Dependence of the sp3 bond fraction on the laser wavelength in thin carbon films prepared by pulsed laser deposition

3 bonds in the carbon films prepared by pulsed laser deposition of carbon obtained from graphite was investigated by electron energy loss spectroscopy (EELS) and X-ray photoelectron spectroscopy (XPS). The fraction of sp³ bonds increased with a decreasing laser wavelength. Energetic C⁺ ion species were effectively produced by using a short-wavelength laser. The sp³ bond fraction increased with an increasing amount of energetic C⁺ ion species. The fractions of sp³ bonds in the carbon film were 80%, 42%, 26% and 15% at wavelengths of 193, 248, 532 and 1064 nm, respectively. Received: 28 October 1997/Accepted:29 October 1997     

Thermal desorption spectroscopy of fullerene films containing endohedral Li@C60

60 and produced by low-energy ion bombardment. The desorption properties of the films were studied as a function of the film thickness, ion energy, and ion dose. The results provide strong corroborative evidence for the endohedral nature of the Li-containing species and also clearly indicate the prescence of Li₂@C₆₀ in the films. The binding energy in these films is significantly larger than in pure C₆₀ films and evidence of degradation of the fullerenes is seen at high ion energies and/or large ion doses. Received: 6 August 1997/Accepted: 8 August 1997     

Generation of superpositions of coherent states of the motion of a trapped ion

We propose a method for preparing superpositions of coherent states of the motion of an ion in an anisotropic two-dimensional trap, in which the ion is tightly bound in the y direction. In the scheme the ion is excited by two resonant laser beams with equal amplitude, propagating along the x and y directions, respectively. In the Dicke-Lamb limit, an initial coherent state of the ion motion can be converted into a superposition of several coherent states on a circle through the laser-ion interactions and state-selective measurements on the ion. Received: 30 May 1997 / Revised: 29 July 1997 / Accepted: 22 October 1997     

Characterization of ablation plumes and carbon nitride films produced by reactive pulsed laser deposition in the presence of a magnetic field

x ) films in a nitrogen atmosphere within the range 5×10^-4–4×10^-1 Torr. In the presence of a magnetic field, the emission intensities of N₂ (second positive system) and CN species in the graphite ablation plumes were altered significantly, depending on the pressure of the N₂ environment. Corresponding to an intense CN emission, a magnetic field-induced enhancement of N incorporation – for example, up to 37% at an N₂ pressure of 300 mTorr – and the formation of sp³ tetrahedral CN bonding were both observed in the films. This suggests that the arrival of CN species at the substrate surface with kinetic energies is important for film deposition. Received: 27 August 1997/Accepted: 8 September 1997     

Nanotube size-dependent melting of single crystals in carbon nanotubes

Received: 15 April 1997/Accepted: 16 April 1997     

Neutral molybdenum atom and dimer formations in laser-induced plasma: emission study of 355-nm pulse laser photolysis of Mo(CO)6 in the gas phase

6 in the gas phase at room temperature. The transient behavior of emissive atoms and dimers were measured by using a time-resolved UV-vis emission spectroscopy. Most atomic emissions were found to consist of two distinct transient components, an “early” component with a similar time profile to the photolysis pulse, and a “late” component with a peak at around 300 ns after the photolysis. The fact that the addition of an electron scavenger decreased the late emission sensibly suggests that the late emission was caused by Mo^* produced in a neutralization process of ionic species. A plausible mechanism involving electron–ion recombination was proposed for the late emission and examined by the numerical simulation of the transient behaviors as a function of some experimental parameters such as total pressure, initial ion concentration, and electron scavenger concentration. Received: 9 April 1997/Revised version: 12 June 1997     

All-diode-laser cooling of single Ca+ ions

+ ions. The Ca⁺ ions are trapped in a miniature rf Paul trap and irradiated by light from a frequency-doubled diode laser at 397 nm and by light from a diode laser at 866 nm. We are able to cool a single ion and observe its fluorescence continuously with the laser diode locked to the external frequency-doubling cavity. Quantum jumps in the fluorescence light of a single ion and of a small cloud of five ions have been induced by driving the “clock” transition at 729 nm. We were able to resolve the influence of the micromotion on the excitation spectrum of the small ion cloud. Received: 10 July 1997/Revised version: 17 November 1997     

Calculation of dielectronic recombination cross sections and rate coefficients for heliumlike carbon

