Gradual weakening down to complete disappearance of the velocity correlated cluster emission effect in keV collisions of C60 with light metallic targets: Microscopic insights via molecular dynamics simulations.

The so-called velocity correlated cluster emission (VCCE) effect is the recently reported emission of large clusters with nearly the same velocity from an atomically heavy target (such as coinage metals) following a single C60- impact at the keV kinetic energy range. The effect was observed to get weaker for a meaningfully lighter target (Al) down to its complete disappearance for C60-Be impact. Microscopic insight into the subpicosecond evolution and thermalization of the impact induced energy spike (driving the effect) is achieved using molecular dynamics simulations. It is shown that the weakening of the VCCE effect for aluminum (toward its complete disappearance for Be) is due to ultrafast decay of the atomic number density within the spike nanovolume, thus not enabling the buildup of sufficient subsurface pressure as required for driving the correlated emission. For the Be target, an extremely rapid decay of nearly 90% of the initial density within 200 fs from impact is observed. This finding provides further support for the conclusion that the emission of the velocity correlated clusters as observed for the heavier targets takes place within an ultra-short time window of only a few hundreds of femtoseconds, roughly extending from 200 to 500 fs from impact. The lower bound is dictated by the requirement for a relatively slow rate of decay of number density, enabling the buildup of a sufficiently intense pressure spike. The upper bound is dictated by the cooling rate of the spike (still maintaining an extremely high temperature of kT ≥ 1 eV, as experimentally observed) and the onset of the evolution of the impact crater.

Velocity correlated emission of secondary clusters by a single surface impact of a polyatomic ion: a new mechanism of cluster emission and subpicosecond probing of extreme spike conditions.

Emission of secondary clusters off clean solid surfaces following the impact of a projectile ion at kiloelectronvolt (keV) kinetic energies is important from both practical and fundamental points of view. Understanding the underlying emission mechanisms using different types and sizes of projectile ions is therefore of high interest. In this perspective article we provide an up-to-date review of our recently observed new mechanism of velocity correlated cluster emission (VCCE) describing the emission of large clusters off different targets following the impact of a large polyatomic ion (C60-). Due to its large collision cross section and large number of light constituent atoms, the incoming C60- disintegrates completely upon an impact resulting in a rather broad and shallow energy deposition, high subsurface energy density and ultrafast evolution of an extreme nonlinear collision cascade dynamics. It is shown that kinetic energy distributions (KEDs) of the emitted clusters behave very differently from the KEDs of cluster ions emitted following the impact of a heavy monoatomic ion. All the large clusters emitted from a given target (following the keV C60- impact) move with nearly the same velocity and their KEDs could be fairly well described by a shifted Maxwellian. Namely, a thermal distribution superimposed on a center-of-mass velocity of a moving precursor which is the source of the emitted clusters. So far we have measured and analyzed KEDs for NbnCn+, TanCn+,Agn+, Cun+, Aun+ and Aln+ clusters emitted from their respective metallic targets, thus demonstrating the general validity of the VCCE effect as a new sputtering mechanism. We have also proposed a simple model for the initial, subpicosecond emission step of the precursor (and the resulting emitted clusters) and supported it by molecular dynamics (MD) simulations of the thermal behavior of the impact induced spike on the subpicosecond timescale. It is shown that each complete family of measured cluster KEDs (for a given target) can serve as a unique diagnostic tool, probing the extreme temperature and pressure evolving in the impact induced spike. We will discuss our accumulated findings from new perspectives and report new observations and additional analysis aspects.

Emission of velocity-correlated clusters in fullerene-solid single collision and diagnostics of the impact energized subsurface nanovolume.

We have measured kinetic energy distributions (KEDs) of large clusters emitted from five different solid targets following a single impact of C60 - ion at 14 keV kinetic energy. It was found that all the large clusters emitted from a given target move with nearly the same velocity and that their KEDs can be described by a thermal distribution riding on a common center-of-mass velocity (shifted Maxwellian) of some precursor. This behavior is in sharp contrast to that observed when the incoming projectile ion is monoatomic. Different trends were observed when comparing the behavior of the KED families of group 5 early transition metal elements (Ta and Nb) with those of group 11 late transition metals (Cu, Ag, and Au). We propose a model for the initial phase of formation of the precursor and show that the measured KEDs can serve as both pressure and temperature probes for the impact excited, highly energized subsurface nanovolume, driving the ejection of the clusters. It is also shown that under the proposed impact scenario, thermally equilibrated conditions (of the atomic subsystem) can be established at the subsurface nanovolume on the early subpicosecond time scale relevant for the emission process. This conclusion is demonstrated both experimentally by the KEDs of the emitted large clusters (very high temperatures and center-of-mass velocity) and by molecular dynamics simulation of the temporal evolution of the thermal characteristics of the impact energized subsurface nanovolume.

Direct experimental observation of a new mechanism for sputtering of solids by a large polyatomic projectile: velocity-correlated cluster emission.

We have measured kinetic energy distributions of Ta(n)C(n)(+) (n=1-10) and Ag(n)(+) (n=1-9) cluster ions sputtered off Ta and Ag targets, following impact of C(60)(-) at 14 keV kinetic energy. A gradual increase of the most probable kinetic energies with increased size of the emitted cluster was observed (nearly the same velocity for all n values). This behavior is in sharp contrast to that reported for cluster emission induced by the impact of a monoatomic projectile. Our observation is in good agreement with a mechanism based on the new concept of a superhot moving precursor as the source of the emitted clusters.

Formation and emission of gold and silver carbide cluster ions in a single C60- surface impact at keV energies: experiment and calculations.

