Dissociation Of Molecular Ions Research Articles

Dissociative electron attachment to 1- and 9-chloroanthracene in the gas phase

Dissociative electron attachment (DEA) to 1-chloroanthracene and 9-chloroanthracene was investigated under gas-phase conditions. In both compounds, the elimination of the chlorine anion is the dominant channel for the dissociation of molecular negative ions (NIs). The second most intense channel leads to the formation of molecular anions (Mˉ). The autodetachment lifetime of Mˉ was measured to be about 170 μs for both compounds. The widths of the Mˉ peaks indicate that molecular anions are formed via two resonances: at thermal electron energies and through a shape resonance at the energy of ∼0.5 eV. Adiabatic electron affinities were estimated in the framework of the simple Arrhenius model to be 0.86 eV for both molecules, the values being close to the theoretical predictions by DFT method of 0.90 eV and 0.93 eV for 1-chloroanthracene and 9-chloroanthracene respectively. Metastable negative ions are observed in the DEA spectra of both molecules, which testifies that the elimination of chlorine anions from molecular NIs appears on a time scale of several microseconds.

0-D composition and performance analysis of an air-breathing radiofrequency ion thruster

AbstractThis paper presents a physics-based 0-D steady-state model of a radiofrequency ion thruster (RIT) that is valid for any propellant blend, and hence, any plasma mixture, and the model is used to predict the drag compensation capability and plasma composition of an air-breathing RIT operating in very-low Earth orbit (VLEO). This study fills a gap in the modeling of air-breathing electric propulsion with an air-breathing RIT model that is validated with experimental data of a RIT operating with an $${\mathrm{N}}_{2}$$ N 2 /$${\mathrm{O}}_{2}$$ O 2 mixture. The model expands upon current 0-D RIT models by accounting for the dissociation of neutral molecules and molecular ions and by applying particle conservation for each ion and neutral species. Atomic oxygen in the atmosphere is assumed to completely recombine into $${\mathrm{O}}_{2}$$ O 2 in the collector, and the model is used to calculate an air-breathing RIT’s thrust, discharge efficiency, mass utilization efficiency, and discharge plasma composition at altitudes between 80 and 150 km. At 1 kW of total input power and an optimal flow rate of 0.17 mg/s, the net thrust was limited to 3 mN at all altitudes considered. At these same operating conditions, the mass utilization efficiency and discharge efficiency were approximately 0.15 and 870 eV/ion, respectively. To increase the performance of the thruster, the magnetic field should be increased in order to increase the beam current. The mole fraction of atomic oxygen in the discharge chamber was found to be between 0.19 and 0.27 depending on the altitude, indicating that grid erosion should be a focus of future studies.

Physics studies of ADITYA & ADITYA-U tokamak plasmas using spectroscopic diagnostics

Several spectroscopic diagnostics encompassing the spectral emission range from the x-ray to near infrared (NIR) have been developed, installed and operated for diagnosing and physics studies in the ADITYA and ADITYA-U tokamaks. The recycling and impurity influxes and plasma Z eff after lithium (Li) coating have been studied using a PMT (photomultiplier tube)-filter based system by capturing H α , O1+, C2+, and visible continuum emissions. Significant reduction in the Z eff values has been observed in the discharge with the Li coated walls. The measured radial profile of H α emission using a filter-PMT array, has been modelled using a neutral transport code. The results show substantial contributions from the molecular hydrogen and molecular hydrogen ion dissociation (∼56%) and charge-exchange (∼30%) processes in the measured H α emission. Furthermore, a high-resolution, 1 m spectrometer with charge coupled device detector capable of multi-track measurements, has been used to study impurity transport, neutral and ion temperature and intrinsic plasma rotation. By modelling the measured radial profile of O4+ spectral line emission using an impurity transport code, substantial contribution of edge fluctuations on the oxygen transport has been observed. The toroidal ( ∼ 20 km s−1 in core) and poloidal ( ∼ 4.5 km s−1 at edge) rotation velocities are measured using C5+ (529 nm) and C2+ (464.7 nm) passive line emissions respectively. The measurement of radial profile of toroidal plasma rotation revealed a reversal of rotation direction depending on the electron density content of the ADITYA-U plasmas. The neutral temperature measurements showed a poloidal asymmetry indicating a presence of asymmetrical source of neutral heating. Moreover, the modelling of measured Fe14+ and Fe15+ vacuum ultraviolet spectral lines has revealed the neo-classical nature of iron transport in ADITYA core. Fast visible camera images captured the formation of filament structures triggered by interchange instabilities during plasma disruptions.

