A. Qayyum

Laser fluence effects on ion emission from a laser-generated Cu plasma

We present details of an experimental facility developed for the diagnostics of highly charged ions produced during pulsed laser ablation of solid targets. A range of laser fluences (2–10 J cm−2) from a Q-switched Nd : YAG laser (wavelength = 1064 nm, pulse duration ~10 ns) was used to generate a copper plasma. The ion diagnostics were based on the time-of-flight (TOF) methods; an ion collector and a 45° parallel plate electrostatic ion energy analyser were used. A channel electron multiplier located 1.31 m away from the Cu target was used to record the energy-resolved TOF ion spectrum. The effect of laser fluence on the total ion charge, average ion energy and charge state distribution was investigated. The estimated threshold fluence for the onset of the plasma was 2.5 J cm−2. About four times increase in both average ion energy and total ion charge was observed in the investigated laser fluence range. The maximum attainable charge state of the Cu ions increased from 1+ to 7+ with the increase in laser fluence. The correlation between relative abundance of the various ion charge states indicated that the formation of Cun+ occurred through ionization from Cu(n−1)+ by the impact of fast electrons or by multiphoton interactions.

A magnetically confined hollow cathode duoplasmatron for the PINSTECH ion implanter

Abstract A duoplasmatron ion source is being reported which operates at low gas pressures, uses small discharge power, has a long life time and is of small dimensions and weight. The source incorporates a magnetically confined hollow cathode with the main plasma constriction between the intermediate electrode and anode. Ring shaped permanent magnets produce the special magnetic field profile in the source that is responsible for the enhanced performance. The source can deliver up to 5 mA of singly charged ion beam at moderate operating conditions.

Photoemission spectroscopy and velocity analysis of sputtered carbon atoms, ions, and clusters Cm0,± (m⩽4)

Diagnostics of the carbon atoms, ions, and clusters Cm0,± (m⩽4) sputtered from a graphite surface under 10 keV Xe+ bombardment reveals the presence of neutral Cm0 and positively charged Cm+ species in the photoemission spectra. The complete absence of Cm+ in the mass spectra by a velocity analyzer is supplemented by the presence of the negatively charged Cm−. The velocity spectra are dominated by C and C2− with peaks due to C3− and C4−. 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 regenerative soot.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.

Influence of source-to-substrate distance on the properties of ZnS films grown by close-space sublimation

ZnS films were deposited on soda glass at various source-to-substrate distances by closespace sublimation. The influence of source-to-substrate distance on the structural and optical properties of the films was investigated. XRD spectra showed that films were crystalline in nature having cubic structure oriented mainly alogn (111) plan. The crystallinity of films increased with the source-to-substrate distance up to 40 mm. The crystallite size increases from 15.76 to 19.06 nm as the source-to-substrate distance increased from 5 to 40 mm. AFM data reveled that RMS roughness decreases and grain size of the film increases with the source-to-substrate distance. The optical transmittance in the visible range was about 70% for all films. The refractive index of a ZnS film decreases with increasing source-to-substrate distance. But source-to-substrate distance seems to have no effect on the energy bandgap and absorption edge of ZnS films. Moreover, it is shown that resisitivity of the ZnS films resuced significantly by Ag doping.

The influence of substrate temperature on the structural and optical properties of ZnS thin films

Thin films of ZnS were deposited on soda lime glass substrates by a modified close-space sublimation technique. The change in optical and structural properties of the films deposited at various substrate temperatures (150–450°C) was investigated. X-ray diffraction spectra showed that films were polycrystalline in nature having cubic structure oriented only along (111) plan. The crystallinity of films increased with the substrate temperature up to 250°C. However, crystallinity decreased with further increase of substrate temperature and films became amorphous at 450°C. The atomic force microscopy data revealed that the films become more uniform and dense with the increase of substrate temperature. Optical properties of the films were determined from the transmittance data using Swanepoel model. It was observed that the energy band gap is increased from 3.52 to 3.65 eV and refractive index of the films are decreased with the increase of substrate temperature. Moreover, considerable improvement in blue response of the films was noticed with increasing substrate temperature.

