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The present paper reports on a merged-beam experiment on mutual neutralization between ${\mathrm{Na}}^{+}$ and ${\mathrm{D}}^{\ensuremath{-}}$. For this experiment, we have used the DESIREE ion-beams storage-ring facility. The reaction products are detected using a position- and time-sensitive detector, which ideally allows for determination of the population of each individual quantum state in the final atomic systems. Here, the $4s, 3d$, and $4p$ final states in Na are observed and in all cases the D atom is in its ground state $1s ^{2}S$. The respective branching fractions of the states populated in Na are determined by fitting results from a Monte Carlo simulation of the experiment to the measured data. The center-of-mass collision energy is controlled using a set of biased drift tubes, and the branching fractions are measured for energies between 80 meV and 393 meV. The resulting branching fractions are found to agree qualitatively with the only available theoretical calculations for this particular system, which are based on a multichannel Landau-Zener approach using dynamic couplings determined with a linear combination of atomic orbitals model.

Spontaneous decays of small, hot silver-cluster anions ${\text{Ag}}_{n}^{\ensuremath{-}}$, $n=4--7$, have been studied using one of the rings of the Double ElectroStatic Ion Ring ExpEriment (DESIREE). Observation of these decays over very long time scales is possible due to the very low residual gas pressure ($\ensuremath{\sim}{10}^{\ensuremath{-}14}$) and cryogenic (13 K) operation of DESIREE. The yield of neutral particles from stored beams of ${\text{Ag}}_{6}^{\ensuremath{-}}$ and ${\text{Ag}}_{7}^{\ensuremath{-}}$ anions were measured for 100 milliseconds and were found to follow single power-law behavior with millisecond time-scale exponential cutoffs. The ${\text{Ag}}_{4}^{\ensuremath{-}}$ and ${\text{Ag}}_{5}^{\ensuremath{-}}$ anions were stored for 60 s and the observed decays show two-component power-law behaviors. We present calculations of the rate constants for electron detachment from and fragmentation of ${\text{Ag}}_{4}^{\ensuremath{-}}$ and ${\text{Ag}}_{5}^{\ensuremath{-}}$. In these calculations, we assume that the internal energy distribution of the clusters are flat and with this we reproduce the early steep parts of the experimentally measured decay curves for ${\text{Ag}}_{4}^{\ensuremath{-}}$ and ${\text{Ag}}_{5}^{\ensuremath{-}}$, which extends to tens and hundreds of milliseconds, respectively. The fact that the calculations reproduce the early slopes of ${\text{Ag}}_{4}^{\ensuremath{-}}$ and ${\text{Ag}}_{5}^{\ensuremath{-}}$, which differ for the two cases, suggests that it is the changes in fragmentation rates with internal cluster energies of ${\text{Ag}}_{4}^{\ensuremath{-}}$ and ${\text{Ag}}_{5}^{\ensuremath{-}}$ rather than conditions in the ion source that determine this behavior. Comparisons with the measurements strongly suggest that the neutral particles detected in these time domains originate from ${\text{Ag}}_{4}^{\ensuremath{-}}\ensuremath{\rightarrow}{\text{Ag}}_{3}^{\ensuremath{-}}+\text{Ag}$ and ${\text{Ag}}_{5}^{\ensuremath{-}}\ensuremath{\rightarrow}{\text{Ag}}_{3}^{\ensuremath{-}}+{\mathrm{Ag}}_{2}$ fragmentation processes.

We apply near-threshold laser photodetachment to characterize the rotational quantum level distribution of OH- ions stored in the cryogenic ion-beam storage ring DESIREE at Stockholm University. We ...

We have measured the spontaneous neutral particle emission from copper-cluster anions (${\mathrm{Cu}}_{n}{}^{\ensuremath{-}}, n=3--6$) stored at cryogenic temperatures in one of the electrostatic ion storage rings of the Double ElectroStatic Ion Ring ExpEriment facility at Stockholm University. The measured rate of emission from the stored ${\mathrm{Cu}}_{3}{}^{\ensuremath{-}}$ ions follows a single power-law decay for about 1 ms but then decreases much more rapidly with time. The latter behavior may be due to a decrease in the density of available final states in ${\mathrm{Cu}}_{3}$ as the excitation energies of the decaying ions approach the electron detachment threshold. The emissions from ${\mathrm{Cu}}_{4}{}^{\ensuremath{-}}, {\mathrm{Cu}}_{5}{}^{\ensuremath{-}}$, and ${\mathrm{Cu}}_{6}{}^{\ensuremath{-}}$ are well described by sums of two power laws that are quenched by radiative cooling of the stored ions with characteristic times between a few and hundreds of milliseconds. We relate these two-component behaviors to populations of stored ions with higher and lower angular momenta. In a separate experiment, we studied the laser-induced decay of ${\mathrm{Cu}}_{6}{}^{\ensuremath{-}}$ ions that were excited by 1.13- or 1.45-eV photons after 46 ms of storage.

