviations.

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.

The subject of this thesis is experimental studies of electron-transfer processes in ion-atom collisions at velocities significantly higher than typical orbital velocities of electrons in bound states of atoms or molecules. The experimental technique applied combines the high beam intensity of heavy-ion storage rings with a supersonic gas-jet target equipped with a recoil-ion-momentum spectrometer. In singleelectron capture to fast protons from helium atoms, we have for the first time achieved a complete separation of the kinematic and Thomas transfer mechanisms and are able to perform a quantitative comparison with the many theoretical results on a much more detailed level than what was previously possible. For the process of transfer ionization in proton-helium collisions we have determined the velocity dependence of the Thomas transfer ionization cross section to be the expected vp-11 when the projectile velocity, vp, is sufficiently high. Further, we have determined the velocity-dependent probability for shake-off of the second electron from helium provided that the first one is transferred in a kinematic capture process. Finally, we have considered collisions between protons and hydrogen molecules. Here we have found a strong variation in the cross section for transfer and excitation processes when the angle between the direction of the incoming projectile and the internuclear axis of the target molecule is varied. The variation can be explained as a result of quantum mechanical interference related to the two indistinguishable atomic centers of the molecule.

We present high-luminosity experimental investigations of the transfer ionization (TI:p+He{yields}H{sup 0}+He{sup 2+}+e{sup -}) process in collisions between fast protons and neutral helium atoms in the earlier inaccessibly high-energy range 1.4-5.8 MeV. The protons were stored in the heavy-ion storage and cooler ring CRYRING, where they intersected a narrow supersonic helium gas jet. We discuss the longitudinal recoil-ion momentum distribution, as measured by means of cold-target recoil-ion momentum spectroscopy and find that this distribution splits into two completely separated peaks at the high end of our energy range. These separate contributions are discussed in terms of the earlier proposed Thomas TI (TTI) and kinematic TI mechansims. The cross section of the TTI process is found to follow a {sigma}{proportional_to}v{sup -b} dependence with b=10.78{+-}0.27 in accordance with the expected v{sup -11} asymptotic behavior. Further, we discuss the probability for shake-off accompanying electron transfer and the relation of this TI mechanism to photodouble ionization. Finally the influence of the initial-state electron velocity distribution on the TTI process is discussed.

The production of carbon cluster ions by injecting C60 fullerene vapor into different electron cyclotron resonance (ECR) ion sources (MONO1001/2.45GHz and CAPRICE∕14GHz) is described. The extracted ion mass spectra show a bimodal distribution, well known from collisions between electrons or ions and fullerenes. In addition to small carbon clusters and even-numbered fullerene ions, odd-numbered clusters with (n>30) are detected with low intensities. In particular, we have analyzed the mass spectra as a function of the rf power applied to the ECR plasma. Optimum power values are found for the production of individual carbon cluster ions, which increase with decreasing cluster size. Whereas at low power the production of fullerene ions dominates, the intensity of the very small carbon clusters is found to strongly increase with the injected power. This difference in the power dependence is used to vary and to determine the beam composition for ions with identical mass/charge ratios.

We have studied collisions between slow highly charged ions and pure C{sub 70} and C{sub 60} molecular targets, and report on measurements of target ionization and fragmentation in electron transfer processes. The intensity distributions in the fragmentation spectra for C{sub 60} and C{sub 70} are rather similar, indicating that similar roles are played by evaporation (neutral C{sub 2} emission) and fission processes (charged particle emission) in the two systems. For C{sub 70}, intact molecular ions are formed in charge states up to 10+, while the maximum charge state for C{sub 60} molecules is 9+ following collisions with Xe{sup 23+} at 69 keV. The kinetic-energy releases measured for asymmetric fission of C{sub 70} ions (C{sub 70}{sup r+}{yields}C{sub 68}{sup (r-1)}+C{sub 2}{sup +}) are mostly close to the corresponding ones for C{sub 60}, while both sets of results are significantly lower than those reported for electron-impact ionization studies of C{sub 60} and C{sub 70}. Kinetic-energy releases and fission barrier heights are estimated for C{sub 60} and C{sub 70} ions using an electrostatic model. An increased intensity of the higher charge states of C{sub 70} ions is observed compared to the C{sub 60} case, which most likely is due to a larger number ofmore » degrees of freedom on which the internal excitation energy may be distributed.« less

