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The following article appeared in Journal of Chemical Physics 139.3 (2013): 034309 and may be found at http://scitation.aip.org/content/aip/journal/jcp/139/3/10.1063/1.4812790, We report experimental results for the ionization and fragmentation of weakly bound van der Waals clusters of n C60 molecules following collisions with Ar2+, He2+, and Xe20+ at laboratory kinetic energies of 13 keV, 22.5 keV, and 300 keV, respectively. Intact singly charged C60 monomers are the dominant reaction products in all three cases and this is accounted for by means of Monte Carlo calculations of energy transfer processes and a simple Arrhenius-type [C60]n+ → C60++(n-1) C60 evaporation model. Excitation energies in the range of only ∼0.7 eV per C60 molecule in a [C60]13 + cluster are sufficient for complete evaporation and such low energies correspond to ion trajectories far outside the clusters. Still we observe singly and even doubly charged intact cluster ions which stem from even more distant collisions. For penetrating collisions the clusters become multiply charged and some of the individual molecules may be promptly fragmented in direct knock-out processes leading to efficient formations of new covalent systems. For Ar2 + and He2 + collisions, we observe very efficient C119+ and C118+ formation and molecular dynamics simulations suggest that they are covalent dumb-bell systems due to bonding between C59+ or C58+ and C60 during cluster fragmentation. In the Ar 2+ case, it is possible to form even smaller C120-2m+ molecules (m = 2-7), while no molecular fusion reactions are observed for the present Xe20+ collisions, This work was supported by the Swedish Research Council (Contract Nos. 621-2008-3773, 621-2009-3468, and 621- 2011-4047). The authors thank Fabien Noury and Stephane Guillous for supplying the ion beam. We acknowledge the COST action CM1204 “XUV/X-ray light and fast ions for ultrafast chemistry (XLIC)” and computer time from CCCUAM and BSC Mare Nostrum. Work partially supported by Project Nos. FIS2010-15127, CTQ2010-17006, CSD2007- 00010 (MICINN), S2009/MAT1726 (CAM), and the CNRS PICS-05356. Studies of interstellar chemistry at Leiden Observatory are supported through advanced-ERC grant (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 report on the first storage of ion beams in the Double ElectroStatic Ion Ring ExpEriment; DESIREE, at Stockholm University. We have produced beams of atomic carbon anions and small carbon anion molecules (C$_n^-$, $n=1,2,3,4$) in a sputter ion source. The ion beams were accelerated to 10 keV kinetic energy and stored in an electrostatic ion storage ring enclosed in a vacuum chamber at 13 K. For 10 keV C$_2^-$ molecular anions we measure the residual-gas limited beam storage lifetime to be 448 s $\pm$ 18 s with two independent detector systems. Using the measured storage lifetimes we estimate that the residual gas pressure is in the 10$^{-14}$ mbar range. When high current ion beams are injected, the number of stored particles does not follow a single exponential decay law as would be expected for stored particles lost solely due to electron detachment in collision with the residual-gas. Instead, we observe a faster initial decay rate, which we ascribe to the effect of the space charge of the ion beam on the storage capacity. %The latter effect becomes insignificant after longer storage times of typically 100-150 seconds and we then observe a constant decay rate due to residual-gas collisions.

We present exact analytical solutions for charge transfer reactions between two arbitrarily charged hard dielectric spheres. These solutions, and the corresponding exact ones for sphere-sphere interaction energies, include sums that describe polarization effects to infinite orders in the inverse of the distance between the sphere centers. In addition, we show that these exact solutions may be approximated by much simpler analytical expressions that are useful for many practical applications. This is exemplified through calculations of Langevin type cross sections for forming a compound system of two colliding spheres and through calculations of electron transfer cross sections. We find that it is important to account for dielectric properties and finite sphere sizes in such calculations, which for example may be useful for describing the evolution, growth, and dynamics of nanometer sized dielectric objects such as molecular clusters or dust grains in different environments including astrophysical ones., 23 pages, 8 figures

We have studied the fragmentation of the polycyclic aromatic hydrocarbon anthracene (C14H10) after double electron transfer to a 5 keV proton. The excitation energies leading to the most relevant dissociation and fission channels of the resulting molecular dication were directly determined experimentally. Density functional theory calculations were performed to explore the potential energy surfaces on which the fragmentation dynamics proceed. There is clear experimental evidence for a dominance of fission into C11H7+-C3H3+ over C2H2+ loss. The energetic ordering of the dissociation and fission channels and the kinetic energy releases are in good agreement with the theoretical results. It can be concluded that the unique combination of experiment and theory presented here is an excellent tool to study the fragmentation of complex molecular ions in unprecedented detail.

