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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 C 60 − 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. [ABSTRACT FROM AUTHOR]

We have measured kinetic energy distributions (KEDs) of large clusters emitted from five different solid targets following a single impact of C 60 − 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. [ABSTRACT FROM AUTHOR]

We report the experimental observation and computational analysis of the binary tin-carbon gas phase species. These novel ionic compounds are generated by impact of C- 60 anions on a clean tin target at some kiloelectronvolts kinetic energies. Positive SnmCn+ (m = 1-12, 1 ≤ n ≤ 8) ions were detected mass spectrometrically following ejection from the surface. Impact induced shattering of the C- 60 ion followed by sub-surface penetration of the resulting atomic carbon flux forces efficient mixing between target and projectile atoms even though the two elements (Sn/C) are completely immiscible in the bulk. This approach of C- 60 ion beam induced synthesis can be considered as an effective way for producing novel metal-carbon species of the so-called non-carbide forming elements, thus exploring the possible onset of molecular level miscibility in these systems. Sn2C2+ was found to be the most abundant carbide cluster ion. Its instantaneous formation kinetics and its measured kinetic energy distribution while exiting the surface demonstrate a single impact formation/emission event (on the sub-ps time scale). Optimal geometries were calculated for both neutral and positively charged species using Born-Oppenheimer molecular dynamics for identifying global minima, followed by density functional theory (DFT) structure optimization and energy calculations at the coupled cluster singles, doubles and perturbative triples [CCSD(T)] level. The calculated structures reflect two distinct binding tendencies. The carbon rich species exhibit polyynic/cummulenic nature (tin end capped carbon chains) while the more stoichiometrically balanced species have larger contributions of metal-metal bonding, sometimes resulting in distinct tin and carbon moieties attached to each other (segregated structures). The Sn2Cn (n = 3-8) and Sn2Cn+ (n = 2-8) are polyynic/cummulenic while all neutral SnmCn structures (m = 3-4) could be described as small tin clusters (dimer, trimer, and tetramer, correspondingly) attached to a nearly linear carbon chain. For example, the 1:1 (Sn:C) Sn3C3 and Sn4C4 clusters are composed of all-tin triangle and rhombus, correspondingly, with a short carbon chain (C3, C4) attached on top. The cationic Sn3Cn+ (n = 1-5) and Sn4Cn+ (n = 1-4) species exhibit various intermediate geometries. Structure calculations at the CCSD(T) level are essential since the segregation effect is not as easily evident based on the most stable structures calculated by DFT alone. Dependences of bond energies (per atom) reflect the evolution of the segregation effect. The mass spectral abundances could be reasonably rationalized in terms of calculated stabilities of the cluster ions with respect to various dissociation channels. [ABSTRACT FROM AUTHOR]

Impact of fullerene ions (C60-) 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, AunCm+ (n = 1-5, 1 ≤ m ≤ 12), AgnCm+ (n = 1-7, 1 ≤ m ≤ 7), AunCm- (n = 1-5, 1 ≤ m ≤ 10), and AgnCm- (n = 1-3, 1 ≤ m ≤ 6), were observed. The Au3C2+ and Ag3C2+ clusters are the most abundant cations in the corresponding mass spectra. Pronounced odd/even intensity alternations were observed for nearly all AunCm+/- and AgnCm+/- series. The time dependence of signal intensity for selected positive ions was measured over a broad range of C60- impact energies and fluxes. A few orders of magnitude immediate signal jump instantaneous with the C60- 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 C60- ion into small Cm 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 AunCm+ (n = 1, 2) clusters is a linear carbon chain with one or two terminal gold atoms correspondingly (except for a bent configuration of Au2C+). The calculated AuCm adiabatic ionization energies showed parity alternations in agreement with the measured intensity alternations of the corresponding ions. The Au3C2+ ion possesses a basic Au2C2 acetylide structure with a π-coordinated third gold atom, forming a π-complex structure of the type [Au(π-Au2C2)]+. The calculation shows meaningful contributions of direct gold-gold bonding to the overall stability of the Au3C2+ complex. [ABSTRACT FROM AUTHOR]

C60- ions were scattered from a gold surface at impact energies of 80–900 eV. The Cn- 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 Cn- 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. [ABSTRACT FROM AUTHOR]