We have measured the time dependence of the fragmentation of protonated amino acids and peptides upon UV excitation in an electrostatic ion storage ring. After absorption of a 266 nm photon, protonated Trp (TrpH+) has a lifetime of 10–20 μs but also a component with a millisecond lifetime is present. The long lifetime may be due to fluorescence, which leads to ions with lower excitation energy, or to the decay of the Trp+˙ radical cation formed after prompt hydrogen loss in the laser interaction region. Only one component with a lifetime of about 10 μs was detected for TyrH+. The lifetime of photoexcited PheH+ is even shorter with an upper limit of a few microseconds. For the singly protonated tripeptides (LysTrpLysH+ and LysTyrLysH+), the decay curves are found to consist of a single component that can be reproduced with an assumption of statistical decay after equilibration of the photon energy among all vibrational modes. The rate constant is expressed in the Arrhenius form in terms of the microcanonical temperature, and the decay rate is obtained by integration over the energy distribution, which has a spread corresponding to the canonical energy distribution at room temperature. The resulting deviation from exponential decay makes it possible to determine the decay parameters from a measurement at a single photon wavelength. Activation energies of Ea = 1.24 ± 0.07 and 1.5 ± 0.4 eV were determined for LysTrpLysH+ and LysTyrLysH+, respectively, with pre-exponential factors of Ad = 1011.1±0.5 and 1012.9±2.6 s−1.

We have measured kinetic energy releases in asymmetric fission, ${\mathrm{C}}_{60}^{r+}\ensuremath{\rightarrow}{\mathrm{C}}_{58}^{(r\ensuremath{-}1)+}+{\mathrm{C}}_{2}^{+}$ $(r=6--9)$ and evaporation ${\mathrm{C}}_{60}^{r+}\ensuremath{\rightarrow}{\mathrm{C}}_{58}^{r+}+{\mathrm{C}}_{2}$ $(r=2,3),$ following multiple-electron removal from ${\mathrm{C}}_{60}$ in ${\mathrm{He}}^{2+}$ and ${\mathrm{Xe}}^{17+}$ collisions at $3q\mathrm{keV}(q=2,17).$ We used the recoil-ion momentum technique and limited the initial momentum distribution of the target molecules by collimation of the effusive ${\mathrm{C}}_{60}$ jet. This yielded a resolution of 3 meV for the final kinetic energies of the charged ${\mathrm{C}}_{58}$ fragments, mapped out as two-dimensional position distributions at the end of a linear time-of-flight mass spectrometer. The present results for asymmetric fission are in agreement with earlier ones deduced from time-of-flight ${\mathrm{C}}_{2}^{+}$ peak-shape and sector-field ${\mathrm{C}}_{58}^{(r\ensuremath{-}1)+}$-energy analysis. Model calculations treating ${\mathrm{C}}_{58}^{(r\ensuremath{-}1)+}$ and ${\mathrm{C}}_{2}^{+}$ as conducting spheres indicate that the autocharge-transfer process, which has been proposed to link asymmetric fission to neutral ${\mathrm{C}}_{2}$ emission, most likely is inactive for all r. Using a charge-independent activation energy for ${\mathrm{C}}_{2}$ emission from ${\mathrm{C}}_{60}^{r+}$ of ${E}_{a}=10\mathrm{eV},$ we deduce fission barriers indicating lower (semiempirical) and upper (model) ${\mathrm{C}}_{60}^{r+}$-stability limits of $r=11$ and $r=18,$ respectively.

Clusters of fullerenes (C60,C70)(n) are produced in a gas aggregation source and are multiply ionized in collisions with highly charged Xe(20+,30+) ions. Their stabilities and decay processes are analyzed with high-resolution time-of-flight mass spectrometry. Fullerene clusters in charge states up to q=5 have been observed and appearance sizes are found to be as small as n(app)=5, 10, 21, and 33 for q=2, 3, 4, and 5, respectively. The analysis of the multicoincident fragmentation spectra indicates a high charge mobility. This is in contrast to charge localization effects which have been reported for Ar(q+)(n) rare gas clusters. Clusters of fullerenes are found to be conducting when multiply charged.