Photostability of the deprotonated forms of the UV filters homosalate and octyl salicylate: molecular dissociation versus electron detachment following UV excitation.

While common molecular anions show a strong propensity to undergo electron detachment upon UV excitation, this process often occurs in competition with molecular ion dissociation. The factors that affect the balance between these two major possible decay pathways have not been well understood to date. Laser photodissociation spectroscopy of the deprotonated forms of the UV filter molecules, Homosalate (HS) and Octyl Salicylate (OS), i.e. [HS − H]− and [OS − H]−, was used to acquire gas-phase UV absorption spectra for [HS − H]− and [OS − H]−via photodepletion from 3.0–5.8 eV. No photofragmentation (i.e. dissociation of the ionic molecular framework) was observed for either [HS − H]− and [OS − H]− following photoexcitation, revealing that electron loss entirely dominates the electronic decay pathways for these systems. High-level quantum chemical calculations were used to map out the excited states associated with [HS − H]− and [OS − H]−, revealing that the minimum-energy crossing points (MECPs) between the S1 and S0 states are located in elevated regions of the potential energy surface, making internal conversion unlikely. These results are consistent with our experimental observation that electron detachment out-competes hot ground state molecular fragmentation. More generally, our results reveal that the competition between molecular dissociation and electron detachment following anion photoexcitation can be determined by the magnitude of the energy gap between the excitation energy and the MECPs, rather than being a simple function of whether the excitation energy lies above the anion's vertical detachment energy.

Nuclear momentum distributions governed by Rabi flopping in the dissociation of hydrogen molecular ions in strong laser fields

We study the Rabi flopping between the $1s{\ensuremath{\sigma}}_{g}$ and $2p{\ensuremath{\sigma}}_{u}$ states during the dissociation of hydrogen molecular ions in strong laser fields by numerically simulating the time-dependent Schr\"odinger equation. Starting either from a certain vibrational state or from coherently superimposed vibrational states weighted with Frank-Condon factors, the dissociative nuclear momentum distributions present multiple angular nodes and maxima due to molecular alignment-dependent Rabi flopping. The Rabi frequency and nuclear angular distribution depend on the laser chirp and laser ellipticity. Rabi flopping also determines the population on the $2p{\ensuremath{\sigma}}_{u}$ state and thus the dissociation probability. The deep understanding of Rabi flopping in molecular dissociation gives clues to control ultrafast molecular dynamics.

Modeling of the Hα Emission from ADITYA Tokamak Plasmas

The spatial profile of Hα spectrum is regularly measured using a high-resolution multi-track spectrometer in ADITYA tokamak to study the neutral particle behavior. The Monte Carlo neutral particle transport code DEGAS2 is used to model the experimental Hα spectral emissions. Through the modeling of the spectral line profile of Hα, it is found that the neutral hydrogen, which is produced from molecular hydrogen and molecular hydrogen ion dissociation processes contributes 56% to the total Hα emission, and the atoms which are produced from charge-exchange process have 30% contribution. Furthermore, the experimentally measured spatial profile of chord integrated brightness was modeled for the two plasma discharges having relatively high and low density to understand the neutral particle penetration. The presence of neutrals inside the core region of the ADITYA tokamak is mainly due to the charge-exchange process. Furthermore, it is observed that neutral particle penetration is lower in higher density discharge.

Electron-Induced Excitation, Recombination, and Dissociation of Molecular Ions Initiating the Formation of Complex Organic Molecules

We review the study of dissociative recombination and ro-vibrational excitation of diatomic and small polyatomic molecular ions initiating complex organic molecules formation. In particular, we show how Multichannel Quantum Defect Theory (MQDT) and R-matrix methods are used to compute cross sections and rate coefficients for cations in well defined ro-vibrational levels of the ground electronic state, from sub-meV up to few eV collision energies. The most recent MQDT results are compared with either other theoretical data, or with measured data obtained in storage-ring experiments.