Endohedral Cs@C[sub 60] molecules were formed by implanting low energy (E[sub 0]=30–220 eV) Cs[sup +] ions into C[sub 60] molecules adsorbed on gold. Both growth and etching experiments of the surface deposited C[sub 60] layer provide clear evidence for a submonolayer coverage. The Cs[sup +] penetration and Cs@C[sub 60] ejection stages are shown to be a combined, single collision event. Thermal desorption measurements did not reveal any Cs@C[sub 60] left on the surface following the Cs[sup +] impact. The Cs@C[sub 60] formation/ejection event therefore constitutes a unique example of a pick-up scattering by endocomplex formation. Kinetic energy distributions (KEDs) of the outgoing Cs@C[sub 60][sup +] were measured for two different Cs[sup +] impact energies under field-free conditions. The most striking observation is the near independence of the KEDs on the Cs[sup +] 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@C[sub 60]. The KEDs are calculated taking into account the competition between the various decay processes: fragmentation and delayed ionization of the neutral Cs@C[sub 60] emitted from the surface, fragmentation of the Cs@C[sub 60][sup +] 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. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The collisional insertion of Cs[sup +] ions into surface adsorbed C[sub 60] molecules was studied by scattering Cs[sup +] ion beams from a C60 layer deposited on gold over the 35-220 eV impact energy range. Both Cs@C[sup +][sub 60] and C[sup +][sub 60] ions were ejected from the surface following the Cs[sup +] impact but each species was characterized by different impact energy dependent yields and internal temperatures. Clear evidence for the endohedral nature of the complex is given. Both the scattering dynamics (at impact energies up to ∼100 eV) and the instant rise of the Cs@C[sup +][sub 60] signal with the Cs[sup +] beam onset clearly demonstrate that the insertion/ejection process is basically a single collision event. The outgoing Cs@C[sup +][sub 60] and C[sup +][sub 60] ions fragment during their flight time, after leaving the surface, via sequential emission of C[sub 2] units down to Cs@C[sup +][sub 50] and C[sup +][sub 44], respectively. Relative impact energy dependent yields were measured for both parent species and for all fragments. The yield curves are kinetically shifted with respect to each other as expected. Comparing the impact energy dependent fragmentation patterns of C[sup +][sub 60] and Cs@C[sup +][sub 60] we conclude that the ejected Cs@C[sup +][sub 60] ion is much hotter than the C[sup +][sub 60] ion. The internal vibrational excitation for both species is reaching a maximal value around 90-110 eV impact energy and than gradually decreases with increase in impact energy. The integrated Cs@C[sup +][sub 60] yield is strongly peaked at around 80 ± 5 eV impact energy. At impact energies above 120 eV also a C[sup -][sub 60] signal is observed but no Cs@C[sup -][sub 60] could be detected. [ABSTRACT FROM AUTHOR]

In this paper we address the issue of electron exchange between a large molecular projectile and a surface during a hyperthermal collision. Hyperthermal neutral C[sub 60] molecules with well-defined average vibrational energy E[sub v] = 8 ± 0.5 eV were scattered from a graphitized (monolayer-covered) nickel surface for both near normal and near grazing incidence angles. The yield of C[sub 60] negative ions was measured for the impact energy range 8-33 eV and was found to scale exponentially with the inverse of the normal component of the scattered molecule velocity. Normal and tangential velocities of the scattered C[sub 60] are very low, (1.3-5.6) x 10[sup -4] and (0.8-3.1) x 10[sup -4] a.u. correspondingly. The fact that nearly the same slope (characteristic velocity) was extracted from the semilog plots for both near normal and near grazing incidence angles shows that tangential velocity effects are negligible. The rate of electron tunneling from C[sub 60] to the surface was calculated quasi-classically assuming that the excess (active) electron in C[sub 60] is moving in the highly screened short range attractive potential of the neutral C[sup 0, sub 60] core, which is approximated by a radial delta function (Dirac bubble). The transmission coefficient was calculated for C[sub 60] located at the center of a spherical metallic cavity with image charge potential barrier for the active electron. The preexponential frequency factor was obtained by solving the quasi-stationary Shrödinger equation for the same system but without the image charge barrier. Finally, the decay rate was corrected for the case of C[sub 60] in front of a conducting plane. The experimental results were analyzed using the calculated tunneling rates and a critical ion formation distance was obtained. Independence of the scattering dynamics and negative ion yield on the incoming charge state is demonstrated by using incident C[sub 60] at the impact energy range of 40-100 eV. The... [ABSTRACT FROM AUTHOR]

