E. Kolodney

The thermal energy dependence (10–20 eV) of electron impact induced fragmentation of C60 in molecular beams: Experiment and model calculations

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...

The thermal stability and fragmentation of C60 molecule up to 2000 K on the milliseconds time scale

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.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.

Impact induced vibrational excitation in surface scattering of hyperthermal neutral C60 molecule

A mass-spectrometry based method for measuring the average vibrational energy (vibrational thermometry) of large and hot polyatomic molecules is presented. The method is applied to C60 inelastically scattered off nickel with impact energies of 10–50 eV. Both the vibrational cooling effect on the supersonically expanded C60 and the collisional excitation upon surface impact are measured within an experimental accuracy of ±0.25 eV. Under nearly normal beam incidence conditions and impact energy of 33.0 eV (out of which 30.6 eV are in the normal energy component), vibrational excitation was found to be below ∼2% of the impact energy, showing that the recoiled C60 is nearly nondeformed at these collision energies. This value is much lower than estimated before for higher energies C60+ ion surface scattering. The implication of this result in relation with the low energy scattering dynamics of C60 is discussed.

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.

Generation and energy analysis of neutral C60 seeded molecular beams up to 60 eV with electrostatic energy analyzer

Abstract We report on the detailed study and energy analysis of aerodynamically accelerated neutral C60 seeded molecular beams. Hyperthermal C60 beams with kinetic energies up to 56 eV in helium and 73 eV in hydrogen were generated and energy analyzed as a function of nozzle temperature and backing pressure. For these measurements a 90° cylindrical energy analyzer was used in a continuous crossed beam mode with energy resolution of ΔE/E = 0.07, instead of the conventional time-of-flight technique. A narrow C60 energy distribution (ΔE/E = 0.11) is observed. The pronounced advantages of the energy analyzer method for 1–100 eV seeded beams are discussed. No evidence was found for the high kinetic energy endohedral He@C60 down to 0.5% detection limit.