International audience; Complex molecular systems such as large molecules or clusters are characterised by a large number of degrees of freedom. Energies well in excess of individual thresholds for fragmentation can be stored for long times and metastable excited states become important. We will concentrate in this paper on the study of the response of such nanoscale systems, i.e. we will study excitation and fragmentation mechanisms induced by highly charged ion radiation, reflecting dynamic energy and charge flow processes. We will illustrate these relaxation processes for different molecular systems from Polycyclic Aromatic Hydrocarbons, water or biomolecule targets and their clusters in collision with multiply charged ions. We will emphasize that slow multiply charged ions provide an efficient way to study the stability of complex systems. Indeed, such ions are known to remove several electrons at large impact parameters resulting in a fast and gentle ionization.

Nearby charges affect the electronic energy levels of chromophores, with the extent of the effect being determined by the magnitude of the charge and degree of charge-chromophore separation. The molecular configuration dictates the charge–chromophore distance. Hence, in this study, we aim to assess how the location of the charge influences the absorption of a set of model protonated and diprotonated peptide ions, and whether spectral differences are large enough to be identified. The studied ions were the dipeptide YK, the tripeptide KYK (Y = tyrosine; K = lysine) and their complexes with 18-crown-6-ether (CE). The CE targets the ammonium group by forming internal ionic hydrogen bonds and limits the folding of the peptide. In the tripeptide, the distance between the chromophore and the backbone ammonium is enlarged relative to that in the dipeptide. Experiments were performed in an electrostatic ion storage ring using a tunable laser system, and action spectra based on lifetime measurements were obtained in the range from 210 to 310 nm. The spectra are all quite similar though there seems to be some changes in the absorption band between 210 and 250 nm, while in the lower energy band all ions had a maximum absorption at ~275 nm. Lifetimes after photoexcitation were found to shorten upon protonation and lengthen upon CE complexation, in accordance with the increased number of degrees of freedom and an increase in activation energies for dissociation as the mobile proton model is no longer operative. Nearby charges affect the electronic energy levels of chromophores, with the extent of the effect being determined by the magnitude of the charge and degree of charge-chromophore separation. The molecular configuration dictates the charge-chromophore distance. Hence, in this study, we aim to assess how the location of the charge influences the absorption of a set of model protonated and diprotonated peptide ions, and whether spectral differences are large enough to be identified. The studied ions were the dipeptide YK, the tripeptide KYK (Y = tyrosine; K = lysine) and their complexes with 18-crown-6-ether (CE). The CE targets the ammonium group by forming internal ionic hydrogen bonds and limits the folding of the peptide. In the tripeptide, the distance between the chromophore and the backbone ammonium is enlarged relative to that in the dipeptide. Experiments were performed in an electrostatic ion storage ring using a tunable laser system, and action spectra based on lifetime measurements were obtained in the range from 210 to 310 nm. The spectra are all quite similar though there seems to be some changes in the absorption band between 210 and 250 nm, while in the lower energy band all ions had a maximum absorption at ∼275 nm. Lifetimes after photoexcitation were found to shorten upon protonation and lengthen upon CE complexation, in accordance with the increased number of degrees of freedom and an increase in activation energies for dissociation as the mobile proton model is no longer operative.

In this thesis a series of experiments on collisions between atomic projectile ions at keV energies and target vapors of either isolated molecules or van der Waals clusters is presented and analyzed. The atomic ions are produced in an Electron Cyclotron Resonance (ECR) ion source, accelerated and guided into the target volume. The charged target collision products are mass-to-charge analyzed in a time-of-flight spectrometer. The Polycyclic Aromatic Hyrdrocarbons (PAHs) Anthracene (C14H10), Coronene (C24H12), two C16H10 isomers, Pyrene and Fluoranthene, and the fullerene C60 are examined.For projectile ions in low charge states, small impact parameter collisions dominate, which leads to internal heating of the target. With isolated molecules as targets, this typically results in ionization and often also in fragmentation. For cluster targets energy and charge are rapidly distributed among the cluster building blocks. This is followed by cluster evaporation and very limited fragmentation of the individual molecules. C119+ and C118+ are observed as products. These are due to the formation of the reactive C58/59+ ions by direct knockout processes, which react with another C60 of the cluster to form dumb-bell shaped molecules.For projectile ions of high charge (Xe20+) larger impact parameters dominate, leading to little internal heating. For isolated molecule targets, intact molecular ions are the main collision products. Charged fragments stem mostly from multifragmentation following ionization to high charge states. For cluster targets, the collision products consist mainly of singly charged monomers. Fragmentation of the individual molecules is comparatively strong. This suggests a quick distribution of charges followed by a Coulomb explosion, which leads to internal heating.The results show that weakly bound clusters do not sustain the impact of keV-ions and that it is possible to form new molecular structures.