Characterization of highly charged titanium ions produced by nanosecond pulsed laser

In this work, plasma is produced by irradiating a Ti target with 10 ns pulsed Nd:YAG (λ= 1064 nm) laser. The laser fluence at the target was varied in the range of 2–20.3 J/cm. The ion signal from freely expanding Ti plasma in vacuum was characterized with the help of ion collector and time-of-flight electrostatic energy analyzer. The ion charge state was found to increase with the laser fluence and maximum available ion charge in this fluence range is Ti. A correlation between the intensities of various ion charge states was observed, which indicates that higher charge states are most probably produced through stepwise ionization mechanism. It is also observed that charge state distribution of plasma can be controlled by variation of the laser fluence. In addition, energy distribution of ion charge states Ti (n= 1–4) is measured by varying back plate voltage of the electrostatic energy analyzer for a fixed laser fluence of 20.3 J/cm. Ions energy distributions were in the range of 0.36–3.0 keV and the most probable ion energy was found to increase linearly with ion charge state. The estimated equivalent potential at the laser fluence of 20.3 J/ cm is about 310 V. These results are in good agreement with the predictions of electrostatic model of ion acceleration in laser plasma.

MULTIPLY CHARGED DIRECT RECOIL SPECTRA FROM AR+ AND KR+ BOMBARDED GRAPHITE

Abstract Measurements of the energy spectra of multiply charged positive and negative carbon ions (Cn ±) 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°, direct recoil (DR) peaks have been observed with singly as well as multiply charged carbon ions Cn ±, where n = 1 to 6. These Cn ± ions have been seen recoiling with the characteristic recoil energy EDR = kE0cos2 θDR, where θDR is the direct recoil angle, k = 4m 1 m 2 (m 1 + m 2 ) 2 , m1 and E0 are projectile mass and energy and m2 is carbon mass. We have observed sharp DR peaks. A collimated projectile beam with divergence ∼ ±0.2° is supplemented with a similar collimation before the energy analyzer to reduce the background of sputtered ions due to scattered projectiles.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.

Characterization of 1064 nm Nd:YAG laser-produced cu plasma

The plasma was produced by focusing Nd:YAG laser pulses of 1064 nm wavelength on to a copper target at laser fluences of 5.35, 6.95, and 9.33 J/cm2. An ion collector placed along the target surface normal was used to record the time-of-flight (TOF) ion signal during plasma expansion in vacuum. The TOF ion pulses were deconvoluted using the Coulomb-Boltzmann-shifted function to estimate the available Cu ion charge states, equivalent plasma ion temperature, and accelerating potential in the nonequilibrium plasma. The maximum available ion charge state, equivalent plasma ion temperature, and accelerating potential are found to increase with laser fluence. In the local thermal equilibrium conditions, the accelerating potential can be supposed to apply across a distance of the order of the Debye length. The Debye length and, hence, the electric field in the laser produced plasma at three laser fluences values were estimated. The electric field was in the range of 1 MV/cm and increased with laser fluence. In the laser fluence range used in this work, the sum of thermal and adiabatic energy of the ion was slightly higher than its Coulomb energy.

Surface-Induced Dissociation of Low Energy H+2 Impact on a Carbon Surface: A Monte Carlo Simulation

A Monte Carlo simulation based on the classical binary collision approximation is performed to investigate the interaction of H+2 ions with the carbon target. The incident H+2 ion is characterized by its translational energy, eigenenergy and population of the vibrational state, and orientation of the ion with respect to the target surface. It is shown that experimentally determined energy resolved mass spectrum of H+2 can be nicely reproduced with the help of the proposed model. These simulations predict that translational to vibrational (T → V) energy transfer efficiency increases nonlinearly with translational energy of the incident ion. T → V energy transfer efficiency along with the initial vibrational energy of the incident H+2 ion found to play an important role in dissociation. Our simulations also show that the fraction of absorbed, reflected, and dissociated ions depends on the translational energy. The average vibrational energy of reflected H+2 increases with its initial translational energy. Moreover, average number of collisions required for dissociation varies inversely with the initial translational energy of the H+2.

Ion-induced kinetic electron emission from 6 LiF, 7 LiF and MgF 2 thin films

Secondary electron yields for Ar+ impact on 6LiF, 7LiF and MgF2 thin films grown on aluminum substrates are measured each as a function of target temperature and projectile energy. Remarkably different behaviours of the electron yields for LiF and MgF2 films are observed in a temperature range from 25 ?C to 300 ?C. The electron yield of LiF is found to sharply increase with target temperature and to be saturated at about 175 ?C. But the target temperature has no effect on the electron yield of MgF2. It is also found that for the ion energies greater than 4 keV, the electron yield of 6LiF is consistently high as compared with that of 7LiF that may be due to the enhanced contribution of recoiling 6Li atoms to the secondary electron generation. A comparison between the electron yields of MgF2 and LiF reveales that above a certain ion energy the electron yield of MgF2 is considerably low as compared with that of LiF. We suggest that the short inelastic mean free path of electrons in MgF2 can be one of the reasons for its low electron yield.