We have investigated the time dependence of the fragmentation of protonated amino acids stored at 22 keV in the electrostatic ring ELISA. The ions were produced in an electrospray source and after bunching in a quadrupole trap they were excited by collisions in a Ne gas. As in earlier studies of metal clusters and fullerenes produced in “hot” ion sources we find that the dissociation of metastable molecules follows approximately a 1/t decay law until a time $\tau$ after which the yield falls off much more rapidly. We interpret this reduction as a result of radiative cooling with a characteristic cooling time, $\tau_c \simeq G\tau$ , where G is the magnitude of the exponent in an Arrhenius expression for the rate of the dominant fragmentation process. The values of $\tau$ obtained from fits to the data are in the range 9-17 ms corresponding to cooling times of a few hundred milliseconds, in good accord with the expected rate of cooling by emission from IR-active vibrations. The power-law behaviour for $t < \tau$ varies somewhat between the different amino acids, with powers between -0.9 and -1.1. We argue that this may be due to a competition between fragmentation channels with different Arrhenius parameters.

We present a static classical over-the-barrier model (OBM) for electron transfer between two isolated, infinitely conducting spheres with arbitrary and different electrical charges. This model is shown to be very useful for first estimates of single- and multiple-electron transfer cross sections in cluster-cluster collisions when the collision velocities are significantly lower than the typical target electron velocities. For faster collisions, more advanced models such as the dynamical OBM, or the time-dependent local density approximation (TDLDA) or the solutions of their semiclassical counterparts---the Vlasov equations have to be used. The latter two methods clearly provide the most detailed information on the electronic response, but they are also computationally very demanding and have, so far, only been used for collisions involving one cluster (and an atomic ion). We compare our static OBM results (in the limit in which one of the sphere radii approaches zero) with TDLDA and Vlasov calculations of cluster charging in ${\mathrm{Ar}}^{8+}\ensuremath{-}{\mathrm{Na}}_{40}$ collisions at different velocities to demonstrate that the present static OBM is valid at sufficiently low velocities. The static OBM is then used in a comparison with experimental target charge state distributions in ${\mathrm{C}}_{60}^{q+}\ensuremath{-}{\mathrm{C}}_{60}$ and ${\mathrm{C}}^{q+}\ensuremath{-}{\mathrm{C}}_{60}$ collisions at $0.01\sqrt{q}$ and $0.06\sqrt{q}{v}_{0},$ respectively. Calculated electronic excitation of the projectile after two-electron transfer in ${\mathrm{C}}_{60}^{4+}\ensuremath{-}{\mathrm{C}}_{60}$ collisions readily explains the recently observed suppression of the transfer ionization channel in this reaction. Finally, we model the total projectile electron loss and dissociation cross section in highly protonated Lysozyme-oxygen $(\mathrm{Lys}\ensuremath{-}{\mathrm{H}}_{9}^{9+}\ensuremath{-}{\mathrm{O}}_{2})$ collisions and make comparisons with recent experimental results at $0.01{v}_{0}.$

We have stored the first beams in one of the rings of the double electrostatic ion-storage ring, DESIREE at cryogenic and at room temperature conditions. At cryogenic operations the following parameters are found. Temperature; T= 13K, pressure; p 10(7) and storage lifetime of a C-2(-) beam; tau = 450 S.

In this work we study collisions between ions and Polycyclic Aromatic Hydrocarbons with the aid of a novel over-the-barrier model and well-established models for nuclear and electronic stopping processes.

We report on the ongoing commissioning of the Double ElectroStatic Ion Ring ExpEriment, DESIREE, at Stockholm University. Beams of atomic carbon anions (C‑) and smaller carbon anion molecules (C‑2, C‑3, C‑4 etc.) have been produced in a sputter ion source, accelerated to 10 keV or 20 keV, and stored successfully in the two electrostatic rings. The rings are enclosed in a common vacuum chamber cooled to below 13 Kelvin. The DESIREE facility allows for studies of internally relaxed single isolated atomic, molecular and cluster ions and for collision experiments between cat- and anions down to very low center-of-mass collision energies (meV scale). The total thermal load of the vacuum chamber at this temperature is measured to be 32 W. The decay rates of stored ion beams have two components: a non-exponential component caused by the space charge of the beam itself which dominates at early times and an exponential term from the neutralization of the beam in collisions with residual gas at later times. The residual gas limited storage lifetime of carbon anions in the symmetric ring is over seven minutes while the 1/e lifetime in the asymmetric ring is measured to be about 30 seconds. Although we aim to improve the storage in the second ring, the number of stored ions are now sufficient for many merged beams experiments with positive and negative ions requiring milliseconds to seconds ion storage.

