R. Shanker

Bio: R. Shanker is an academic researcher from Banaras Hindu University. The author has contributed to research in topics: Ionization & Ion. The author has an hindex of 8, co-authored 31 publications receiving 224 citations. Previous affiliations of R. Shanker include Bielefeld University.

L X-ray anisotropy and L3-subshell alignment of heavy atoms induced by ion impact

Abstract: The anisotropy emission of the Ll X-ray line from heavy atoms (54

Impact parameter dependence of Ar L-shell excitation in slow Ar-Ar collisions

Abstract: We report here on studies of the impact-parameter dependence of Ar L-shell excitation in slow Ar-Ar collisions. The molecular-orbital (MO) correlation diagram for the Ar-Ar collision system is shown in Fig. 1. This diagram suggests that L-shell excitation of both collision partners should occur when the internuclear distance reaches values smaller than or equal to about 0.5 a.u.. Then, due to dynamic couplings, electrons promoted via the steeply rising 4fσ MO (3σu, 4σu in the adiabatic notation of Fig. 1) can be transferred into initially empty bound MO or into the continuum. The probability for excitation of one 4fσ electron via this promotion mechanism is expected to be practically unity. Thus, the number of electrons excited per collision through this channel should attain the value of two in the entire impact-parameter range 0 < b< 0.5 a.u. At small impact parameters an additional promotion mechanism2 for Ar L-shell electrons becomes operative when the initially filled 3dπ(1πg) and 3dσ(4σg, 3σg, 2σg) MO are rotationally coupled to the initially empty 3dδ(lδg) MO.

Momentum mapping spectrometer for probing the fragmentation dynamics of molecules induced by keV electrons

Abstract: We describe a new experimental setup for studying the fragmentation dynamics of molecules induced by the impact of keV electrons using the well-known technique of recoil ion momentum spectroscopy. The apparatus consists of mainly a time- and position-sensitive multi-hit particle detector for ion analysis and a channel electron multiplier detector for detecting the ejected electrons. Different components of the setup and the relevant electronics for data acquisition are described in detail with their working principles. In order to verify the reliable performance of the setup, we have recorded the collision-induced ionic spectra of the CO2 molecule by the impact of keV electrons. Information about the ion pairs of CO+:O+, C+:O+ and O+:O+ resulting from dissociative ionizing collisions of 20 and 26 keV electrons with a dilute gaseous target of CO2 molecules has been obtained. Under conditions of the present experiment, the momentum resolutions of the spectrometer for the combined momenta of CO+ and O+ ions in the direction of the time-of-flight axis and perpendicular to the direction of an electron beam are found to be 10.0 ± 0.2 and 15.0 ± 0.3 au, respectively.

Coincidence measurements of M-shell excitation in slow Xe-Xe collisions

Abstract: Ion-photon and ion-Auger-electron coincidence measurements have been performed to study the impact parameter dependence of Xe M-shell excitation in 1.05 MeV Xe3+-Xe collisions. The experimental results are found to be consistent with the prediction of the molecular orbital model of atomic collisions. The average fluorescence yield for the Xe M shell is found to be strongly dependent on the impact parameter. This is ascribed to the production of highly charged Xe ions in close collisions.

Modelling of electron beam absorption in complex geometries

Abstract: Computational modelling of processes that involve highly energetic electrons like electron beam melting, welding, drilling or electron beam lithography, to name but a few, requires information about the attenuation of the electron beam as it passes through the sample. Depth–dose curves as a function of electron energy, target material as well as local surface obliquity have to be provided in situ during the calculation. The most efficient way to address this issue is by employing mathematical expressions. Therefore, we propose an electron beam model based on a set of semi-empirical equations available from different published literature and on theoretical considerations. Particular stress is thereby put on accuracy and the range of validity of the theoretical approach by comparison with experimental data. Finally, we apply our model to powder-bed based additive manufacturing. The numerical results demonstrate that electron beam absorption and depth of penetration have a strong influence on the quality of the fabricated product.

2013 Atomic spectrometry update—A review of advances in X-ray fluorescence spectrometry

Abstract: This review comments on the ever expanding range of work using the XRF group of techniques published approximately between April 2012 and March 2013. It presents innovative work as opposed to the wealth of contributions that combine XRF spectrometry in conjunction with other analytical techniques and those descriptions of the valuable routine applications published during the review period. New specialised laboratory instruments, X-ray sources and data processing procedures have been introduced as well as further detector development. Portable and hand-held systems have been increasingly used and the literature represents the widening application of μ-SR techniques for XRF mapping and insights of the sub-cellular metabolisms of animals and plants. Applications using TXRF were published dealing with samples from the pharmaceutical industry, biological tissues, extraterrestrial samples and the analysis of nuclear fuels. Nano-particles feature in many applications and this review is enlivened by reports on more space missions. Climate change, environmental studies and green chemistry feature along with work on archaeological and cultural heritage samples including the authentication of an eighteenth century harpsichord varnish.

Higher order processes in L-shell ionization

Abstract: The time-dependent perturbation theory has been applied to the description of the L-shell ionization of atoms by heavy charged particles in the independent-particle model approximation. A second order correction factor to cross sections calculated in a first order theory (e.g. PWBA) has been derived considering transitions at an average impact parameter and with minimum energy transfer as dominant ionization processes in low-velocity collisions. Numerical calculations have been performed for light and heavy (Z1 ⩽ 8) ion impact ionization of gold in the energy range 0.15—2.0 MeV u . The results are in satisfactory agreement with the experimental data for the L3-to-L2 and L1-to-L2 subshell ionization cross section ratios. The model seems to account also for the anomalous behaviour of the L3-subshell alignment observed recently at heavy-ion impact.

L X-ray anisotropy and L3-subshell alignment of heavy atoms induced by ion impact

Abstract: The anisotropy emission of the Ll X-ray line from heavy atoms (54