Pragya Bhatt

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

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.

Kinematics and dissociation dynamics of a water molecule under the impact of 10 keV electrons

The kinematics and dissociation dynamics of a H2O molecule induced by 10 keV electrons are studied using a time-of-flight mass spectrometer in conjunction with a position-sensitive detector in multi-hit coincidence mode. Five dissociative channels arising from the complete as well as the incomplete Coulomb explosions of H2Oq+ (q = 2, 3) ions are observed and identified. The dissociation mechanisms (concerted and/or sequential) for these channels are examined. Further, the angular correlation of different fragment ions and the geometrical structure of the precursor ion are studied. The kinetic energy release distributions for the observed channels are also determined. It is found that the pure Coulomb explosion model is insufficient to explain the observed kinetic release distributions. The mean kinetic energy release for these channels is compared with the available data reported by earlier workers who have employed different charged projectiles and sources of photons.

Study of K-line radiation of thick titanium produced in collisions of keV electrons.

The characteristic K-line yields Y(E₀) of a pure thick titanium (Z=22) element target are measured for 8-18 keV electron impact and compared with the simulation calculations using PENELOPE code. A fair agreement between experiment and simulation results is found within the existing experimental uncertainty of measurements. The ratio F of indirectly produced characteristic Ti K X-ray yield to its total (directly+indirectly) yield is determined by employing an approximate analytical formulation of Hanson and Cowan (Hanson, H.P., Cowan, D.J., 1961. Phys. Rev. 124, 22-26). It is found that F changes strongly with impact energy E₀ for normal angle of incidence in contrast to a mild change predicted by the simulation calculations. Furthermore, experimental and simulation results for peak to effective continuum ratio R of Ti K-line are compared and discussed for the energy range of impact of the present investigation.

Three-body dissociation of OCS3+: Separating sequential and concerted pathways

Events from the sequential and concerted modes of the fragmentation of OCS3+ that result in coincident detection of fragments C+, O+, and S+ have been separated using a newly proposed representation. An ion beam of 1.8 MeV Xe9+ is used to make the triply charged molecular ion, with the fragments being detected by a recoil ion momentum spectrometer. By separating events belonging exclusively to the sequential mode of breakup, the electronic states of the intermediate molecular ion (CO2+ or CS2+) involved are determined, and from the kinetic energy release spectra, it is shown that the low lying excited states of the parent OCS3+ are responsible for this mechanism. An estimate of branching ratios of events coming from sequential versus concerted mode is presented.

Three-body dissociation of OCS3+ : separating sequential and concerted pathways

Three body fragmentation pathways of OCS3+ molecular ion leading to detection of C + + O + + S + in coincidence have been studied using the technique of recoil ion momentum spectroscopy. This fragmentation can occur either via a one step concerted process or a two step sequential process. In the first step of the sequential pathway either CO2+ or CS2+ is formed. By separating the events coming from the two processes, branching ratios are determined and the possible electronic states associated with the intermediate CO2+ or CS2+ have been identified.

Angular distributions in two body breakup of OCSq+ ions

The angular distribution of the correlated dissociation fragments with respect to the beam direction has been topic of interest for both experimentalists and theoreticians. This is of fundamental importance in various areas of science and technology (see, e.g. [1]). In the past decades, many experimental methods have been used to study the anisotropy in the angular distributions of fragments of multiply ionized molecules formed by impact of highly charged ion (e.g. [2] and references therein).

Measurement of Plasma Potential using Deceleration technique

We report a novel technique to measure the plasma potential. In this technique, we extract an ion beam into the chamber with few keV of energy and then decelerate it using series of electrodes till the deceleration potential nullifies the extraction potential and measure the retaining energy of the beam using retarding plate analyzer (RPA) to determine the plasma potential.

Ionic fragmentation of CO and H2O under impact of 10 keV electrons: kinetic energy release distributions

Dissociative ionization of COq+ (q=2-4) and H2Oq+ (q=2-3) molecular ions produced from the collisions of CO and H2O with 10 keV electrons is studied using time-of-flight mass spectrometer and position sensitive detector with multi-hit ability, respectively. The kinetic energy release distributions for these channels are obtained. We found that a pure Coulomb explosion model is insufficient to explain the observed kinetic release distributions for the Coulomb explosion channels. A detail of this study is given in references [3, 4].

Dissociative-ionization cross sections for 12-keV-electron impact on CO{sub 2}

The dissociative ionization of a CO{sub 2} molecule is studied at an electron energy of 12 keV using the multiple ion coincidence imaging technique. The absolute partial ionization cross sections and the precursor-specific absolute partial ionization cross sections of resulting fragment ions are obtained and reported. It is found that {approx}75% of single ionization, 22% of double ionization, and {approx}2% of triple ionization of the parent molecule contribute to the total fragment ion yield; quadruple ionization of CO{sub 2} is found to make a negligibly small contribution. Furthermore, the absolute partial ionization cross sections for ion-pair and ion-triple formation are measured for nine dissociative ionization channels of up to a quadruply ionized CO{sub 2} molecule. In addition, the branching ratios for single-ion, ion-pair, and ion-triple formation are also determined.