We have used the ion storage ring CRYRING and its internal gas-jet target and recoil-ion-momentum spectrometer to measure absolute cross sections for transfer ionization (TI: p + He → H 0 + He 2 + + e - ) in 2.5-4.5 MeV p-He collisions with separate Thomas (TTI) and kinematic (KTI) TI contributions. The probability for electron emission in kinematical capture decreases with increasing velocity and appears to approach the photoionization shakeoff value (1.63%) [T. Aberg, Phys. Rev. A 2, 1726 (1970)]. The velocity dependence of the TTI cross section is consistent with the theoretically predicted v - 1 1 scaling [J. S. Briggs and K. Taulbjerg, J. Phys. B 12, 2565 (1979)].

We discuss hollow atom formation and stabilization of electrons on ${\mathrm{Ar}}^{q+}$ following ${\mathrm{Ar}}^{q+}+{\mathrm{C}}_{60}\ensuremath{\rightarrow}{\mathrm{Ar}}^{(q\ensuremath{-}s)+}+\ensuremath{\cdot}\ensuremath{\cdot}\ensuremath{\cdot}$ collisions at $3.3q \mathrm{keV}$ $(q=4\char21{}18).$ The experimental information consists of the final projectile charge-state distributions ${f}_{q}^{s},$ i.e., the relative distributions of the number of stabilized electrons, s, and the corresponding mean values $〈s〉={\ensuremath{\sum}}_{s=1}^{q}{\mathrm{sf}}_{q}^{s}$ as functions of q. We use the classical over-the-barrier model to deduce sequences of effective principal quantum numbers and find that the hollow atom formation is completed $3\char21{}{4a}_{0}$ above the surface of the ${\mathrm{C}}_{60}$ cage for all q. For $ql~8$ (filled L shells), the last electrons are transferred from delocalized outer ${\mathrm{C}}_{60}$ orbitals directly to the projectile M shell (side-feeding), while several intermediate shells are left open for larger q leading to further electron transfer at intermediate distances and simultaneous electron emission from higher projectile shells. At still closer distances, localized carbon K-shell electrons are transferred directly to the argon L $(q=10$ and 11) and M shells $(qg~12).$ The direct transfer to the L shell of ${\mathrm{Ar}}^{10+}$ and ${\mathrm{Ar}}^{11+}$ is manifested as significant enhancements of $〈s〉$ for $q=10$ and $q=11.$

In this work, we report the absorption spectra in the Soret band region of isolated Fe(III)-heme+ and Fe(III)-heme+(His) ions in vacuo from action spectroscopy. Fe(III)-heme+ refers to iron(III) coordinated by the dianion of protoporphyrin IX. We find that the absorption of the five-coordinate complex is similar to that of pentacoordinate metmyoglobin variants with hydrophobic binding pockets except for an overall blueshift of about 16 nm. In the case of four-coordinate iron(III), the Soret band is similar to that of five-coordinate iron(III) but much narrower. These spectra serve as a benchmark for theoretical modeling and also serve to identify the coordination state of ferric heme proteins. To our knowledge this is the first unequivocal spectroscopic characterization of isolated 4c ferric heme in the gas phase.

We have investigated collisions between Xe20+ ions and the isolated polycyclic aromatic hydrocarbon (PAH) C16H10 isomers pyrene and fluoranthene or pure clusters of either pyrene or fluoranthene. We show fragment distributions and intensities for both cases and rationalize the results by means of a classical over-the-barrier (COB) model in conjunction with quantum chemistry calculations of vertical ionization energies of pyrene and fluoranthene monomers. We find that comparatively small differences in the first and second ionization energies for these isomers have significant effects on both the monomer and cluster fragment distributions.

We have studied multiply ionization for a range of Polycyclic Aromatic Hydrocarbons (PAHs) and their charge dependent stabilities with respect to H−, H+-, C2H2- and C2H2+-emissions by means of Density Functional Theory. The adiabatic dissociation energies reveal information about the competition between these channels as functions of PAH charge state and provide predictions of the thermodynamical stability limits, while the sequences of ionization energies may e.g. be used to extract absolute ionization cross sections of astrophysical relevance.

We describe the status of the DESIREE facility, which is an electrostatic ion-storage device consisting of two storage rings with a common section, where ion beams of opposite charges stored in the two rings are merged. The electrostatic storage rings are mounted in a single common vacuum vessel, which is isolated inside an outer vacuum chamber and cooled by means of four cryogenerators. This unique system will allow for studies of interaction among ions of opposite charges at well controlled conditions both in terms of internal degrees of freedom and relative velocity. An advanced ion-beam production system for intense pulses of cold complex molecular ions is also presented.