Positive-ion accelerator mass spectrometry at ATLAS: Peaks and pits

Electron Cyclotron Resonance (ECR) ion sources and the production of multiple-charge positive ions with high efficiency in combination with a heavy-ion accelerator have opened the way to an alternative and complementary version of accelerator mass spectrometry (AMS). A notable strength of positive-ion over traditional AMS is the capability of ultra-high sensitivity detection of radioactive isotopes of noble gases, in particular 37Ar (t1/2 = 35 d) and 39Ar (269 y). The complete dissociation of molecular ions in the ECR and in particular of hydride ions of neighboring stable isotopes results in superior isotopic separation. However, the use of high charge states, necessary for acceleration to high energy, entails the existence of severe transmission degeneracies with stable ions having nearly equal mass-to-charge ratios, in addition to that of stable isobaric ions. Separation or discrimination of these parasitic ions require powerful and sophisticated dispersive systems at detection stage. We review here work performed and in progress at the ATLAS facility of Argonne National Laboratory (ANL) where an ECR ion source, a Radio-Frequency Quadrupole (RFQ), a superconducting linear accelerator and a Gas-Filled Magnet (GFM) are used as an AMS setup.

Studying divertor relevant plasmas in the Pilot-PSI linear plasma device: experiments versus modelling

Predictions for the operation of tokamak divertors are reliant on edge plasma simulations typically consisting of a fluid plasma code in combination with a Monte-Carlo (MC) code for neutral species. Pilot-PSI is a linear device operating with a cascaded arc plasma source that produces plasmas comparable to those expected during the inter-ELM phase in the ITER divertor (Te ∼ 1 eV, ne ∼ 1020 m−3). In this study, plasma discharges in Pilot-PSI have been modelled using the Soledge2D fluid plasma code (Bufferand et al 2015 Nucl. Fusion 55) coupled to the Eirene neutral MC code (Reiter et al 2005 Fusion Sci. Technol. 47, 172–186) in order to (a) investigate which phenomena need to be included in the modelling to reproduce experimental trends and (b) provide new insights to the interpretation of experiments. The simulations highlight the key role of ion/molecule elastic collisions in determining the ion flux reaching the target. Recombination is likely to play a role at high molecular background pressure. However, even with the most advanced atomic and molecular model used in this work, Te at the target is overestimated with respect to the measurements using TS and spectroscopy. Te in the simulations appears to saturate at 0.7 eV for a wide range of parameters, while experimentally values of 0.1–0.3 eV are found. As a consequence, in the simulations the volume recombination is underestimated, which is a strong function of Te when it is below 1 eV. Further analysis of simulation results using a two-point formalism shows that inelastic collisions between electrons and neutral background particles remove most of the energy flux, mainly via dissociation of molecules and molecular ions. However this happens mostly in the upstream region of the beam where Te > 1 eV. For Te < 1 eV, there seems to be no significant energy removal mechanism in the simulated cases. The results also indicate that conclusions on the importance of volume processes, e.g. recombination, cannot be solely based on Te or the dominance of certain reaction rate coefficients over others, but rather the complete transport picture, including macroscopic flow, has to be taken into account. In the cases studied here, the plasma is typically advected to the wall too fast for recombination to remove a significant fraction of the particle flux.

Remote control of the dissociative ionization of H2 based on electron- H2+ entanglement

The single ionization of ${\mathrm{H}}_{2}$ in strong laser fields creates the correlated electron-${{\mathrm{H}}_{2}}^{+}$ pair. Based on such a correlation, we conceive a strategy to control the energy spectra of the freed electron or dissociative fragments by simulating the time-dependent Schr\odinger equation. Two attosecond pulses in a train produce the replica of electron-${{\mathrm{H}}_{2}}^{+}$ pairs, which are to be steered by a time-delayed phase-stabilized (mid)infrared laser pulse. By controlling the behavior of the freed electron, the dissociation of ${{\mathrm{H}}_{2}}^{+}$ can be controlled even though there is no direct laser-${{\mathrm{H}}_{2}}^{+}$ coupling. On the other hand, the photoelectron energy spectra can be manipulated via laser-${{\mathrm{H}}_{2}}^{+}$ coupling. This study demonstrates the entanglement of molecular quantum wave packets, and affords a route to remotely control molecular dissociative ionization.

Toward accurate composition analysis of GaN and AlGaN using atom probe tomography

With scaling of semiconductor devices showing no signs of abating and three-dimensional structures now being developed, new metrologies to meet these demands are being sought. Atom probe tomography offers the potential to meet these challenges, and here, the authors present an in-depth study focused on finding useable conditions for accurate stoichiometric analysis of GaN and AlGaN. By varying the laser energy/power, changes in the average tip field were induced, and the resulting impact on the measured stoichiometry was investigated. A strong variation in the GaN stoichiometry as a function of the average tip field was found, although a range of conditions that enable accurate stoichiometry were determined. Moreover, the stoichiometric variation as a function of tip field was highly reproducible across instruments and laser wavelengths. However, for AlGaN, the N concentration was always underestimated. To try and establish the underlying cause of the N underestimation, potential loss mechanisms which include N2 sublimation, N2 neutral generation from molecular ion dissociation, and differences in the field of evaporation between the matrix elements and multihits were considered and are reported herein.