A simplified relativistic configuration interaction method is used to calculate the dielectronic recombination cross sections and rate coefficients for heliumlike carbon. In this method, the infinite resonant doubly excited states can be treated conveniently in the frame of Quantum Defect Theory. Our calculated cross sections are in agreements with the experimental measurements except for the 1s2lnl'(n=6,7) resonances. The total energy-integrated cross sections and rate coefficients over all dielectronic resonances are in agreements with the experimental measurements within percent. Received: 7 July 1997 / Revised: 7 October 1997 / Accepted: 8 December 1997     

Cluster ion emission in the interaction of slow highly charged ions with surfaces

Cluster ion emission from a variety of surfaces upon impact of highly charged ions is investigated by time-of-flight secondary ion mass spectrometry. The yield of cluster ions as a function of cluster size for and surface follow a power law decline with exponent approaching the -2 limit of the ”equilibrium” and ”shock wave” cluster emission models. While the decline of the cluster ion emission with cluster size is an exponential decay for highly oriented pyrolytic graphite upon impact, the decline is more gradual than for impact, such that at the relative cluster yield is 1000 times higher. Received: 22 April 1997 / Revised: 29 December 1997 / Accepted: 19 January 1998     

Diagnostics of mixed van der Waals clusters

We present two complementary techniques that provide detailed diagnostics of supersonic beams involving several species. First, surface scattering, together with quadrupole mass spectrometer detection, yields the monomer percentage for each species within the beam. Second, analyses of beam profiles for different masses after scattering by a buffer gas permit determination of mixed cluster presence and, if any, of cluster sizes and compositions. The two techniques are applied to supersonic expansions of an argon-nitrogen mixture. We discuss the results that provide new insight in binary nucleation processes. Received: 6 October 1997 / Revised: 4 November 1997 / Accepted: 13 November 1997     

Formation of metallic nanophases in silica by ion-beam mixing Part I: Mixing mechanisms

2 matrix is studied by means of the Rutherford backscattering technique. Two different mechanisms are observed: at low temperature (300 K and below) the variance of the mixed profile varies with the square of the ion fluence, whereas at higher temperature (400 K to 620 K) a linear variation is found. The low-temperature kinetics are accounted for by the migration of Ag-defect complexes after introduction of Ag atoms into the silica matrix by a ballistic process. A combination of ballistic and radiation-enhanced diffusion processes explains the results obtained at high temperature. This work emphasizes the role of the presence of metallic clusters on the migration of metal atoms in silica. Received: 10 June 1997/Accepted: 29 September 1997     

Hardnes and resistivity of amorphous carbon thin films deposited by laser ablation

Amorphous carbon thin films were deposited by laser ablation of a graphite target, using the fundamental line of a 5 ns Nd:YAG laser. Deposition was carried out as a function of the plasma parameters (mean kinetic ion energy and plasma density), determined by means of a planar probe. In the selected working regimes the optical emission from the plasma is mainly due to atomic species, namely C⁺ (426.5 nm); however, there is also emission from other atomic species and molecular carbon. The hardness and resistivity could be varied in the range between 10 and 25 GPa, and 10⁸ and 10¹¹ Ω cm, respectively. The maximum values were obtained at a 200 eV ion energy and 6×10¹³ cm⁻³ plasma density, where the maximum quantity of C–C sp³ bonds was formed, as confirmed by Raman spectroscopy.     

Ionization of metal clusters by ions in the Fermi velocity range

We simulate excitation of metal clusters by highly charged, energetic ions, analyzing electron emission in terms of discrete ionization probabilities. Our test case is the collision of on the cluster at velocities around the electronic Fermi velocity of bulk sodium. The calculations are performed with a density-functional approach, using the time-dependent local density approximation. We find that ionization takes place on an extremely short time scale of less than 5 fs. The preferred final charge state depends sensitively on the impact parameter. High ionization can easily be achieved in sufficiently close collisions. Direct trapping through the by-passing ion is found to be of little importance at the velocities considered. Received: 28 July 1997 / Received in final form: 23 December 1997 / Accepted: 8 January 1998     

Optical studies on the deposition of carbon nitride films by laser ablation

2 radicals when the 355 and 1064 nm outputs of a Nd:YAG laser were applied. While for the 532 nm ablation, a relatively higher concentration of excited atomic carbon was obtained. Different Raman and FTIR spectral features were observed from the deposited films with different ablation wavelengths. The 532 nm laser ablation is proposed for the synthesis of high quality carbon nitride films. Received: 16 October 1996 / Accepted: 11 April 1997     

