A.H. Dogar

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.

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.

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.

Ion-induced kinetic electron emission from 6LiF, 7LiF and MgF2 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.