A. Bekkerman

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.

Negative ion formation in near grazing surface scattering of hyperthermal neutral C60: Image charge effects

Formation of negatively charged C60− was observed in the near-grazing scattering of hyperthermal (10–50 eV) neutral C600 from a carbon-covered (monolayer graphite) nickel surface. Under these conditions the normal energy component of the scattered particle (0.3–1.5 eV) is of the same order of magnitude as the attractive potential well. We have measured both energy and angle distributions of the scattered C60− and studied the electron transfer process by comparing the energy and angular distributions of the scattered negatively charged and neutral C60 as a function of primary energy. The shifts observed between the angular and energy distributions maxima of the neutral and negative ion could be analyzed and explained in terms of image charge effects on the outgoing trajectory (deflection) and exit energy (retardation) of the C60−. The angular deflection analysis yield image charge barrier of 0.28±0.02 eV corresponding to a rather large “ion formation” critical distance of 12.9±0.9 A. We believe that this i...

The formation and ejection of endohedral Cs@C60+ by low energy collisions (35–220 eV) of Cs+ ions with surface adsorbed C60 molecules

The collisional insertion of Cs+ ions into surface adsorbed C60 molecules was studied by scattering Cs+ ion beams from a C60 layer deposited on gold over the 35–220 eV impact energy range. Both Cs@C60+ and C60+ ions were ejected from the surface following the Cs+ 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@C60+ signal with the Cs+ beam onset clearly demonstrate that the insertion/ejection process is basically a single collision event. The outgoing Cs@C60+ and C60+ ions fragment during their flight time, after leaving the surface, via sequential emission of C2 units down to Cs@C50+ and C44+, 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 expect...

Direct experimental evidence for the thermal nature of delayed electron emission from a superhot C60 molecule

Delayed electron emission from superhot neutral C60 was found to strongly depend on the initial molecular thermal energy. This dependence is clearly demonstrated by measurements of delayed ionization efficiency curves following electron impact for widely different C60 average vibrational energies in the range of 8–14 eV. These measurements provide direct experimental evidence for the thermal nature of delayed ionization of superhot clusters. A simulation based on decay kinetics with thermal rate constants was found to be in good agreement with experiment.

Surface scattering of hyperthermal (10–50 eV) neutral C60: Angular and energy distributions

The scattering dynamics of hyperthermal C60 from a carbonized nickel surface at impact energies E0=10–50 eV was studied by high resolution angular and energy distributions. The scattered energy scales linearly with E0 and kinetic energy losses vary with scattering angle from ∼85 to ∼40%. Nearly complete decoupling between normal and tangential energy losses was found. The tangential losses are described in terms of various models of rotational excitation and the involvement of translational slip is concluded.

Surface scattering of hyperthermal (10–50 eV) neutral C60 molecules

Abstract We have studied the scattering of neutral C 60 molecules from surfaces at the impact energy range of 10–50 eV. The reactive interaction of hyperthermal C 60 with a clean polycrystalline nickel surface was found to be strongly dependent upon surface temperature and C 60 beam dose. Following adsorption and decomposition of C 60 on the substrate, two different types of carbon overlayer phases were identified. The high temperature (passivated) phase gave rise to direct inelastic scattering in the form of sharply-peaked angular and energy distributions. The scattering dynamics of C 60 molecular beams from this surface was probed by high resolution angular and energy distributions and vibrational energy measurements at various impact energies, surface temperatures and scattering and incidence angles. The scattered energy scales linearly with impact energy and kinetic energy losses vary with scattering angle from ∼85% to ∼40% (peak values). We have developed an improved method for measuring average vibrational energy of large polyatomic molecule or cluster and applied it to the surface scattered C 60 . Vibrational excitation was found to be below 2% of impact energy, showing that C 60 is nearly non-deformable at these collision energies. This value is much lower than estimated before for higher energies C 60 + ion surface scattering. Analyzing the results we find nearly complete decoupling between normal and tangential energy losses. The agreement obtained between model prediction and experiment provides evidence in favor of spherical potential barrier geometry. The tangential losses are described in terms of various models of rotational excitation, and the involvement of translational slip during the scattering process is concluded. The C 60 surface interaction well depth was determined from the scattering results and compared with the measured desorption energy. Our detailed results describe the first real “bouncing” regime for C 60 scattering off surfaces, in agreement with recent molecular dynamics calculation.

Infrared multiphoton ionization of superhot C60: Experiment and model calculations

We address, both experimentally and theoretically, the issue of infrared (IR) resonance enhanced multiphoton ionization (IR-REMPI) and thermally induced redshifts of IR absorption lines in a very large and highly vibrationally excited molecular system. Isolated superhot C60 molecules with well defined and variable average vibrational energy in the range of 9-19 eV, effusing out of a constant flux thermal source, are excited and ionized after the absorption of multiple (500-800) infrared photons in the 450-1800 cm(-1) spectral energy range. Recording the mass-selected ion signal as a function of IR wavelength gives well resolved IR-REMPI spectra, with zero off-resonance background signal. An enhancement of the ion signal of about a factor of 10 is observed when the temperature is increased from 1200 to 1800 K under otherwise identical conditions. A pronounced temperature dependent redshift of some of the IR absorption lines is observed. The observations are found to be in good agreement with a model which is based on the sequential absorption of single photons, always followed by instantaneous vibrational energy redistribution. The mass spectra (C60(+) fragmentation pattern) are found to be strongly excitation wavelength dependent. Extensive fragmentation down to C32(+) is observed following the absorption of 1350-1400 cm(-1) as well as 1500-1530 cm(-1) photons while negligible fragmentation is observed when exciting around 520 cm(-1).

Thermally activated decay channels of superhot C60−: delayed electron emission and dissociative attachment studied by hyperthermal negative surface ionization

Abstract Superhot C 60 − ions were formed by attachment of low energy free electrons to superhot C 60 0 molecules in effusive beam. We have studied the thermally activated decay of the C 60 − ion as a function of its vibrational temperature. In addition to the main channel of delayed electron emission C 60 − → C 60 0 + e − , we give evidence for a secondary channel (only weakly competitive) of dissociative attachment C 60 0 + e − → C 58 0 + C 2 − . We have found no evidence for the nearly isoenergetic complementary channel C 60 0 + e − → C 58 − + C 2 0 . In order to measure the initial C 60 − beam flux (which partially decays to C 60 0 during the flight time) independently for each nozzle temperature we have used a unique detection method that is completely insensitive to the initial charge state and therefore to the ratio of charge states in the primary beam. The method is based on collisional electron exchange and negative ion (C 60 − ) formation in a hyperthermal collision with a surface. Analyzing the kinetics of the delayed electron emission we have obtained a straight Arrhenius plot with a slope (activation energy) of 2.64 ± 0.07 eV and intersection ( A value) of 1.3 × 10 11 s −1 . This value of the pre-exponential factor is in good agreement with former measurements but is several orders of magnitude lower than current models’ predictions. A possible explanation for this difference is discussed.

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.

Charge transfer in hyperthermal surface collisions of C600 and C60−: Experiment and model calculations

In this paper we address the issue of electron exchange between a large molecular projectile and a surface during a hyperthermal collision. Hyperthermal neutral C60 molecules with well-defined average vibrational energy Ev=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 C60− 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 C60 are very low, (1.3–5.6)×10−4 and (0.8–3.1)×10−4 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 C60− to the surface was calculated quasi-classically assuming that the excess (active) electron in ...