Lokesh C. Tribedi

Role of surface roughness in hard-x-ray emission from femtosecond-laser-produced copper plasmas

Hard x-ray emission in the energy range of 30-300 keV from copper plasmas produced by 100-fs, 806-nm laser pulses at intensities in the range of 10 1 5 -10 1 6 W cm - 2 is investigated. We demonstrate that surface roughness of the targets overrides the role of polarization state in the coupling of light to the plasma. We further show that surface roughness has a significant role in enhancing the x-ray emission in the above mentioned energy range.

On the Photoabsorption by Permanganate Ions in Vacuo and the Role of a Single Water Molecule. New Experimental Benchmarks for Electronic Structure Theory

We report electronic spectra of mass-selected MnO4(-) and MnO4(-)⋅H2O using electronic photodissociation spectroscopy. Bare MnO4(-) fragments by formation of MnO3(-) and MnO2(-), while the hydrated complex predominantly decays by loss of the water molecule. The band in the visible spectral region shows a well-resolved vibrational progression consistent with the excitation of a Mn-O stretching mode. The presence of a single water molecule does not significantly perturb the spectrum of MnO4(-). Comparison with the UV/Vis absorption spectrum of permanganate in aqueous solution shows that complete hydration causes a small blueshift, while theoretical models including a dielectric medium have predicted a redshift. The experimental data can be used as benchmarks for electronic structure theory methods, which usually predict electronic spectra in the absence of a chemical environment.

Fast-electron impact ionization of molecular hydrogen: energy and angular distribution of double and single differential cross sections and Young-type interference

We report the energy and angular distribution of absolute double differential cross sections (DDCSs) of ejected electrons in collisions of 8 keV projectile electrons with molecular hydrogen. The ejected electrons with energy between 1 eV and 400 eV and ejection angles between 30° and 150° are detected. The measured data are compared with the theoretical calculations based on two-effective centre (TEC) model. The first-order interference is derived from the energy distribution of DDCS and the resulting ratio spectra (H2 to 2H) exhibit oscillating behaviour. The signature of first-order interference is also demonstrated in the DDCS spectra as a function of the ejection angle. We have shown that the constructive interference prevails in soft- and binary-collision regions. The single differential cross sections (SDCS) are deduced by integrating the DDCS over the solid angle as well as ejection energy. We demonstrate that the SDCS and corresponding ratio spectra also preserve the signature of interference.

Efficiency determination of a Si(Li) X-ray detector in the energy range 1–20 keV

Abstract The efficiency of a Si(Li) X-ray detector has been measured in the energy range 1–20 keV employing standard radioactive sources and the “reference” PIXE cross sections with the intention to check the application of these cross sections for efficiency measurement in the energy region below 5 keV.

Absolute total ionization cross sections of uracil (C4H4N2O2) in collisions with MeV energy highly charged carbon, oxygen and fluorine ions

Ionization and fragmentation of uracil molecules (C4H4N2O2, m = 112 amu) in collisions with fast highly charged C, O and F ions have been investigated using a time-of-flight mass spectrometer. The measurement of total ionization cross sections (TCS) is reported for different charge states (q), such as Fq + with q = 5?8; Oq + with q = 5,7; Cq + with q = 5 and 6. These studies reveal a (q/v)?1.5 dependence of TCS, in contrast, to the well-known q2-dependence in ion?atom collisions. Scaling properties of the TCS with projectile energy and charge states are obtained. The experimental results for TCS measurements are compared with the theoretical calculations performed within classical and quantum mechanical frameworks. The trends in energy dependence of the TCSs is qualitatively well reproduced by the different models and more specifically by the classical description, which provides the best agreement with measurements.

Experimental study of K-shell ionization of low-Z solids in collisions with intermediate-velocity carbon ions and the local plasma approximation

K-shell vacancy production in low-atomic-number (Z t = 17–29) solid targets ha sb een measured in collisions of highly charged carbon ions with energies of 1.5–6 MeV u −1 .T he K-shel li onization cross sections of Cl, K, Ti, Fe and Cu are derived from the measured K x-ray cross sections. The present data-set has been used to test the predictions of a theoretical model based on the local plasma approximation (LPA). This theory takes into account the response of solid core electrons working within the dielectric formalism. We find that this ab initio ion–solid model gives very good agreement with the measured data for Fe and Cu targets, while it tends to under-estimate the data for the most symmetric collision systems studied here. We discuss the range of validity of the LPA in terms of the symmetry parameter and the impact velocity. On the other hand, am odel based on the perturbed stationary state approximation, designed for ion–atom collisions (ECPSSR) is found to give excellent agreement with the measured data for all target elements over the whole energy range. All the measured cross sections for different targets are found to follow a universal scaling rule predicted by the ECPSSR.