Impact of fullerene ions (C(60)(-)) on a metallic surface at keV kinetic energies and under single collision conditions is used as an efficient way for generating gas phase carbide cluster ions of gold and silver, which were rarely explored before. Positively and negatively charged cluster ions, Au(n)C(m)(+) (n = 1-5, 1 ≤ m ≤ 12), Ag(n)C(m)(+) (n = 1-7, 1 ≤ m ≤ 7), Au(n)C(m)(-) (n = 1-5, 1 ≤ m ≤ 10), and Ag(n)C(m)(-) (n = 1-3, 1 ≤ m ≤ 6), were observed. The Au(3)C(2)(+) and Ag(3)C(2)(+) clusters are the most abundant cations in the corresponding mass spectra. Pronounced odd/even intensity alternations were observed for nearly all Au(n)C(m)(+/-) and Ag(n)C(m)(+/-) series. The time dependence of signal intensity for selected positive ions was measured over a broad range of C(60)(-) impact energies and fluxes. A few orders of magnitude immediate signal jump instantaneous with the C(60)(-) ion beam opening was observed, followed by a nearly constant plateau. It is concluded that the overall process of the fullerene collision and formation∕ejection of the carbidic species can be described as a single impact event where the shattering of the incoming C(60)(-) ion into small C(m) fragments occurs nearly instantaneously with the (multiple) pickup of metal atoms and resulting emission of the carbide clusters. Density functional theory calculations showed that the most stable configuration of the Au(n)C(m)(+) (n = 1, 2) clusters is a linear carbon chain with one or two terminal gold atoms correspondingly (except for a bent configuration of Au(2)C(+)). The calculated AuC(m) adiabatic ionization energies showed parity alternations in agreement with the measured intensity alternations of the corresponding ions. The Au(3)C(2)(+) ion possesses a basic Au(2)C(2) acetylide structure with a π-coordinated third gold atom, forming a π-complex structure of the type [Au(π-Au(2)C(2))](+). The calculation shows meaningful contributions of direct gold-gold bonding to the overall stability of the Au(3)C(2)(+) complex.

A magnetically focused molecular beam of ortho-water.

Like dihydrogen, water exists as two spin isomers, ortho and para, with the nuclear magnetic moments of the hydrogen atoms either parallel or antiparallel. The ratio of the two spin isomers and their physical properties play an important role in a wide variety of research fields, ranging from astrophysics to nuclear magnetic resonance (NMR). Unlike ortho and para H(2), however, the two water isomers remain challenging to separate, and as a consequence, very little is currently known about their different physical properties. Here, we report the formation of a magnetically focused molecular beam of ortho-water. The beam we formed also had a particular spin projection. Thus, in the presence of holding magnetic fields, the water molecules are hyperpolarized, laying the foundation for ultrasensitive NMR experiments in the future.

Above the surface multifragmentation of surface scattered fullerenes.

C(60) (-) ions were scattered from a gold surface at impact energies of 80-900 eV. The C(n) (-) fragments abundance distribution (odd and even) and the sharp fragmentation threshold observed, point at a prompt shattering event. The measured angle and energy distributions of the C(n) (-) fragments (n=2-12) provide clear evidence for a multifragmentation process where the superheated fullerenes leave the surface "intact" and disintegrate away from the surface.

The dynamics of endohedral complex formation in surface pick-up scattering as probed by kinetic energy distributions: experiment and model calculation for Cs@C60+.

Endohedral Cs@C60 molecules were formed by implanting low energy (E0 = 30-220 eV) Cs+ ions into C60 molecules adsorbed on gold. Both growth and etching experiments of the surface deposited C(60) layer provide clear evidence for a submonolayer coverage. The Cs+ penetration and Cs@C60 ejection stages are shown to be a combined, single collision event. Thermal desorption measurements did not reveal any Cs@C60 left on the surface following the Cs+ impact. The Cs@C60 formation/ejection event therefore constitutes a unique example of a pick-up scattering by endocomplex formation. Kinetic energy distributions (KEDs) of the outgoing Cs@C60+ were measured for two different Cs+ impact energies under field-free conditions. The most striking observation is the near independence of the KEDs on the Cs+ impact energy. Both KEDs peak around 1.2 eV with similar line shapes. A simple model for the formation/ejection/fragmentation dynamics of the endohedral complex is proposed. The model leads to a strong correlation between the vibrational and kinetic energy of the outgoing Cs@C60. The KEDs are calculated taking into account the competition between the various decay processes: fragmentation and delayed ionization of the neutral Cs@C60 emitted from the surface, fragmentation of the Cs@C60+ ion, and radiative cooling. It is concluded that the measured KEDs are heavily biased by the experimental breakdown function. Good agreement between experimental and calculated KEDs is obtained.

Diffusion-limited extraction of organic ions by a track-membrane interfaced vacuum inlet.

Glycerol-wetted track membranes (polyethylene terephthalate) were used to interface a low-vacuum facility (approximately (10(-3) Torr) to an ambient pressure liquid analyte. High-field charge extraction conditions were routinely maintained between the liquid samples and a grid collector. The latter was positioned just near to the vacuum-facing side of such membranes. Upon establishing a steady-state charge extraction regime, the collector currents were monitored and recorded at various solute concentration levels. The collector currents, which depend on solute concentration, were found to agree with recent theoretical treatments of such processes. Both positively- and negatively-charged species from organic solutions were routinely extracted. Ion injection for the low- and the high-mobility species has favored the diffusion-limited and the evaporation-limited schemes, respectively. Variable concentrations of 1-pyrenoyl-methylpyridinium bromide as well as naphthylacetic and anthracenecarboxylic acids in glycerol were used.