The intrinsic radiative lifetimes of the $5{d}^{10}6{s}^{}^{2}S_{1/2}$ and $5{d}^{9}6{s}^{2} ^{2}D_{3/2}$ bound excited states in the platinum anion ${\text{Pt}}^{\ensuremath{-}}$ have been studied at cryogenic temperatures at the Double ElectroStatic Ion Ring Experiment (DESIREE) facility at Stockholm University. The intrinsic lifetime of the higher-lying $5{d}^{10}6{s}^{} ^{2}S_{1/2}$ state was measured to be $2.54\ifmmode\pm\else\textpm\fi{}0.10\phantom{\rule{0.16em}{0ex}}\mathrm{s}$, while only a lifetime in the range of 50\char21{}200 ms could be estimated for the $5{d}^{9}6{s}^{2} ^{2}D_{3/2}$ fine-structure level. The storage lifetime of the ${\text{Pt}}^{\ensuremath{-}}$ ion beam was measured to be a little over 15 min at a ring temperature of $13\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The present study reports the lifetime of an atomic negative ion in an excited bound state with an electron configuration different from that of the ground state.

We present experimental and theoretical results on single- and multiple-electron capture, and fragmentation, in C6+ + C-60 collisions at velocities in the v(col) = 0.05 - 0.4 a.u. range. We use time-of-flight mass spectrometry and coincidence detection of charged fragments to separate pure target ionization from processes in which the C-60 target is both ionized and fragmented. The coincidence technique allows us to identify different types of fragmentation processes such as C-60(q+) -> C-58(q+) + C-2 and C-60(q+) -> C-58((q-1)+) + C-2(+). A quasimolecular approach is employed to calculate charge transfer and target excitation cross sections. First-order time-dependent perturbation and statistical methods are used to treat the postcollisional processes: the calculated rate constants for C-2 and C-2(+) emission from the excited and charged fullerene are then used to evaluate the fragmentation dynamics. We show that the target ionization cross section decreases with the induced target charge state and the impact energy. C-2 emission from C-60(q+) is found to dominate when q = 5, in agreement with the present and previous experimental results.

International audience; We have studied collisions between 22.5 keV He2+ ions and mixed clusters [(C60)m(C24H12)n] of m C60 and n coronene molecules where m and n range up to about ten. Surprisingly, the cluster fragmentation behavior in distant collisions is dramatically different for pure coronene clusters (m=0) and clusters containing a single C60 molecule (m=1). In the latter case, the clusters may be ionized without also being fragmented on the experimental time scale of tens of microseconds. This does not occur for pure coronene clusters, but is a main characteristic of pure fullerene clusters. For ion trajectories penetrating the mixed cluster, we observe covalent bond formations between C59 or C58 and C60, but not between coronene fragments and C60, or between C60 fragments and coronene. These results are explained by means of classical molecular dynamics simulations of collisions inside the fragmenting mixed clusters.

Dumbbell-shaped fullerene dimers C-118(+) and C-119(+) have recently been observed in mass spectra resulting from collisions between clusters of C-60 molecules and keV He2+ or Ar2+ ions [H. Zettergren et al., Phys. Rev. Lett. 110, 185501 (2013) and F. Seitz et al., J. Chem. Phys. 139, 034309 (2013)]. To unveil the formation mechanisms of these fullerene dimers, systematic molecular dynamics (MD) simulations based on the self-consistent charge density functional tight-binding method have been performed for C-n(+) + C-60 (n = 58,59,60) collisions following prompt atom knockouts by the fast ions. The statistics from the MD simulations indicate a much higher reactivity of C-59(+) and C-58(+) fragments compared to that of C-60(+). It is found that the covalently bonded dumbbell-shaped fullerene dimers C-118(+) and C-119(+) can be formed at very low-collision energies within 1 ps and are stable enough to survive on the microsecond time scale of the experiment. The thermodynamic and kinetic stabilities, as well as the bonding features, have been investigated for the most stable dumbbell dimers C-118(+), C-119(+), and C-120(+).