Helium adsorbed on C60+ and C70+ exhibits phenomena akin to helium on graphite. Mass spectra suggest that commensurate layers form when all carbon hexagons and pentagons are occupied by one He each, but that the solvation shell does not close until 60 He atoms are adsorbed on C60+, or 62 on C70+. Molecular dynamics simulations of C 60Hen+ at 4 K show that the commensurate phase is solid. Helium added to C60He32+ will displace some atoms from pentagonal sites, leading to coexistence of a registered layer of immobile atoms interlaced with a nonregistered layer of mobile atoms, This work was supported by MICINN projects FIS2010-15127, ACI2008-0777, CTQ2010-17006, Consolider-Ingenio CSD2007-00010, CAM program NANOBIOMAGNET S2009/MAT1726, the Austrian Science Fund, Wien (FWF, projects P19073, L633, and I200 N29), the European Commission, Brussels (ITS-LEIF), and the European COST Action CM0702.

The loss of C(2)H(2) is a low activation energy dissociation channel for anthracene (C(14)H(10)) and acridine (C(13)H(9)N) cations. For the latter ion another prominent fragmentation pathway is the loss of HCN. We have studied these two dissociation channels by collision induced dissociation experiments of 50 keV anthracene cations and protonated acridine, both produced by electrospray ionization, in collisions with a neutral xenon target. In addition, we have carried out density functional theory calculations on possible reaction pathways for the loss of C(2)H(2) and HCN. The mass spectra display features of multi-step processes, and for protonated acridine the dominant first step process is the loss of a hydrogen from the N site, which then leads to C(2)H(2)/HCN loss from the acridine cation. With our calculations we have identified three pathways for the loss of C(2)H(2) from the anthracene cation, with three different cationic products: 2-ethynylnaphthalene, biphenylene, and acenaphthylene. The third product is the one with the overall lowest dissociation energy barrier. For the acridine cation our calculated pathway for the loss of C(2)H(2) leads to the 3-ethynylquinoline cation, and the loss of HCN leads to the biphenylene cation. Isomerization plays an important role in the formation of the non-ethynyl containing products. All calculated fragmentation pathways should be accessible in the present experiment due to substantial energy deposition in the collisions.

We report on measurements of the ionization and fragmentation of polycyclic aromatic hydrocarbon (PAH) targets in Xe(20+) + C(16)H(10) and Xe(20+) + [C(16)H(10)](k) collisions and compare results for the two C(16)H(10) isomers: pyrene and fluoranthene. For both types of targets, i.e., for single PAH molecules isolated in vacuum or for isomerically pure clusters of one of the molecules, the resulting fragment spectra are surprisingly similar. However, we do observe weak but significant isomer effects. Although these are manifested in very different ways for the monomer and cluster targets, they both have at their roots small differences (2.5 eV) between the total binding energies of neutral, and singly and multiply charged pyrene and fluoranthene monomers. The results will be discussed in view of the density functional theory calculations of ionization and dissociation energies for fluoranthene and pyrene. A simple classical over-the-barrier model is used to estimate cross sections for single- and multiple-electron transfer between PAHs and ions. Calculated single and multiple ionization energies, and the corresponding model PAH ionization cross sections, are given.

We discuss the ionization and fragmentation of isolated monomers and cold clusters of polycyclic aromatic hydrocarbon (PAH) molecules in collisions with keV ions in low or high charge states. With low charge state projectile ions, PAH cluster or monomer targets are thermally excited through electronic stopping processes directly in close peripheral or penetrating collisions while only single or few electrons are removed. With high charge state projectiles, electrons are very effectively removed from both the cluster and the monomer target already at very large distances with very little direct target heating. Singly charged and internally very hot PAH monomers are dominant fragmentation products following collisions between Xe20+ ions and PAH clusters. We suggest that this due to an unusually strong dominance of multiple-ionization over single ionization for PAH clusters interacting with highly charged ions. Here, charge and excitation energy is very rapidly redistributed within the clusters before they Coulomb explode and we suggest that these Coulomb explosions induce strong internal heating in the individual PAH molecules. We thus conclude that PAH cluster fragmentation always dominates strongly for all ionization processes regardless if these are due to interactions with ions in high or low charge states. These findings are discussed in view of simple models for cluster evaporation or single and multiple ionizations of PAH clusters.

We have performed density functional theory calculations for a range of neutral, singly, and multiply charged polycyclic aromatic hydrocarbons (PAHs), and their fragmentation products for H-, H(+)-, C(2)H(2)-, and C(2)H(2)(+)-emissions. The adiabatic and vertical ionization energies follow linear dependencies as functions of charge state for all five intact PAHs (naphthalene, biphenylene, anthracene, pyrene, and coronene). First estimates of the total ionization and fragmentation cross sections in ion-PAH collisions display markedly different size dependencies for pericondensed and catacondensed PAH species, reflecting differences in their first ionization energies. The dissociation energies show that the PAH(q+)-molecules are thermodynamically stable for q ≤ 2 (naphthalene, biphenylene, and anthracene), q ≤ 3 (pyrene), and q ≤ 4 (coronene). PAHs in charge states above these limits may also survive experimental time scales due to the presence of reaction barriers as deduced from explorations of the potential energy surface regions for H(+)-emissions from all five PAHs and for C(2)H(2)(+)-emission from naphthalene--the smallest PAH.