We present analytical solutions for the interaction energies between a static point charge and a metal- or a dielectric sphere. These solutions include polarization effects to infinite orders in the inverse of the distance between the point charge and the spheres. Further, we present Density Functional Theory (DFT) calculations of interaction energies for a point charge in ranges of fixed positions outside a neutral or a singly charged C(60) molecule. Based on these DFT results, we conclude that the metal sphere model describes the electronic response of the C(60) molecule much better than the dielectric sphere model. These findings are particularly important for calculations of energy barriers for charge transfer in reactions involving C(60) molecules. The metal- and dielectric models should further be useful for descriptions of, for example, the polarizabilities of and interactions with conducting and insulating spherical clusters or particles.

In this proceedings I will describe the design of a new storage device currently under construction at Stockholm University, Sweden. This device uses purely electrostatic focussing and deflection elements and allows ion beams of opposite charge to be confined under extreme high vacuum and cryogenic conditions in separate "rings" and then merged over a common straight section. This Double ElectroStatic Ion Ring ExpEriment (DESIREE) apparatus allows for studies of interactions between cations and anions at low and well-defined centre-of-mass energies. I discuss the design of the DESIREE facility, highlighting some of the technical advantages of using purely electrostatic over magnetic elements, as well as the issues that have arisen during its development and construction. Finally, the advantages of this design are a boon to fundamental experimental studies and I finish by discussing an example of such potential research.

We have studied fragmentation of the hydrated nucleotide anion adenosine 5'-monophosphate after collisions with neutral sodium atoms. Regardless of the initial number (≤ 16) of water molecules attached to the ion, dianion formation always leads to water evaporation as well as loss of a single hydrogen atom, possibly from the adenine part. This damaging effect becomes more important as the number of initially attached water molecules increases, which is just the opposite to the protective behavior observed for collision induced dissociation processes without electron transfer.

We present a detailed study of the electronic structure and the stability of C(60) dianions in the gas phase. Monoanions were extracted from a plasma source and converted to dianions by electron transfer in a Na vapor cell. The dianions were then stored in an electrostatic ring, and their near-infrared absorption spectrum was measured by observation of laser induced electron detachment. From the time dependence of the detachment after photon absorption, we conclude that the reaction has contributions from both direct electron tunneling to the continuum and vibrationally assisted tunneling after internal conversion. This implies that the height of the Coulomb barrier confining the attached electrons is at least approximately 1.5 eV. For C(60)(2-) ions in solution electron spin resonance measurements have indicated a singlet ground state, and from the similarity of the absorption spectra we conclude that also the ground state of isolated C(60)(2-) ions is singlet. The observed spectrum corresponds to an electronic transition from a t(1u) lowest unoccupied molecular orbital (LUMO) of C(60) to the t(1g) LUMO+1 level. The electronic levels of the dianion are split due to Jahn-Teller coupling to quadrupole deformations of the molecule, and a main absorption band at 10,723 cm(-1) corresponds to a transition between the Jahn-Teller ground states. Also transitions from pseudorotational states with 200 cm(-1) and (probably) 420 cm(-1) excitation are observed. We argue that a very broad absorption band from about 11,500 cm(-1) to 13,500 cm(-1) consists of transitions to so-called cone states, which are Jahn-Teller states on a higher potential-energy surface, stabilized by a pseudorotational angular momentum barrier. A previously observed, high-lying absorption band for C(60)(-) may also be a transition to a cone state.

The inside cover picture shows gas-phase absorption spectra of four p-nitrophenolates. The ions cover the entire visible spectrum when isolated in vacuo and are located within a narrower region in solution. On p. 1207 A. Rubio, M. Brondsted Nielsen, S. Brondsted Nielsen et al. use the gas-phase absorption maxima to benchmark excitation energy calculations (blue lines).

We present a systematic study of experimental kinetic energy release distributions for the asymmetric fission processes Cq+60 → C(iq-1 4.

