R. Ehlich

Experimental and theoretical study of third-order harmonic generation in carbon nanotubes

Third-harmonic generation from solid samples of carbon nanotubes has been studied experimentally, using ultrashort pulses generated by a Cr:Forsterite laser, at a wavelength of 1250 nm. The results show an unusual nonperturbative behavior of the third-harmonic yield, for relatively low input laser fields, of ∼1010 W/cm2. This strong nonlinearity of the laser interaction with carbon nanoubes is also confirmed theoretically, in a full quantum-mechanical theory for harmonics generation from a single-walled carbon nanotube.

Photofragmentation of C60

Photo-ionisation and -fragmentation ofC60 by 15 ns excimer laser pulses at 308 nm and 193 nm as well as 0.8 ps laser pulses at 193 nm has been studied with reflectron time-of-flight mass spectrometry. The initial fragmentation process is ejection ofCn,n>2, as opposed to successiveC2 evaporation. Studies of the relative intensities of metastable fragmentation processes compared with direct fragmentation provide new insight into the fragmentation mechanism and provide a thermometer for the internal energy ofC60+ prior to fragmentation. The proposed mechanism is in agreement with measurements of the fragment ion kinetic energies. The results are compared with molecular dynamics simulations.

Collision energy dependence of He and Ne capture by C60

Recently, reports of the formation of endohedral cluster compounds He@Cn+ and Ne@Cn − 4/+ (n even and ≤ 60) have been published for high energy collisions between C60+ and rare gases [1–4]. Here we present the collision energy dependence for the formation of He@C60+ and Ne@C60+ in the region of the energetic threshold for the capture process. The threshold for He capture lies at 6±2 eV in the centre of mass reference frame whereas that for Ne lies somewhat higher at 9±1 eV.

Collisional dynamics of C60 with noble-gas-atoms studied by molecular dynamics with empirical two- and three-body forces

The dynamics of C60-rare gas collisions is studied using molecular dynamics with empirical two- and three-body forces. The carbon potential is chosen to be able to reproduce the experimentally determined bond lengths and cluster radius of C60 as well as the structure of small carbon clusters. The reaction channels observed can be divided into four categories: deep inelastic scattering, fragmentation, capture and inelastic scattering. The temperature dependence of the threshold energy for capture is studied and compared with available experimental data. The calculations predict a maximum in the lowest tail of the kinetic-energy distribution of the projectile with a transition from a single- to a double-humped maximum with increasing collision energy and, in addition, may provide a natural interpretation for the, as yet unexplained, structure in the experimental shapes.

Growth of nanotubes by decomposition of C60 on transition metal surfaces

Abstract A new method for the growth of carbon nanotubes was discovered which is based on C 60 . The C 60 was heated in a stainless steel oven under high vacuum. The material which was evaporated at temperatures between 360 and 510 °C in the presence of metal catalysts was deposited on a HOPG substrate. Measurements with STM, SEM and TEM gave evidence for the formation of multiwalled nanotubes. Nanotubes were found on the HOPG Substrate and within the oven. No nanotubes were found using a quartz oven or a gold substrate.

Collision induced fragmentation of mass-selected (CO2) n + clusters

The collisional velocity dependence of the cross sections for fragmentation of mass-selected (CO2)n+ (n+2...7) clusters in collisions with Ar atoms is presented. Interesting structure can be observed in the cross sections which indicate that the collision occurs between the Ar atom and one CO2 molecule within the cluster. The results may be explained by assuming that the collision leads to either vibrational excitation of a loosely bound CO2 monomer which then leaves the cluster or excitation of the entire cluster to a dissociative state.

Collision experiments with fullerenes

Relative fragmentation cross sections for fullerene ion collisions with rare gas atoms and SF6 are presented over a range of collision energies. Structure in the cross sections and threshold energy determinations can shed some light on the fragmentation dynamics. Cluster cluster collisions with fullerenes are also described which show evidence of fusion reactions.

Third-harmonic generation in carbon nanotubes: theory and experiment

The nonlinear optical response of carbon nanotubes (CNTs) to the interaction with intense ultrashort laser pulses was studied theoretically and experimentally. A full quantum-mechanical theory for harmonics generation from a single-walled CNT has been developed, using the quantum kinetic equations for π-electrons with both intraband and interband transitions taken into account. In the regime of strong driving fields, a non-perturbative approach with the numerical solution of the quantum kinetic equations in the time domain was used to calculate the density of the axial electric current in CNTs. The results of this theory are compared to experiments performed on samples of multi-walled CNTs, using pulses of 160 fs generated by a Cr:Forsterite laser, at a wavelength of 1250 nm. The experimental results show indeed an unusual nonperturbative behavior of the third-harmonic yield, for relatively low input laser fields of ~ 1010 W/cm2, in very good agreement with the theoretical predictions. The interaction of CNTs with strong laser fields results not only in the generation of harmonics, but also in the generation of a broad spectral background. Generation of a continuous background in the vicinity of the third-harmonic of the laser field was also obtained from the quantum-mechanical calculations, however, with lower intensities than observed experimentally. Possible explanations for this discrepancy will be discussed.