We show that the relative surface charge distribution from classical electrostatics cannot be used to discriminate between different assumptions about the dielectric properties of fullerenes interacting with external charges.

4 páginas, 7 figuras.-- et al., Double vs triple: How is the electronic coupling between the donor and acceptor affected by alkene vs alkyne bridges? To address this question the authors have elucidated the intrinsic optical properties of p-nitrophenolates by using state-of-the-art gas-phase spectroscopy (see picture) and excited-state calculations., The San Sebastian group acknowledges funding by the Spanish MEC (FIS2007–65702-C02–01), “Grupos Consolidados UPV/EHU del Gobierno Vasco” (IT-319–07), the European Community through the e-I3 ETSF project (Contract Number 211956), Ikerbasque and by the Barcelona Supercomputing Center, “Red Espanola de Supercomputacion”, SGIker ARINA (UPV/EHU), ACI-Promociona (ACI2009–1036), and ETORTEK (inanoGUNE).

We find that the most stable fullerene isomers, C-70-C-94, form efficiently in close-to central collisions between keV atomic ions and weakly bound clusters of more than 15 C-60-molecules. We observe extraordinarily high yields of C-70 and marked preferences for C-78 and C-84. Larger even-size carbon molecules, C-96-C-180, follow a smooth log-normal (statistical) intensity distribution. Measurements of kinetic energies indicate that C-70-C-94 mainly are formed by coalescence reactions between small carbon molecules and Coo, while C-n with n andgt;= 96 are due to self-assembly (of small molecules) and shrinking hot giant fullerenes. Original Publication:H Zettergren, H A B Johansson, H T Schmidt, Jens Jensen, P Hvelplund, S Tomita, Y Wang, F Martin, M Alcami, B Manil, L Maunoury, B A Huber and H Cederquist, Magic and hot giant fullerenes formed inside ion irradiated weakly bound C-60 clusters, 2010, JOURNAL OF CHEMICAL PHYSICS, (133), 10, 104301.http://dx.doi.org/10.1063/1.3479584Copyright: American Institute of Physicshttp://www.aip.org/

International audience; Electron-transfer and -capture dissociations of doubly protonated peptides gave dramatically different product ions for a series of histidine-containing pentapeptides of both non-tryptic (AAHAL, AHAAL, AHADL, AHDAL) and tryptic (AAAHK, AAHAK, AHAAK, HAAAK, AAAHR, AAHAR, AHAAR, HAAAR) type. Electron transfer from gaseous Cs atoms and fluoranthene anions triggered backbone dissociations of all four N-C(alpha) bonds in the peptide ions in addition to loss of H and NH(3). Substantial fractions of charge-reduced cation-radicals did not dissociate on an experimental time scale ranging from 10(-6) to 10(-1) s. Multistage tandem mass spectrometric (MS(n)) experiments indicated that the non-dissociating cation-radicals had undergone rearrangements. These were explained as being due to proton migrations from N-terminal ammonium and COOH groups to the C-2' position of the reduced His ring, resulting in substantial radical stabilization. Ab initio calculations revealed that the charge-reduced cation-radicals can exist as low-energy zwitterionic amide pi* states which were local energy minima. These states underwent facile exothermic proton migrations to form aminoketyl radical intermediates, whereas direct N-C(alpha) bond cleavage in zwitterions was disfavored. RRKM analysis indicated that backbone N-C(alpha) bond cleavages did not occur competitively from a single charge-reduced precursor. Rather, these bond cleavages proceeded from distinct intermediates which originated from different electronic states accessed by electron transfer. In stark contrast to electron transfer, capture of a free electron by the peptide ions mainly induced radical dissociations of the charge-carrying side chains and loss of a hydrogen atom followed by standard backbone dissociations of even-electron ions. The differences in dissociation are explained by different electronic states being accessed upon electron transfer and capture.

International audience; We report the first experimental study of ions interacting with clusters of polycyclic aromatic hydrocarbon (PAH) molecules. Collisions between 11.25 keV 3He+ or 360 keV 129Xe20+ and weakly bound clusters of one of the smallest PAH molecules, anthracene, show that C14H10 clusters have much higher tendencies to fragment in ion collisions than other weakly bound clusters. The ionization is dominated by peripheral collisions in which the clusters, very surprisingly, are more strongly heated by Xe20+ collisions than by He+ collisions. The appearance size is k = 15 for [C14H10]2+k

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.

Multiply ionized fullerene dimers ([C(60)](2) (q+),q=1-6) have been studied by means of state-of-the-art density functional theory methods. We found that the singly charged dimer is more strongly bound than the corresponding neutral van der Waals dimer at the binding distance of the latter; in contrast, multiply charged dimers (q>or=2) are unstable. For the latter dimers, the calculated kinetic energy release in the binary fission yielding intact fullerenes is lower than those reported in recent experimental work. This implies that, in such experiments, there are significant internal excitations in the separating monomers. We also show that electron transfer within the charged dimers occurs on the subfemtosecond time scale, in accordance with the high charge mobility observed in dimers and larger clusters of fullerenes. This provides an explanation for the even-odd effects in the measured multiple ionization yields of fullerene dimers.