We have studied the stabilities of multiply charged van der Waals dimers in slow Xe30+ + [C60]2([C60C70], [C70]2) → Xe(30-s)+ + [C60]2r+([C60C70]r+, [C70]2r+) + (r-s)e-electron-transfer collisions (v = 0.4 a.u. and r ≤ 7). The relative ionization cross sections display even-odd variations as functions of r for [C60]2r+, [C60C70]r+, and [C70]2r+. This is in clear contrast to the typical smooth decreasing behavior of the cross sections for multiple ionization of fullerene monomers, which can be explained within the framework of the classical over-the-barrier model. In addition, we report the branching ratios and the kinetic energy releases for the dominant fragmentation processes that yield intact fullerenes. The experimental results are discussed in view of a simple electrostatic model for dimer ionization and recent results from high level density functional theory calculations.

We have calculated the electronic energies and optimum geometries of C(70) (q+) and C(68) (q+) fullerenes (q=0-14) by means of density functional theory. The ionization energies for C(70) and C(68) fullerenes increase more or less linearly as functions of charge, consistent with the previously reported behavior for C(60) and C(58) [S. Diaz-Tendero et al., J. Chem. Phys. 123, 184306 (2005)]. The dissociation energies corresponding to the C(70) (q+)--C(68) (q+)+C(2), C(70) (q+)--C(68) ((q-1)+)+C(2) (+), C(70) (q+)--C(68) ((q-2)+)+C(+)+C(+), C(70) (q+)--C(68) ((q-3)+)+C(2+)+C(+), and C(70) (q+)--C(68) ((q-4)+)+C(2+)+C(2+) decay channels show that C(70) (q+) (like C(60) (q+)) is thermodynamically unstable for qor=6. However, the slope of the dissociation energy as a function of charge for a given decay channel is different from that of C(60) (q+) fullerenes. On the basis of these results, we predict q=17 to be the highest charge state for which a fission barrier exists for C(70) (q+).

We report on studies of collisions between 3 keV Ar$^+$ projectile ions and neutral targets of isolated 1,3-butadiene (C$_4$H$_6$) molecules and cold, loosely bound clusters of these molecules. We identify molecular growth processes within the molecular clusters that appears to be driven by knockout processes and that could result in the formation of (aromatic) ring structures. These types of reactions are not unique to specific projectile ions and target molecules, but will occur whenever atoms or ions with suitable masses and kinetic energies collide with aggregates of matter, such as carbonaceous grains in the interstellar medium or aerosol nanoparticles in the atmosphere., Comment: 8 pages, 6 figures, Accepted for publication in Physical Chemistry Chemical Physics 2017

Abstract Radiative cooling of carbon cluster anions C2n+1− (n = 3–5) is investigated using the cryogenic electrostatic ion storage ring DESIREE. Two different strategies are applied to infer infrared emission on slow (milliseconds to seconds) and ultraslow (seconds to minutes) timescales. Initial cooling of the ions over the millisecond timescale is probed indirectly by monitoring the decay in the yield of spontaneous neutralization by thermionic emission. The observed cooling rates are consistent with a statistical model of thermionic electron emission in competition with infrared photon emission due to vibrational de-excitation. Slower cooling over the seconds to minutes timescale associated with infrared emission from low-frequency vibrational modes is probed using time-dependent action spectroscopy. For C9− and C11−, cooling is evidenced by the time-evolution of the yield of photo-induced neutralization following resonant excitation of electronic transitions near the detachment threshold. The cross-section for resonant photo-excitation is at least two orders of magnitude greater than for direct photodetachment. In contrast, C7− lacks electronic transitions near the detachment threshold. Graphical abstract

We have investigated the transfer ionization process in fast collisions between protons, stored in the heavy‐ion storage and cooler ring CRYRING, and a cold supersonic helium gas‐jet target: H+ + He → H + He2+ + e−. We have refined the COLTRIMS (COLd Target Recoil‐Ion‐Momentum Spectroscopy) technique by applying time‐varying fields in the recoil‐ion‐momentum spectrometer to block random singly charged helium ions from single‐ionization events.

We have studied electron transfer processes and collision‐induced fragmentation in collisions between slow highly charged ions and C60 molecules. Our newly developed reaction microscope provides information on the dynamics in the charge transfer and the fragmentation processes. By using a position‐sensitive detector, located after a time‐of‐flight tube, we obtain recoil energies of fragments resulting from the disintegration of multiply charged C60 ions. Thereby we deduce kinetic energy releases (KER) for the dissociation processes where multiple charged C60 ions emits either a single C2+ ion or a neutral C2‐unit following ionization by slow highly charged ions. We find that C584+ ions predominately are due to the evaporation of a neutral C2 molecule.