This work was supported by the Swedish Research Council (Contracts No. 621-2008-3773, No. 621-2009- 3468, and No. 621-2011-4047). The work is partially supported by the Projects No. FIS2010-15127, No. CTQ2010-17006 CSD2007-00010 (MICINN), No. S2009/MAT1726 (CAM), and the CNRS PICS-05356. Studies of interstellar chemistry at Leiden Observatory are supported through advanced-ERC Grant No. 246976 from the European Research Council, through a grant by the Dutch Science Agency, NWO, as part of the Dutch Astrochemistry Network, and through the Spinoza premie from the Dutch Science Agency, NWO

We present a systematic study of the stability of highly charged cationic and anionic fullerenes whose most stable neutral counterparts follow the isolated pentagon rule (IPR). In agreement with recent studies, we have found that, for many highly charged fullerenes, non-IPR isomers are significantly more stable than the IPR ones. To understand this behavior, we compare the results of elaborate density-functional theory (DFT) calculations to those of a simple H\"uckel molecular-orbital theory in which the DFT energies of the corresponding neutral systems are used as a reference. The model leads to a reasonable estimate of the relative stability of the IPR and non-IPR isomers as a function of charge, which can be used to identify, among the thousands of possible isomers and charge states, the non-IPR species that are likely more stable than the IPR isomers.

We present experimental kinetic-energy-release distributions in the asymmetric fission processes $\mathrm{C}_{60}{}^{q+}\ensuremath{\rightarrow}\mathrm{C}_{58}{}^{(q\ensuremath{-}1)+}+\mathrm{C}_{2}{}^{+}$ and $\mathrm{C}_{70}{}^{q+}\ensuremath{\rightarrow}\mathrm{C}_{68}{}^{(q\ensuremath{-}1)+}+\mathrm{C}_{2}{}^{+}$ for highly excited mother ions in charge states $q=4\char21{}8$. We find that the distributions for $\mathrm{C}_{70}{}^{q+}$ are considerably narrower and peak at lower energies than for $\mathrm{C}_{60}{}^{q+}$ in the corresponding charge state when $qg4$. Further, semiempirical values for $\mathrm{C}_{2}{}^{+}$ fission barrier heights were extracted for $q=4\char21{}6$ by means of a statistical approach and the measured intensity ratios between fission and ${\mathrm{C}}_{2}$ evaporation.

We present density functional theory calculations of C{sub 60}{sup q-} and C{sub 70}{sup q-} (q=0-2) isomers that contain zero to three pairs of adjacent pentagons. The first- and second-electron affinities for the archetype structures of C{sub 60} and C{sub 70} are close to the experimental results, while isomers with pentagon adjacencies have significantly higher values. The results are rationalized in view of the numbers and locations of pentagon adjacencies in the fullerene cages and the spherical aromaticity for the magic number C{sub 70}{sup 2-} with a closed (l=5) electronic shell in the spherical gas model.

We report strong even-odd effects in multiple ionization yields of van der Waals dimers in slow ${\mathrm{Xe}}^{30+}+[{\mathrm{C}}_{60}{]}_{2}([{\mathrm{C}}_{60}{\mathrm{C}}_{70}])\ensuremath{\rightarrow}\ensuremath{\cdots}+{[{\mathrm{C}}_{60}{]}_{2}}^{r+}({[{\mathrm{C}}_{60}{\mathrm{C}}_{70}]}^{r+})$ electron-transfer collisions as functions of $r\ensuremath{\le}7$. This behavior may be due to even-odd variations in the sequences of dimer ionization energies as calculated with an electrostatic model including an electrical fullerene-fullerene contact at the $19{a}_{0}$ center-center separation in ${[{\mathrm{C}}_{60}{]}_{2}}^{+}$. Prompt dissociations predominantly yield intact fullerenes ${[{\mathrm{C}}_{60}{]}_{2}}^{r+}\ensuremath{\rightarrow}{{\mathrm{C}}_{60}}^{{r}_{1}+}+{{\mathrm{C}}_{60}}^{{r}_{2}+}$ in the same (${r}_{1}={r}_{2}$, even $r$) or nearby (${r}_{1}={r}_{2}\ifmmode\pm\else\textpm\fi{}1$, odd $rg1$) charge states.

Fragmentation of singly charged anions of adenosine 5'-monophosphate (AMP-) induced by collisions with neutral atoms (Ne, Na) has been studied at a collision energy of 50 keV. The experiments were performed with isolated AMP- as well as with AMP- anions nanosolvated in a cluster with a given number m of water molecules. In the first case, the dominant fragmentation channels concern the loss of adenine, PO3- and H2PO4-. In the latter, loss of water molecules becomes the dominating process, and the AMP- ion is fully protected when m is larger than approximately 13. The observed fragment distributions are well described with the model of an evaporative ensemble.

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.

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.

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 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.

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 $q=11.$