We report on evaporation studies on positively charged water clusters (H(+)(H(2)O)(N)) and negatively charged mixed clusters (X(-)(H(2)O)(N)) with a small core ion X (X=O(2), CO(3), or NO(3)), in the size range N=5-300. The clusters were produced by corona discharge in ambient air, accelerated to 50 keV and mass selected by an electromagnet. The loss of monomers during the subsequent 3.4 m free flight was recorded. The average losses are proportional to the clusters' heat capacities and this allowed the determination of size-dependent heat capacities. The values are found to increase almost linearly with clusters size for both species, with a rate of 6k(B)-8k(B) per added molecule. For clusters with N21 the heat capacities per molecule are lower but the incremental increase higher. For N21 the values are intermediate between the bulk liquid and the solid water 0 degrees C values.

Electron capture by peptide dications results in N–C α bond cleavage to give c + and z or c and z + fragments. In this work we have investigated how crown ether (18-crown-6 = CE) complex formation and a change in the internal energy affect the charge division between the z and c fragments. Both complex formation and a high temperature have the effect of breaking internal ionic hydrogen bonds. The crown ether complex also lowers the probability of internal proton transfer between the two fragments, and reduces the recombination energy of the charged group it targets. The systems under study were doubly protonated di- and tripeptides, [AK+2H] 2+ , [AR+2H] 2+ , [KK+2H] 2+ and [GHK+2H] 2+ (A = alanine, K = lysine, R = arginine, G = glycine and H = histidine). For crown ether complexes the formation of z + ions was always preferred over c + ions. In the case of [GHK+2H] 2+ , the bare ion dissociated into z 2 + + c 1 and z 1 + c 2 + from cleavage of the first and second N–C α bond, respectively, whereas z 1 + fragment ions had higher yield than c 2 + for [GHK+2H] 2+ (CE). The internal energy of the ions was changed by storing them in a 22-pole ion trap in which they were equilibrated to a temperature between −60 and 90 °C in collisions with helium gas. The average internal energy increased by about 0.4 eV from the lowest to the highest temperature for the dipeptides and 0.6 eV for the tripeptide. More fragmentation occurred at the higher temperature, as observed by an increase in the formation of b + and y + ions after breakage of the peptide bond of vibrationally hot even-electron cations and from secondary reactions of z + radical cations within the time window of the experiment. However, the z + to c + partitioning was not found to depend significantly on temperature in the measured range. In addition the decay of [GHK+H] + /[GHK+2H] + and [AK+H] + formed after electron capture by [GHK+2H] 2+ and [AK+2H] 2+ was found to occur on a microsecond to millisecond timescale. The data are well described by a power-law decay, which implies that the internal energy distribution is broad after electron capture and/or hydrogen loss.

The results from an experimental study of bare and microsolvated peptide monocations in high-energy collisions with cesium vapor are reported. Neutral radicals form after electron capture from cesium, which decay by H loss, NH(3) loss, or N-C(alpha) bond cleavage into characteristic z(*) and c fragments. The neutral fragments are converted into negatively charged species in a second collision with cesium and are identified by means of mass spectrometry. For protonated GA (G = glycine, A = alanine), the branching ratio between NH(3) loss and N-C(alpha) bond cleavage is found to strongly depend on the molecule attached (H(2)O, CH(3)CN, CH(3)OH, and 18-crown-6 ether (CE)). Addition of H(2)O and CH(3)OH increases this ratio whereas CH(3)CN and CE decrease it. For protonated AAA ([AAA+H](+)), a similar effect is observed with methanol, while the ratio between the z(1) and z(2) fragment peaks remains unchanged for the bare and microsolvated species. Density functional theory calculations reveal that in the case of [GA+H](+)(CE), the singly occupied molecular orbital is located mainly on the amide group in accordance with the experimental results.

Photodissociation of isolated Fe(III)-heme(+) and Fe(III)-heme(+)(His) ions in the gas phase has been investigated using an electrostatic storage ring. The experiment provides three pieces of information, namely fragmentation channels, dissociation times, and absorption spectra. After photoexcitation with either 390 or 532 nm light, we find that the fragmentation takes place on a microsecond to millisecond time scale, and the channels are CH(2)COOH loss (beta-cleavage reaction) and histidine loss from Fe(III)-heme(+) and Fe(III)-heme(+)(His), respectively. These channels were also observed by means of collision-induced dissociation. Significant information on the nonradiative processes that occur after photoexcitation was revealed from the decay spectra. At early times (first two to three milliseconds), the decay of the photoexcited ions is well-described by a statistical model based on an Arrhenius-type expression for the rate constant. The activation energy and preexponential factor are 1.9 +/- 0.2 eV and 1 x 10(17) to 1 x 10(21) s(-1) for heme(+) and 1.4 +/- 0.2 eV and 1 x 10(16) to 1 x 10(19) s(-1) for heme(+)(His). Decay on a longer time scale was also observed and is ascribed to the population of lower-lying states with higher spin multiplicity because intersystem crossing back to the electronic ground-state is a bottleneck for the dissociation. The measurements give lifetimes for these lower-lying states of about 10 ms after 390 nm excitation and we estimate the probability of spin flip to be 0.3 and 0.8 for heme(+) and heme(+)(His), respectively.

