H.-G. Busmann

Near specular reflection of C60 ions in collisions with an HOPG graphite surface

Abstract Collisions of Co + 60 ions with an HOPG surface are studied in an energy range between 140 and 450 eV for a geometry which allows to change the angle of incidence with respect to the normal direction of the surface, keeping the total deflection angle at 40°. The kinetic energy of the scattered ions is found to be about 10 to 20 eV and nearly independent of the impact energy. The angular distribution peaks at nearly specular reflection for 140 eV, but much closer to the normal direction as the impact energy is increased.

AN INTENSE, SIMPLE CARBON CLUSTER SOURCE

An intense source of positive, negative, and neutral carbon clusters using excimer laser ablation of polyimide is described. The detection probability for large masses using a standard channel plate configuration is shown to increase exponentially with ion velocity. The large clusters are formed via aggregation of atomic carbon or small carbon molecules. The small ‘‘clusters’’ may be molecular fragments from the polymer which rearrange on leaving the surface to form stable structures.

Collision induced fragmentation and resilience of scattered C60+ fullerenes

Abstract Fragmentation of positively charged Buckminster fullerenes scattered from a surface of pyrolytic graphite is studied. The ions are produced by laser desorption, accelerated, directed onto the surface and detected after the collision by time of flight mass spectroscopy. Velocity distributions of the scattered C60+ ions and of dissociated scattered ions are observed to be bimodal. The slower part has a narrower velocity distribution and is scattered more towards the surface normal in comparison to the faster part. Two interaction mechanisms are proposed to describe the experimental findings. Common to both is the transfer of incident kinetic energy into final thermal energy followed by substantial fragmentation. The differences between both mechanisms are explainable under the assumption that either chemical bonds are formed between cluster and surface and broken again when the cluster receeds (low velocity) or not (higher velocity).

Laser mass spectroscopic investigations of purified, laboratory-produced C60/C70

Abstract Laser desorption mass spectra of Kratschmers purified “fullerite” have been studied in a reflectron time-of-flight mass spectrometer. Desorption with 308 or 248 nm yields direct ions emerging from the substrate with high efficiency. At low fluence, a distribution of pure C + 60 (76%), C + 70 (23%) and, perhaps, a small amount C + 84 (⩽ 1.5%) ions are observed. As the fluence is increased, fragmentation of these species and aggregation to larger clusters are observed. Post-ionisation of neutral desorbed species with 248 or 308 nm is much less efficient and leads to substantial fragmentation both immediately on ionisation as well as in the field-free region of the mass spectrometer (a timescale of up to 100 μs).

Energy partition in collisions of C60+ ions with diamond (111) and graphite (0001) surfaces

Velocity distributions and fragmentation of C60 molecules after collision with type IAa diamond (111) and graphite (0001) surfaces are studied for primary energies up to 500 eV at a scattering angle of 140° using time‐of‐flight mass spectrometry. The absolute scattering yield is much lower for diamond due to polishing induced surface corrugation. However, the final velocity distribution of C60+ and its rate of metastable fragmentation after collision are similar for scattering off the two targets, and is interpreted by their similar lateral atomic structure. At a particular impact energy, the final internal energy of C60+, as monitored by the rate of metastable fragmentation, increases with final kinetic energy, but the energy transferred to the target decreases. These results are understood when considering the collision as a two‐step mechanism, in which the impact energy is transformed into thermal and deformational energy of the target and C60. This latter energy is subsequently transformed to final ki...

Stability of fragmentation of carbon clusters

Abstract Metastable evaporation of carbon cluster ions produced from excimer laser ablation of polyimide has been investigated in a reflection time-of-flight (TOF) mass spectrometer. Rate constants for the decay process are extracted from the parent and fragment peak intensities. The rate constants increase with decreasing delay between laser pulse and TOF extraction field for a given laser fluence and decrease with increasing laser fluence for a given delay.

Growth mechanisms of C60-molecular beam epitaxy on mica

The surface morphology of heteroepitaxial C60-films on mica has been studied by atomic force microscopy as a function of nominal film thickness at constant substrate temperature. Irregular islands grow in the early stage until they touch each other. Coalescence then forms larger, well-oriented islands of near thermodynamical equilibrium form. The size of the islands does not increase on further deposition; rather new islands are formed. A bare mica surface between the islands shows that growth at this stage continues by a Volmer-Weber mechanism. When complete surface coverage is achieved, a continuous film with a layer-by-layer growth mechanism is observed. The films finally obtained are of single-crystal quality.

Dynamics of C 60 + surface impact: Rolling, deformation, disintegration, and deposition on HOPG graphite

Cluster surface collisions of C+60 (Buckminsterfullerene) ions with pyrolytic graphite are studied by time of flight mass spectroscopy in the energy range of 100 eV to 1500 eV at oblique angles of incidence extending our previous studies with steep angles of incidence. Analysis of the velocity dependence of the scattered cluster ions reveals that the normal and tangential component of the ion velocity have different significance for the collision dynamics. The tangential component is partially transformed into rotational energy of the C+60 during the interaction with the surface as may be explained by an intuitive and astonishingly simple rolling ball model. In contrast, the normal component of the velocity appears to determine the amount of energy transferred into vibrational and deformational energy of projectile and target. At high “normal energies” the C+60 disintegrate significantly and can also be partially deposited onto the surface.

The influence of the surface nature on scattering, fragmentation and deposition processes in C60+ collisions with solid surfaces

Abstract Surface collision experiments with C 60 + ions under UHV conditions were carried out to study the influence of the surface atomic mass, structure, and chemical reactivity of a variety of crystalline surfaces on different collision processes. The impact energy was varied from about 100 to 700 eV. Collisions with graphite (0001), diamond (111), and tungsten diselenide (0001) yield scattered C 60 + ions which were analyzed with respect to their final kinetic energy and rate of metastable fragmentation by means of velocity selective time-of-flight mass spectroscopy. With increasing mass of the surface atoms, the final kinetic energy decreases along with a rise of the final internal energy as deduced from the rate of metastable fragmentation. In strong contrast, C 60 + collisions with Ni(100) and Cu(100) did not result in a detectable yield of scattered ions. Auger electron spectroscopy (AES) showed carbon deposition on these two surfaces. However, the surface carbon concentration as a function of the C 60 + exposure dose depicts remarkable differences. On Ni(100), the carbon concentration rapidly rises at low exposure doses until saturation is reached. By means of low energy electron diffraction (LEED), a c(2×2) carbon superstructure was found. On Cu(100), the carbon concentration steadily rises with increasing slope. The blurring of the Cu(100) electron diffraction spots gives evidence that the surface becomes damaged under C 60 + bombardment resulting in carbon-binding defects. Finally we show that C 60 + collisions with fullerite surfaces are nearly elastic for impact energies up to 250 eV. Rainbow scattering was observed for this surface. For higher impact energier sputtering becomes dominant.

Laser induced fission of C60 with kinetic energy release

A neutral C60 fullerene beam is ionised by 308 nm laser pulses. For each cluster sizeCn+, 0<n≦60 of the typical bimodal mass distributions known from the literature [1] velocity distributions have been determined by a time of flight method. A consistent interpretation of the measured mean velocities is obtained when binary fission of the parent molecule is assumed to be responsible for the fragmentation patterns, the total kinetic energy release being 0.45±0.1 eV independent of fragment mass and of laser fluence.