Unexpected and delayed fragmentation dynamics of the organometallic ferrocene induced by ion-collision

We have investigated the fragmentation dynamics of the organometallic ferrocene molecule after interaction with multiply charged ions using multicoincidence mass spectrometry and quantum chemistry calculations. We observed unexpected fragmentation dynamics of the two-body breakup channels from ferrocene dications revealing a charge screening effect from the iron atom and delayed fragmentation dynamics. These observations are rationalized through the population of a specific long-lived excited state, where one positive charge is located on each cyclopentadienyl ring.

Multiple electron capture from isolated protein poly-anions in collision with slow highly charged ions

Multiply charged ions very efficiently capture several electrons from trapped mass/charge selected protein poly-anions, producing dominantly charge-reduced proteins and small neutral losses.

Fragmentation of pure and hydrated clusters of 5Br-uracil by low energy carbon ions: observation of hydrated fragments

The fragmentation of the isolated 5-bromouracil (5BrU) molecule and pure and nano-hydrated 5BrU clusters induced by low energy ¹²C⁴⁺ ions has been studied.

Determination of Energy-Transfer Distributions in Ionizing Ion-Molecule Collisions

The ionization and fragmentation of the nucleoside thymidine in the gas phase has been investigated by combining ion collision with state-selected photoionization experiments and quantum chemistry calculations. The comparison between the mass spectra measured in both types of experiments allows us to accurately determine the distribution of the energy deposited in the ionized molecule as a result of the collision. The relation of two experimental techniques and theory shows a strong correlation between the excited states of the ionized molecule with the computed dissociation pathways, as well as with charge localization or delocalization.

Formation of H2 from internally heated polycyclic aromatic hydrocarbons: excitation energy dependence

We have investigated the effectiveness of molecular hydrogen (H2) formation from Polycyclic Aromatic Hydrocarbons (PAHs) which are internally heated by collisions with keV ions. The present and earlier experimental results are analyzed in view of molecular structure calculations and a simple collision model. We estimate that H2 formation becomes important for internal PAH temperatures exceeding about 2200 K, regardless of the PAH size and the excitation agent. This suggests that keV ions may effectively induce such reactions, while they are unlikely due to, e.g., absorption of single photons with energies below the Lyman limit. The present analysis also suggests that H2 emission is correlated with multi-fragmentation processes, which means that the [PAH-2H](+) peak intensities in the mass spectra may not be used for estimating H2-formation rates.

Absolute fragmentation cross sections in atom-molecule collisions: scaling laws for non-statistical fragmentation of polycyclic aromatic hydrocarbon molecules

We present scaling laws for absolute cross sections for non-statistical fragmentation in collisions between Polycyclic Aromatic Hydrocarbons (PAH/PAH(+)) and hydrogen or helium atoms with kinetic energies ranging from 50 eV to 10 keV. Further, we calculate the total fragmentation cross sections (including statistical fragmentation) for 110 eV PAH/PAH(+) + He collisions, and show that they compare well with experimental results. We demonstrate that non-statistical fragmentation becomes dominant for large PAHs and that it yields highly reactive fragments forming strong covalent bonds with atoms (H and N) and molecules (C6H5). Thus nonstatistical fragmentation may be an effective initial step in the formation of, e.g., Polycyclic Aromatic Nitrogen Heterocycles (PANHs). This relates to recent discussions on the evolution of PAHNs in space and the reactivities of defect graphene structures.

Formations of dumbbell C118 and C119 inside clusters of C60 molecules by collision with α particles

We report highly selective covalent bond modifications in collisions between keV alpha particles and van der Waals clusters of C(60) fullerenes. Surprisingly, C(119)(+) and C(118)(+) are the dominant molecular fusion products. We use molecular dynamics simulations to show that C(59)(+) and C(58)(+) ions--effectively produced in prompt knockout processes with He(2+)--react rapidly with C(60) to form dumbbell C(119)(+) and C(118)(+). Ion impact on molecular clusters in general is expected to lead to efficient secondary reactions of interest for astrophysics. These reactions are different from those induced by photons.