On the detection of multiple events in atom probe tomography

In atom probe tomography (APT), multiple events can arise as a consequence of e.g. correlated field evaporation and molecular ion dissociation. They represent challenging cases for single-particle detectors and can cause compositional as well as spatial inaccuracies. Here, two state-of-the-art atom probe microscopes (Cameca LEAP 5000 XS and 5000 XR) were used to investigate cemented tungsten carbide, which exhibits high amounts of multiple events. By advanced data analysis methods, the natural character of the multiple events, as well as the performance of the APT detectors, are assessed. Accordingly, possible signal loss mechanisms are discussed.

Estimating electron affinity from the lifetime of negative molecular ions: Cycloheptatriene derivatives

Cycloheptatriene derivatives are studied by means of resonance electron capture negative ion mass spectrometry (REC NIMS). The average lifetimes of molecular negative ions (NIs) are measured with respect to electron autodetachment. Using the Arrhenius approach, electron affinity EAa of the molecules under study is estimated, and the effective temperature of the resulting negative molecular ions is determined as a function of the electron energy. It is assumed that the dissociation of negative molecular ions in the ground electronic state is a process similar to that of the thermal degradation of molecules.

Ab initio potential energy surfaces of HCS+: A study of the ground and the low-lying excited electronic states

Three dimensional ab initio potential energy surfaces (PESs) have been computed for the ground state and low-lying excited states of HCS+ molecular ion using the internally contracted multi-reference (single and double) configuration interaction and augmented correlation consistent polarized valence quadruple zeta (aug-cc-pVQZ) basis sets. Ground state global PES is analyzed as dissociation of molecular ion into H+CS+. The ground state PES (H+CS+) has been fitted by the inverse power series expansion function. The anisotropy of the surface has been analyzed in terms of the multipolar expansion coefficients for the rigid-rotor surface. The surface will be useful for detailed understanding of collision dynamics in terms of ro-vibrational cross sections and rate coefficients.

Molecular cascade Auger decays following Si KL23L23 Auger transitions in SiCl4

Cascade Si LVV Auger electron spectra at the photoexcitation of the Si 1s electron in a SiCl4 molecule have been measured using an electron spectrometer combined with monochromatized undulator radiation. In the instance of the resonant excitation of the Si 1s electron into the vacant molecular orbital a peak with high yield is observed at about 106 eV, an energy considerably higher than the energies of the normal LVV Auger electron. This peak is presumed to originate from the participator decay from the state with two 2p holes and one excited electron into the state with one 2p hole and one valence hole. Following the normal KL23L23 Auger transition, the cascade spectrum shows several peak structures, e.g. 63 eV, 76 eV and 91 eV. The peak at 91 eV is probably assigned to the second step Auger decay into states having a 2p hole together with two valence holes. These findings are similar to experimental results of SiF4. The former two peaks (63 eV and 76 eV) are ascribed to Auger transitions of Si atomic ions produced through molecular ion dissociation after the first step cascade decays, although the peak heights of atomic ions are lower than those of SiF4.

Dissociation of Molecular Ions During the DC Field Evaporation ZnO in Atom Probe Tomography

In atom probe tomography, a tip-shaped sample is evaporated atom by atom in a high DC field. The atoms are field ionized and accelerated to a two-dimensional detector, and their measured impact position and time-of-flight enable the three-dimensional reconstruction of the evaporated volume [I]. In some materials, a large fraction of the material evaporates as molecular ions, which can dissociate in the high DC electric field [2). The study of such dissociations can give valuable information in the field of field induced chemistry [3], to better understand the effect of the electric field on molecules stability. In addition, such dissociations can have an effect on the quality of atom probe data and may induce composition artifacts. The understanding of the way such dissociations affect the data are necessary to predict and maybe correct these artifacts. In this work, we first study experimentally the field evaporation of a ZnO sample. In the figure I .a, a two-dimensional histogram shows the correlation between the time-of-flight difference and the distance between impacts, for pairs of ZnO+ ions detected at the same evaporation event. Different regions can be identified. The horizontal line (region I) mostly corresponds to correlated evaporation [4). The two ions reach the detector at the same time but with spacing on the detector that reflects their original spacing on the tip surface (Figure l .b.I). The diagonal line in the histogram, however, evidences the presence of dissociation of Zn202 2 + ions into ZnO+ ions, with different molecular orientation with respect to the electric field. