arXiv subjects

Shoaib Ahmad

Publications and source records attributed to Shoaib Ahmad.

At least 37 records · Page 2Linked to original sources

arXiv2016

Carbon cluster diagnostics-I: Direct Recoil Spectroscopy (DRS) of Ar+ and Kr+ bombarded graphite

Measurements of the energy spectra of multiply charged positive and negative carbon ions recoiling from graphite surface under 100 and 150 keV argon and krypton ion bombardment are presented. With the energy spectrometer set at recoil angle of 79.5 degrees, direct recoil (DR) peaks have been observed with singly as well as multiply charged carbon ions , where n = 1 to 6. These monatomic and cluster ions have been observed recoiling with the characteristic recoil energy E(DR) . We have observed sharp DR peaks. A collimated projectile beam with small divergence is supplemented with a similar collimation before the energy analyzer to reduce the background of sputtered ions due to scattered projectiles.

physics.ins-det↗

arXiv2016

Carbon cluster diagnostics-III: Direct Recoil Spectroscopy (DRS) versus ExB Velocity Analysis

A comparative study is being reported on the identification of heavy carbon clusters using two different techniques namely the Direct Recoil Spectroscopy (DRS) and velocity spectrometry using a Wien ExB velocity filter. In both the techniques carbon clusters are formed in situ either under energetic heavy ion bombardment of graphite surfaces for which DRS is employed or these are emitted from the regenerative sooting discharges and analyzed using ExB velocity analysis.

cond-mat.mes-hall↗

arXiv2016

Carbon cluster diagnostics-II: ExB velocity filter for large carbon clusters

An ExB velocity filter is described that has been used for the detection and diagnostics of large carbon clusters Cm (where m is up to ten thousand). The velocity and momentum analyses are compared for our special experimental arrangement. We describe the variability of the resolving power of our compact velocity filter as the main advantage over other comparable mass analysis techniques.

cond-mat.mes-hall↗

arXiv2016

Regenerative Soot: A cusp field, graphite hollow cathode, carbon cluster ion source

An all carbon hollow cathode ion source with a specially designed cusp magnetic field Bz as a function of radius and theta is described. Velocity spectra from this source has shown that carbon cluster synthesis is taking place as a function of the source design, discharge parameters and the state of sooting. The cusp magnetic field seems to establish two distinct discharge regions within the source. The first is a narrow annular ring that is 2 mm thick and 15-20 mm long with restricted axial and radial extent of the cusp field. Here the source gas ionizes. The carbon atoms sputter of the cathode walls and subsequently get ionized. The other one is a cylindrical plasma region with the entire axial range of Bz. In this region the dynamics of the soot formation from the plasma constituent on the cathode walls seems to be related to the carbon clustering processes

cond-mat.mes-hall↗

arXiv2016

Regenerative Soot-VI: The excited states of neutral and ionized carbon in regenerative sooting discharges

We report the mechanisms of production and the state of excitation of the neutral and singly charged monatomic carbon in the regenerative soot as a function of the discharge parameters in graphite hollow cathode (HC) sources. Two distinctly different source configurations have been investigated. Comparisons of the level densities of various charged states of C1 have identified the regenerative properties of the C radicals in graphite HC soot.

cond-mat.mes-hall↗

arXiv2016

Regenerative Soot-VII: Population inversion of He I and Ne I in regenerative sooting discharges

The mechanisms for population inversion among the excited states of He I and Ne I in regenerative sooting discharges have been investigated as a function of the geometry and the physical parameters of the graphite hollow cathode sources. The effect of the state of sooting of the hollow cathode, the gas pressure and the role of the cycling of the discharge current idis on the population inversion of the excited He I and Ne I levels have been investigated.

cond-mat.mes-hall↗

arXiv2016

Regenerative Soot-V: Spectroscopy of the regenerative sooting discharges

The mechanisms and processes of the formation of the regenerative soot in a graphite hollow cathode discharge that produces and emits carbon clusters are presented. Mass spectrometry with a designed ExB velocity filter analyzes the entire range of the charged clusters from monatomic Carbon C1 to as large clusters as C4300. The state of the carbon vapor within the source is evaluated by using the characteristic line emissions from the carbonaceous discharge whose formative mechanisms depend upon the kinetic and potential sputtering of the sooted cathode. The carbonaceous discharge generates atomic and ionic C and its clusters Cm where m is equal to or greater than 2, noble gas metastable atoms and ions, energetic electrons and photons in the cavity of the graphite hollow cathode. The parameters of soot formation and its recycling depend critically on the discharge parameters, the geometry of the hollow cathode and 3D profile of the cusp magnetic field contours.

cond-mat.mes-hall↗

arXiv2016

Regenerative Soot-IV: The state of the carbon vapour in the regenerative sooting discharge

Formation of the regenerative soot is the process of recycling and introduction of the cathode deposited carbon clusters into the discharge. The agglomerates of carbon clusters on the cathode release their constituents into the plasma that goes from the pure sputtering mode to the sooting one. The process of the regeneration of the soot that emits large carbon clusters is discussed by evaluating the state of the carbon vapour by using the characteristic line emissions from the discharge.

cond-mat.mes-hall↗

arXiv2016

Regenerative Soot-II: Emission of carbon clusters from sooting plasma

A hollow cathode source with a specially designed cusp magnetic field B whose z component is a function of radius and the azimuth angle, provides clustering environment to the sputtered carbon atoms and ions. The distinctive features of the source are its sooting properties that depend upon the two well-defined regions within the discharge. The cusp field drives both of these regions but in different modes. One region ionizes the source gas, introduces the sputtered carbon from the graphite hollow cathode while the other is the collision dominated gyration motion of the positive carbon ions along the z component of the magnetic field contours. Tunability of plasma discharge characteristics provides the tool to investigate clustering in the sooting plasma

cond-mat.mes-hall↗

arXiv2016

Regenerative Soot-III: Role of the kinetic and potential sputtering in the regeneration of the soot

We have used the photoemission spectroscopy of the graphite hollow cathode sooting discharge to identify the roles played by the kinetic and potential sputtering, respectively. Our indicators are the relative densities of the sputtered carbon C, the Ne metastable atoms, and the ionized components (Ne+) of the regenerative sooting plasma. We find that the metastable atoms are the main agents for the regeneration of the soot.

cond-mat.mes-hall↗

arXiv2016

Regenerative Soot-I: Carbon cluster formation in regenerative sooting plasmas

Laboratory formation of large carbon clusters with m C atoms where m could be up to few thousand, in carbonaceous plasma, has been studied by using an especially designed ion source. Carbon is introduced into the glow discharge plasma by sputtering of the graphite electrode. Soot dominated plasma is created whose constituents are carbon clusters. It produces and recycles cluster containing plasma. Regenerative sooting plasma creates the environment in which the entire spectrum of clusters that contain the linear chains, rings and fullerenes. Velocity spectra of the extracted clusters have been measured with an ExB filter. These spectra indicate and identify the mechanisms operating in the soot.

cond-mat.mtrl-sci↗

arXiv2016

Evolution of clusters in energetic heavy ion bombarded amorphous graphite-II: formation and fragmentation phenomena

A study has been conducted into the mechanisms of evolution of clusters and their subsequent fragmentation under energetic heavy ion bombardment of amorphous graphite. The evolving clusters and their subsequent fragmentation under continuing ion bombardment are revealed by detecting various clusters in the energy spectra of the Direct Recoils emitted as a result of collisions between ions and surface constituents. The successive Direct Recoil spectra reveal that the energetics of Carbon Carbon bond formation as well as any subsequent fragmentation can be related to the processes of energy dissipation in a cylindrical volume of a few Angstrom surrounding the ion path. The dependence of cluster with m carbon atoms formation or its subsequent fragmentation into cluster with m-2 carbon atoms and diatomic carbon C2 is seen to be a function of the ionic stopping powers in this cylindrical volume.

cond-mat.mes-hall↗

arXiv2016

Evolution of clusters in energetic heavy ion bombarded amorphous graphite-III: Photo-emission and velocity analysis of sputtered carbon clusters

Diagnostics of the carbon atoms, ions, and clusters sputtered from a graphite surface under 10 keV Xe ion bombardments reveals the presence of neutral carbon clusters and positively charged species in the photo-emission spectra. The observed absence of the positively charged clusters in the mass spectra by a velocity analyzer is supplemented by the presence of the negatively charged monatomic, diatomic, triatomic and quatratomic clusters.. The velocity spectra are dominated by the negative C1, C2, C3 and C4. The latter two are observed having much reduced peak intensities . These results may help us to understand the contribution of neutral and charged species in the heavy-ion sputtering of graphite and the energetics of soot formation. Xe is chosen being the heaviest noble gas ion irradiating at not too high energy i.e 10 keV.

cond-mat.mes-hall↗

arXiv2016

Evolution of clusters in energetic heavy ion bombarded amorphous graphite-III: Energy spectra of charged clusters recoiling from Xe+ irradiations

We report on a novel technique of heavy ion induced surface modification and clustering mechanisms in amorphous graphite using Direct Recoil Spectrometry. The charged clusters emanating as Direct Recoils from 50-150 keV Xe+ irradiated graphite surface are detected in successive energy spectra at large recoil angles. Ion beams irradiated surface is shown to provide an environment of sputtered carbon atoms and clusters. The clustering occurs on the surface where the most stable ones accumulate. The spectra have identified clustering mechanisms operating in amorphous graphite under high fluence irradiations at grazing incidence that are related to the reconstructed surface features. Heavy ion irradiation induces the twin effects of producing a wide range of clusters and simultaneously ejecting these as DRs.

cond-mat.mes-hall↗

arXiv2016

Mass spectrometric identification of C60 fragmentation regimes under energetic Cs+ bombardment

Three C60 fragmentation regimes in fullerite bombarded by Cs+ are identified as a function of its energy. C2 is the major species sputtered at all energies. For E(Cs+) < 1 keV C2 emissions dominate. C2 and C1 have highest intensities between 1 and 3 keV with increasing contributions from C3 and C4. Intensities of all fragments maximize around 2 keV. Above 3 keV, fragments densities stabilize. The roles of and the contributions from direct recoils and collision cascades are determined. Maximum direct recoil energy delivered to the C60 fullerite cage is 210 eV at which only C2 emissions occur is identified and an explanation provided. The three fragmentation regimes under continued Cs+ bombardment eventually lead to complete destruction of the C60 cages transforming fullerite into amorphous carbon

physics.atm-clus↗

arXiv2016

Evolution of clusters in energetic heavy ion bombarded amorphous graphite

Carbon clusters have been generated by a novel technique of energetic heavy ion bombardment of amorphous graphite. The evolution of clusters and their subsequent fragmentation under continuing ion bombardment is revealed by detecting various clusters in the energy spectra of the direct recoils emitted as a result of collision between ions and the surface constituents.

physics.atm-clus↗

arXiv2016

Modelling of C2 addition route to the formation of C60

To understand the phenomenon of fullerene growth during its synthesis, an attempt is made to model a minimum energy growth route using a semi-empirical quantum mechanics code. C2 addition leading to C60 was modelled and three main routes, i.e. cyclic ring growth, pentagon and fullerene road, were studied. The growth starts with linear chains and, at n = 10, ring structures begins to dominate. The rings continue to grow and, at some point n > 30, they transform into close-cage fullerenes and the growth is shown to progress by the fullerene road until C60 is formed. The computer simulations predict a transition from a C38 ring to fullerene. Other growth mechanisms could also occur in the energetic environment commonly encountered in fullerene synthesis, but our purpose was to identify a minimal energy route which is the most probable structure. Our results also indicate that, at n = 20, the corannulene structure is energetically more stable than the corresponding fullerene and graphene sheet, however a ring structure has lower energy among all the structures up to n 6 40. Additionally, we have also proved that the fullerene road is energetically more favored than the pentagon road. The overall growth leading to cage closure for n = 60 may not occur by a single route but by a combination of more than one route.

physics.chem-ph↗

arXiv2016

The degenerate Fermi gas of $π$ electrons in fullerenes and the $σ$ surface instabilities

The departure from perfect spherical symmetry in the case of fullerenes, C60 being the sole exception, induces instabilities due to the stresses generated by the pentagonal protrusions in the sigma-bonded surfaces. By assuming sigma and pi separability and treating pi electrons as a degenerate Fermi gas in the two shells around the central sigma structure, the resulting degeneracy pressures can further enhance the sigma-surface initiated instabilities for non-icosahedral structures especially for those smaller than C60 as well as the icosahedral fullerenes that are larger than C60, with large protrusions. Under certain circumstances the net degeneracy pressure across the sigma surface might have structure-stabilizing effect. The role of the pi electron degeneracy in a broad range of fullerenes from C20 to C1500 and its effects on fullerene stability is investigated.

cond-mat.mes-hall↗