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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]

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]

Examines image charge effects of negative ion formation in near grazing surface scattering of hyperthermal neutral C[sub 60] surface. Setup of molecular beam-surface scattering apparatus; Analysis of negative ion spectrum; Energy distributions of scattered beams generated.

Presents an evidence for the thermal nature of delayed electron emission from a superhot carbon[sub 50] molecule. Measurement of delayed ionization efficiency curves; Dependence of delayed electron emission on the initial molecular thermal energy; Agreement of the experiment with simulations based on decay kinetics with thermal rate constants.

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.

Presents a mass-spectrometry based method for measuring the average vibrational energy of large and hot polyatomic molecules. Application to carbon molecules inelastically scattered off nickel; Measurement of vibrational cooling and collisional excitation upon surface impact.

In this letter we address the issue of the relation between critical distances of negative and positive ion formation during a collisional charge exchange between a large polyatomic projectile and a metallic surface. Based on the ‘freezing distance’ approximation, we have derived a simple expression relating the distances of negative and positive ion formation with the corresponding ionization potential and electron affinity of the neutral molecule. We test this expression by a comparison with experimental values measured for C60–surface scattering, the only system for which distances of formation of both molecular ionic states have been measured. A good agreement with experiment is found. [ABSTRACT FROM PUBLISHER]