We have studied the dependence of electron impact induced ionization and fragmentation of C60 molecules in effusive molecular beams upon the initial thermal excitation in the temperature range of 1190–1875 K, corresponding to an average vibrational energy of 10–20 eV. This is the largest energy range of parent molecule thermal excitation ever reported for electron-impact mass-spectrometric studies. The normalized curves of electron energy (Ee) dependent ion currents of C+60 and C+58 were measured and analyzed for the temperatures (T0) of 1190, 1435, 1570, 1695, and 1875 K. Similar measurements were done for C+2n (n=26–28) fragments for T0=1190 and 1875 K. We have developed an expression for the dependence of C+58 fragment ion current i58(Ee,T0), formed via the decay process C+60→C+58+C2, on electron energy and initial temperature. Using this expression and the strong temperature dependence observed, we have proposed a simple experimental method for estimating the energy deposition function—the probability density of vibrational excitation ε by an ionizing electron of energy Ee. The effective (apparent) value of maximum deposited energy was found to be εm(Ee)=Ee-E*, where E*=30±5 eV. Possible interpretations for this surprisingly low value are discussed. Comparing the experimental i58(Ee,T0) curves with the calculated ones over the range of Ee=30–80 eV we find that for T0≤1600 K, good agreement is obtained assuming that the C60 initial internal excitation is determined by the source temperature alone.For the higher temperature range 1600 K≤T0≤1900 K, we had to use a modified calculation taking into account radiative cooling and ensemble evaporative cooling processes along the molecular beam flight path. As a result, we have obtained an accurate simulation of the complete family of i58(Ee,T0) curves over all the temperature range measured, using a single set of independently measured... [ABSTRACT FROM AUTHOR]

A method of generating effusive molecular beams of C60 with vibrational temperatures up to 2000 K is presented. Direct measurement of the thermal stability and fragmentation kinetics of C60 in the range of 1100–1970 K, on the milliseconds time scale, provides thermal rate constants k(T)=10–300 s-1 (for T=1720–1970 K, respectively) and activation energy of E0=4.0±0.3 eV. [ABSTRACT FROM AUTHOR]

Molecule–surface collision-induced dissociation (CID) has been studied for n-C3F7NO and i-C3F7NO molecular beams scattered from MgO(100) at incident kinetic energies (Eincident) up to 7 eV. The NO fragment was detected state selectively using two-photon, two-frequency ionization, and rotational and spin–orbit distributions are reported for several Eincident values. State and angle-resolved signals were integrated to give CID yields, which increased sharply with Eincident . In most cases, rotational excitation could be described by separate temperatures for each spin–orbit state. The upper 2∏3/2 state was underpopulated relative to statistical predictions (e.g., for n-C3F7NO at Eincident =5.0 eV, the spin–orbit temperature was ∼170 K, while Trot was ∼500 K). The CID results are compared to NO state distributions derived from the photodissociation of expansion-cooled molecules under collision-free conditions, at different energies (E°) above D0. These distributions were measured for both n-C3F7NO and i-C3F7NO up to E°∼4500 cm-1, and rotational excitation within each spin–orbit state was statistical, except at E°≥3000 cm-1. As with CID, a low [2∏3/2]/[2∏1/2] ratio was observed, and the reaction mechanism is probably unimolecular decomposition via the lowest triplet surface T1 with little or no exit channel barrier. The pronounced similarities between the CID and photodissociation results suggest that common reaction mechanisms may be operative. All of the CID results are compatible with direct inelastic scattering followed by unimolecular reaction on the S0 and/or T1 potential surfaces. [ABSTRACT FROM AUTHOR]

Focuses on the scattering dynamics of hyperthermal C[sub 60] from a carbonized nickel surface at impact energies by high resolution angular and energy distributions. Nearly complete decoupling between normal and tangential energy losses; Description of tangential losses in terms of models of rotational excitation and involvement of transcriptional slip.

The title reaction was examined for incident kinetic energies in the range 3–7 eV and surface temperatures in the range 500–800 K; dissociation probabilities as high as 3±1% were measured. NO state distributions were obtained near the specular angle using two-photon, two-frequency laser ionization, and spin–orbit ‘‘temperatures’’∼170 K were observed, in contrast to higher rotational excitations. Possible mechanisms are mentioned. [ABSTRACT FROM AUTHOR]