Double-differential distributions of electron emission in a pure three-body collision: ionization of atomic hydrogen by highly charged ions

We have studied the fundamental process of ionization in a pure three-body ion - atom collision involving atomic hydrogen as a target and a bare heavy-ion as a projectile. We report the measurements of the double-differential cross section (DDCS) of ionization for . The measurements of the energy and angular distributions of the double-differential cross sections of the low-energy electrons and a comparison with the CDW-EIS and FBA calculations provide a detailed understanding of the two-centre mechanism of electron emission. The forward - backward asymmetry parameters have also been studied. The DDCS ratio between molecular and atomic hydrogen shows a prominent structure.

Double differential distribution of electron emission in the ionization of water molecules by fast bare oxygen ions

The doubly differential distributions of low-energy electron emission in the ionization of water molecules under the impact of fast bare oxygen ions with energy of 48 MeV are measured. The measured data are compared with two quantum-mechanical models, i.e. the post and prior versions of the continuum distorted wave–eikonal initial state (CDW-EIS) approximation, and the first-order Born approximation with initial and final wavefunctions verifying correct boundary conditions (CB1). An overall excellent qualitative agreement is found between the data and the CDW-EIS models whereas the CB1 model showed substantial deviation. However, the detailed angular distributions display some discrepancies with both CDW-EIS models. The single differential and total cross-sections exhibit good agreement with the CDW-EIS models. The present detailed data set could also be used as an input for modeling highly charged ion induced radiation damage in living tissues, whose most abundant component is water. Similar measurements are also carried out for a projectile energy of 60 MeV. However, since the double differential cross-section data show similar results the details are not provided here, except for the total ionization cross-sections results.

Temperature diagnostics of ECR plasma by measurement of electron bremsstrahlung.

The x-ray bremsstrahlung spectrum emitted by the electron population in a 14.5 GHz ECR plasma source has been measured using a NaI(Tl) detector, and hence the electron temperature of the higher energy electron population in the plasma has been determined. The x-ray spectra for Ne and Ar gases have been systematically studied as a function of inlet gas pressure from 7 × 10(-7) mbar to 7 × 10(-5) mbar and for input microwave power ∼1 W to ∼300 W. At the highest input power and optimum pressure conditions, the end point bremsstrahlung energies are seen to reach ∼700 keV. The estimated electron temperatures (T(e)) were found to be in the range 20 keV-80 keV. The T(e) is found to be peaking at a pressure of 1 × 10(-5) mbar for both gases. The T(e) is seen to increase with increasing input power in the intermediate power region, i.e., between 100 and 200 W, but shows different behaviour for different gases in the low and high power regions. Both gases show very weak dependence of electron temperature on inlet gas pressure, but the trends in each gas are different.

Investigation of the interference effect in the case of low energy electron emission from O2 in collisions with fast bare C-ions

We have measured the double differential cross sections (DDCSs) for low energy electron emission from O2 under the impact of 51 MeV bare carbon ions. This study is aimed at investigating the Young-type interference in electron emission from a multi-electronic diatomic molecule. The DDCS spectra, differential in emission energy and angle, are compared with state-of-the-art continuum distorted wave-eikonal initial state (CDW-EIS) calculations. The DDCS ratios (i.e. O2/2O) do not produce any oscillatory behaviour due to the interference effect unlike that observed in the case of H2. The forward–backward angular asymmetry parameter, deduced from the measured DDCS values, is found to be a monotonically increasing function of electron velocity and does not show any oscillation. These observations are in qualitative agreement with the prediction of the molecular CDW-EIS model which uses a linear combination of atomic orbitals. The apparent absence of the oscillation in the spectra is qualitatively explained in terms of cancellation of contributions arising from different molecular orbitals.