Collision induced dissociation of the heteronuclear alkali ion .Comparison with the and dissociations

An experimental study of the dissociation of the heteronuclear Na ion by collisions with He and H₂ targets at a collision energy of 2.4 keV is presented. The use of a technique based on the measurement of the velocity vectors of the fragments allows investigating in detail the two basic dissociation mechanisms: impulsive and electronic. In the present asymmetric case, one is able to assess the relative role of close encounters with each of the Na⁺ and atomic cores of the molecule. Electronic transitions at a surface crossing are proposed to explain processes which happens in close collisions between the core and the He target. Received: 3 October 1997 / Revised: 1 December 1997 / Accepted: 16 January 1998     

Is NaI soluble in water clusters?

clusters (solvents being , or ) have been studied by resonance enhanced two photons ionization, leading to the detection of clusters. When water is the solvent, large clusters up to n>50 can be observed, whereas for and no clusters larger than 10 could be evidenced. Because the first step in the ionization process is the excitation from the ground solvated () ion pair state to a covalent excited state, the differences in the cluster size distribution for different solvent may be interpreted as a difference in cluster structures leading to a difference in the charge separation in the ground state. Received: 30 September 1997 / Revised in final form: 30 October 1997 / Accepted: 30 October 1997     

Micromechanical properties of carbon nitride films deposited by radio-frequency-assisted filtered cathodic vacuum arc

Super-hard and elastic carbon nitride films have been synthesized by using an off-plane double-bend filtered cathodic vacuum arc combined with a radio-frequency nitrogen-ion beam source. A nanoindenter was used to determine the micromechanical properties of the deposited films. X-ray photoelectron spectroscopy was used to study the composition and bonding structure of the deposited films. The influence of nitrogen ion energy on the structure and micromechanical properties of the deposited films was systematically studied. As the nitrogen ion energy is increased, the microhardness, Young’s modulus and elastic recovery also increase, reaching a maximum of 47 GPa, 400 GPa, and 87.5%, respectively, at a nitrogen ion energy of 100 eV. Further increase in nitrogen ion energy results in a decrease in microhardness, Young’s modulus and elastic recovery of the deposited films. The formation of five-membered rings, as indicated by XPS, which causes bending of the basal planes and forms a three-dimensional rigid covalent bond network, contributes to the super-hardness, Young’ s modulus and high elastic recovery of the films deposited at a nitrogen ion energy of 100 eV. Revised version: 29 October 2001 / Accepted: 7 November 2001 / Published online: 2 May 2002     

Ionic transitions and oscillator strengths in Debye plasma - a case study

Stability and energy of the excited states of the ion in plasmas are investigated theoretically using the Debye model. The transition energies of and transitions are seen to follow completely opposite trends of variation with the plasma screening strength. The dependence of absorption oscillator strength values on the screening strength is also discussed. Received: 22 October 1997 / Accepted: 9 January 1998     

Band gap variation in laser irradiated polytypic crystals of cadmium iodide

Highly purified single crystals of cadmium iodide obtained through repeated zone refining have been subjected to laser beam exposure (Argon ion laser), both for various time durations and to various beam intensities, and then subjected to band gap determination by UV spectroscopy. The band gap has been found to decrease gradually with increase in the laser beam intensity, whereas its variation with increase in time of exposure shows an unusual behaviour such that it initially falls sharply followed by a gradual rise back to its original value. The results have been analysed and interpreted in terms of indirect band gap character of the material and an unusual variation in phonon frequency. The interpretation is well supported by X-ray diffraction and scanning electron microscopy (SEM) studies. Received: 26 May 1997 / Revised: 6 August 1997 / Accepted: 15 September 1997     

Study on formation and shape of carbon nanotips depending on ion bombardment

Carbon nanotips were grown from carbon film deposited on silicon substrate by plasma-enhanced hot filament chemical vapor deposition. The carbon film and carbon nanotips were investigated by scanning electron microscopy and micro-Raman spectrometry, respectively. The results indicate that the carbon film is composed of amorphous carbon and the carbon nanotips are characteristic of nanographite, and their formation and shapes depend on ion bombardment strongly. Simultaneously, the number of forming the carbon nanotips is increased by the ion bombardment. Because there are ion deposition and sputtering-etching in the process of growing of the carbon nanotips, the theory related to ion deposition and sputtering was used to obtain their formation condition and analyze effects of the ion bombardment on their shapes and the number of forming them.