Ions colliding with cold polycyclic aromatic hydrocarbon clusters

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 [C ₁₄H₁₀](k)2+.

Electron capture induced dissociation of nucleotide anions in water nanodroplets

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.

Shape deformations of surface-charged microdroplets

We present the deformation pathway of critically charged glycol and water droplets from the onset of the Rayleigh instability and compare it to numerical results, obtained for perfectly conducting inviscid droplets. In this simple model presented here, the time evolution of the droplet shape is given by the velocity potential equation. The Laplace equation for the velocity potential is solved by expanding the potential onto harmonic functions. For the part of the pathway dominated by electrostatic pressure, the calculations reproduce the experimental data nicely, obtained for both, glycol and water microdroplets. We find that the droplet shape and in particular the tips, just before charge emission, are well fitted by a lemon shape. We stress that the tip is tangent to a cone of 39 degrees and thus significantly narrower than a Taylor cone.

Afterglow mode and the new micropulsed beam mode applied to an electron cyclotron resonance ion source

An increasing number of experiments in the field of low energy ion physics (<25 keV/charge) requires pulsed beams of highly charged ions. Whereas for high-intensity beams (greater than microampere) a pulsed beam chopper, installed downstream to the analyzing dipole, is used. For low-intensity beams (<100 nA) the ion intensity delivered during the pulse may be increased by operating the electron cyclotron resonance discharge in the afterglow mode. This method gives satisfactory results (i.e., average current during the beam pulse is higher than the current in the cw mode) for high charge state ions. In this paper, we report on results of the afterglow mode for beams of (22)Ne(q+), (36)Ar(q+), and (84)Kr(q+) ions. Furthermore, a new promising "micropulsed beam" mode will be described with encouraging preliminary results for (84)Kr(27+) and (36)Ar(17+) ions.

Lifetimes of C60(2-) and C70(2-) dianions in a storage ring

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

Collision-induced dissociation of hydrated adenosine monophosphate nucleotide ions: protection of the ion in water nanoclusters

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.

Highly charged clusters of fullerenes: charge mobility and appearance sizes

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.

Shape oscillations and stability of charged microdroplets

Classical Rayleigh theory predicts an instability of a surface charged liquid sphere, when the Coulomb energy E(C) exceeds twice the surface energy E(S). Previously, electrified liquid droplets have been found to disintegrate at a fissility X=E(C)/2E(S) well below one, however. We determine the stability of charged droplets in an electrodynamic levitator by observing the amplitude and phase of their quadrupolar shape oscillations as a function of the fissility. With this novel approach, which does not rely on an independent determination of the charge and surface tension of the droplets, we are able to confirm for the first time the Rayleigh limit of stability at X=1 for micrometer sized droplets of ethylene glycol.

Rayleigh instabilities in multiply charged sodium clusters

The stability of multiply charged sodium clusters Na(q+)(n) (q< or =10) produced in collisions between neutral clusters and multiply charged ions A(z+) ( z = 1 to 28) is experimentally investigated. Multiply charged clusters are formed within a large range of temperatures and fissilities. They are identified by means of a high-resolution reflectron-type time-of-flight mass spectrometer (m/deltam approximately 14 000). The maximum fissility of stable clusters is obtained for z = 28 and is X approximately 0.85+/-0.07, slightly below the Rayleigh limit (X = 1). It is mainly limited by the initial cluster temperature (T approximately 100 K).

Deep and Bottom Water of the Weddell Sea's Western Rim

Oceanographic observations from the Ice Station Weddell 1 show that the western rim of the Weddell Gyre contributes to Weddell Sea Bottom Water. A thin (< 300 meters), highly oxygenated benthic layer is composed of a low-salinity type of bottom water overlying a high-salinity component. This complex layering disappears near 66 degrees S because of vertical mixing and further inflow from the continental margin. The bottom water flowing out of the western rim is a blend of the two types. Additionally, the data show that a narrow band of warmer Weddell Deep Water hugged the continental margin as it flowed into the western rim, providing the continental margin with the salt required for bottom-water production.