We report an unusual prototropic rearrangement in gas-phase radicals formed by collisional electron transfer from cesium atoms to protonated peptides HAL, AHL, and ALH at 50 keV. The rearrangement depends on the peptide amino acid sequence and presence or steric accessibility of a free carboxyl group. Upon electron transfer, protonated HAL and ALH rearrange to tautomers that are detected as nondissociated anions in charge-reversal mass spectra. The isomerization is minor in protonated ALH and virtually absent in HAL amide. Electron structure calculations indicate that the gas-phase ions are preferentially protonated in the His imidazole ring and consist of multiple conformers that differ in their hydrogen bonding patterns. Electron transfer to protonated HAL and AHL triggers an exothermic and dynamically barrierless transfer of the carboxyl proton onto the C-2' position of the His ring that occurs on a 120-240 ns time scale. The kinetics of this isomerization are controlled by internal rotations in the radicals to assume conformations favoring the proton transfer. The radical conformations also affect subsequent proton migrations in zwitterionic His imidazoline intermediates that reform the COOH group and result in His ring isomerization. This autocatalytic prototropic rearrangement in gas-phase peptide radicals is analogous to enzyme catalytic reactions involving His and acidic amino acid residues. In contrast to HAL and AHL, the C-2' position is sterically inaccessible in ALH radicals. These radicals undergo proton migrations to the His ring C-5' positions that have moderate energy barriers and are less efficient. RRKM calculations on the combined B3LYP and PMP2/6-311++G(2d,p) potential energy surface of the ground doublet electronic state of the peptide radicals provided rate constants that were quantitatively consistent with the dissociations observed in the gas phase. The formation of minor sequence z(1) and z(2) fragments from AHL was interpreted as occurring in the first excited state of the radical.

Photodissociation of protonated Tyr, Ala-Tyr, Tyr-Ala (Ala = alanine, Tyr = tyrosine), and their complexes with 18-crown-6-ether (CE) was performed in an electrostatic ion storage ring using a tunable laser system. While the three bare ions all absorb strongly at 222 nm, absorption at higher wavelengths was barely visible from sampling the neutrals formed in delayed dissociation. A band at 270 nm was introduced, however, as a consequence of CE attachment to the bare ions. To understand the difference between bare ions and complexes, electronically excited states are considered: The initially reached pipi* state on phenol couples with the dissociative pisigma* state on ammonium, which leads to direct hydrogen loss. Cold radical cations are formed that at high wavelengths do not have enough energy for further dissociation. Excitation within the 222-nm band on the other hand leads to delayed dissociation of stored radical cations that is monitored in the present setup. The pisigma* state moves out of the spectral region upon CE attachment, and instead statistical dissociation is sampled on the microsecond to millisecond time scale at all wavelengths. Our data demonstrate the strength of using supramolecular complexes for action spectroscopy experiments to prevent erroneous spectra as a result of undesired dissociation (H loss) from electronically excited states. The gas-phase absorption spectra firmly establish the perturbations of the phenol electronic structure by a water solvent: The 270-nm band red shifts by approximately 5 nm, whereas the 222-nm band changes by approximately 3 nm. Both transitions occur in the phenol group. These results may be useful for protein dynamics experiments that rely on electronic excitations. Product ion mass spectra of [Tyr + H]+, [Ala-Tyr + H]+, [Tyr-Ala + H]+, [Ala-Tyr + H]+(CE), and [Tyr-Ala + H]+(CE) significantly depend on the excitation wavelength from 210 to 310 nm and on whether the ionizing proton is mobile or not.

International audience; We have studied fragmentation of water embedded adenosine 5'-monophosphate (AMP) anions after collisions with neutral sodium atoms. At a collision energy of 50 keV, loss of water molecules from the collisionally excited cluster ions is the dominant process and fragmentation of the AMP itself is almost completely prohibited if the number of attached water molecules is larger than 13. However, regardless of the initial number of water molecules attached to the ion, capture of an electron, i.e. formation of a dianion, always leads to loss of a single hydrogen atom accompanied by evaporation of water molecules. This damaging effect becomes more important as the size of the water cluster increases, which is just the opposite to the protective behavior observed for collision induced dissociation (CID) without electron transfer. For both cases, the loss of water molecules within the experimental time frame is qualitatively well described by means of a common model of an evaporative ensemble. These simulations, however, indicate that characteristically different distributions of internal energy are involved in CID and electron capture induced dissociation.

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.

The most abundant fullerenes, C(60) and C(70), and all the pure carbon fullerenes larger than C(70), follow the isolated-pentagon rule (IPR). Non-IPR fullerenes containing adjacent pentagons (APs) have been stabilized experimentally in cases where, according to Euler's theorem, it is topologically impossible to isolate all the pentagons from each other. Surprisingly, recent experiments have shown that a few endohedral fullerenes, for which IPR structures are possible, are stabilized in non-IPR cages. We show that, apart from strain, the physical property that governs the relative stabilities of fullerenes is the charge distribution in the cage. This charge distribution is controlled by the number and location of APs and pyrene motifs. We show that, when these motifs are uniformly distributed in the cage and well-separated from one other, stabilization of non-IPR endohedral and exohedral derivatives, as well as pure carbon fullerene anions and cations, is the rule, rather than the exception. This suggests that non-IPR derivatives might be even more common than IPR ones.

A new method for time-resolved daughter ion mass spectrometry is presented, based on the electrostatic ion storage ring in Aarhus, ELISA. Ions with high internal energy, e.g., as a result of photoexcitation, dissociate and the yield of neutrals is monitored as a function of time. This gives information on lifetimes in the microsecond to millisecond time range but no information on the fragment masses. To determine the dissociation channels, we have introduced pulsed supplies with switching times of a few microseconds. This allows rapid switching from storage of parent ions to storage of daughter ions, which are dumped into a detector after a number of revolutions in the ring. A fragment mass spectrum is obtained by monitoring the daughter ion signal as a function of the ring voltages. This technique allows identification of the dissociation channels and determination of the time dependent competition between these channels.

We have studied the outcome of collisions between the hydrated nucleotide anion adenosine 5'-monophosphate (AMP) and sodium. Electron capture leads to hydrogen loss as well as water evaporation regardless of the initial number m of water molecules attached to the parent ion (m< or =16). The yield of dianions with microsecond lifetimes increases strongly with m, which is explained from dielectric screening of the two charges by the water nanodroplet. For comparison, collision induced dissociation results in water losses with no or very little damage of the AMP molecule itself.

The fragmentation of doubly protonated AK dipeptide ions has been investigated after collisional electron transfer. Electron capture leads to three dominant channels, H loss, NH3 loss, and N–C? bond breakage to give either c+ or z+ fragment ions. The relative importance of these channels has been explored as a function of ion velocity, the degree of complexation with crown ether, and collision gas. Our results indicate that H loss and NH3 loss are competing channels whereas the probability of N–C? bond breakage is more or less constant.

The behavior of protonated tryptophan (TrpH(+)) and its complex with 18-crown-6-ether (CE) after photoexcitation has been explored based on measurements of dissociation lifetimes, fragmentation channels, and absorption spectra using an electrostatic ion storage ring. A recent implementation of pulsed power supplies for the ring elements with microsecond response times allows us to identify the daughter ion fragment masses and to disentangle fragmentation that occurs from excited states immediately after photoexcitation from that occurring on a longer time scale of several microseconds to milliseconds. We find that attachment of crown ether significantly alters the dissociation channels since it renders the pisigma(*)(NH(3)) state inaccessible and hence prevents the N-H bond breakage which is an important fragmentation channel of TrpH(+). As a result, on a long time scale (10 micros), photoexcited TrpH(+)(CE) decays exponentially whereas TrpH(+) displays a power-law decay. The only ions remaining in the latter case are Trp(+) radical cations with a broad internal energy distribution caused by the departing hydrogen. Large changes in the fragment branching ratios as functions of excitation wavelength between 210 and 290 nm were found for both TrpH(+) and TrpH(+)(CE).

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.

The authors find even-odd variations as functions of r (or=7) for multiple ionization of van der Waals dimers in slow Xe(30+)+[C60]2([C60C70])--...+[C60]2(r+)([C60C70](r+)) electron-transfer collisions. This even-odd behavior is in sharp contrast to the smooth one for fullerene monomers and may be related to even-odd effects in dimer ionization energies in agreement with results from an electrostatic model. The kinetic energy releases for dimer dissociations [predominantly yielding intact fullerenes [C60]2(r+)--C60(r1+)+C60(r2+) in the same (r1=r2) or nearby (r1=r2+/-1) charge states] are found to be low in comparison with the corresponding model results indicating that internal excitations of the separating (intact) fullerenes are important. Experimental appearance sizes for the heavier clusters of fullerenes [C60]n(r+) (n3 and r=2-5) compare well with predictions from a new nearest-neighbor model assuming that r unit charges in [C60]n(r+) are localized to r C60 molecules such that the Coulomb energy of the system is minimized. The system is then taken to be stable if (i) two (singly) charged C60 are not nearest neighbors and (ii) the r C60(+) molecules have binding energies to their neutral nearest neighbors which are larger than the repulsive energies for the (r-1) C60(+)-C60(+) pairs. Essential ingredients in the nearest-neighbor model are cluster geometries and the present results on dimer stabilities.

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.

The high electrical resistance of a microchannel plate at cryogenic temperatures presents a severe limitation on the detection count rate at low temperatures. We are currently investigating the detector properties of a novel atomic layer deposition activated MCP developed by Gorelikov et al [1]. This is of particular interest to us as MCPs are used for particle detection at the cryogenic DESIREE facility at Stockholm University.

The study is focused on investigation of stereospecific interaction of amino acid enantiomers with chiral targets in collisions of 1 keV and ~ 1 eV energy using tandem mass spectrometry.

We have measured the rates of neutrals produced from 10 keV Cn− or CnH− (n=2, 4, 6, 8, and 10) ion beams stored in one of DESIREE's 14 K storage rings. For n=4, 6, and 8 we observe marked differences between Cn− and CnH− cooling rates as inverse internal conversion [cf. S. Martin et al (2013) Phys. Rev. Lett. 110, 063003] processes are effective for the Cn− ions only. Knowledge of the cooling rates of these ions are important for estimates of their formation and destruction rates in cold interstellar environments.

We investigate collisions of Polycyclic Aromatic Hydrocarbon cations with noble gases at center-of- mass energies close to 100 eV and below. Charged fragments are stored in a new version of the electrostatic ion beam trap ConeTrap. After storage for different well-defined times we extract the ions and use a reflectron based time-of-flight mass spectrometer to analyse the ion mass distributions. This will give information on secondary decay channels and the lifetime of the initially trapped fragment.

We have studied the fragmentation of Pyrene or hydrogenated Pyrene cations (C16H+10+x with x = 0, 5, 6 and 16) in collision with He. We find that the carbon backbone fragmentation cross section increases with the number of H atoms added. This suggests that hydrogenation does not protect PAHs in, e.g., harsh interstellar environments.

Systematic molecular dynamics simulations based on the self-consistent charge density functional tight-binding method have been performed for the molecular collisions inside clusters of fullerene molecules, after prompt atom knockouts by the keV ions. 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

In this work we study statistical fragmentation of Polycyclic Aromatic Hydrocarbon (PAH) molecules following collisions with keV ions. Dissociation and transition state energies for H- and H2-emissions from PAHs have been calculated and a simple electronic stopping model has been used to calculate collision-induced internal PAH temperatures. We find that H2 may be formed efficiently from pristine PAHs for internal ion temperatures above 2200 K.

We have performed an extensive set of experiments with focus on collisions between PAH or biomolecular cations and He atoms at center-of-mass energies around 100 eV. In this energy range, non-statistical fragmentation processes are important.

This paper highlight results from studies of keV-ion impact on complex molecules and molecular clusters, which have been carried out at the ARIBE facility in Caen (France) during the last decade. Studies of fullerenes, Polycyclic Aromatic Hydrocarbons (PAHs), and biomolecules are reviewed with focus on the effect of a surrounding environment when ions interact with weakly bound clusters of theses species. One common result is that charge and energy are rapidly shared between the individual molecules in the clusters, in contrast to e.g. weakly bound atomic clusters where the charge stay localized to a few atoms from which the electrons are removed during the collisions. Another important finding is that ion collisions may induce reactions within clusters such as e.g. proton transfer and different types of molecular growth processes. In the latter case, these processes may be driven by prompt non-statistical atom knockouts in billiard-ball like atom-atom collisions favouring highly reactive fragments. In contrast, statistical fragmentation in general yields different and less reactive fragments.

We report experimental indications of chemical reactions inside clusters of pyrene (C16H10) molecules following collisions with 11.25 keV He+ - and 12.0 keV Ar2+ ions. It appears that bond-forming reactions are more likely with the heavier projectile. We have also performed classical molecular dynamics simulations of these processes where we treat the interaction between the projectiles and all atoms in the cluster as well as non-dispersive and dispersive forces between all atoms in the cluster before, during and after the collision. The time step is typically 10−17 s and the total simulation time 1 picosecond. The simulations were performed for a fixed cluster size with 36 pyrene molecules, although there is a broad range of cluster sizes in the experiment. Still, there is good qualitative agreement between the experimental and the simulated mass spectra exhibiting reaction products with masses between those of the C16H10- monomer and dimer in both cases. Additional studies of the influence of the projectile charge and mass is planned as well as simulations on longer time scales and as functions of cluster size.

Here we will briefly describe the commissioning of the Double ElectroStatic Ion Ring ExpEriment (DESIREE) facility 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 apparatus allows for studies of interactions between cations and anions at very low and well-defined centre-of-mass energies (down to a few meV) and at very low internal temperatures (down to a few K).

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.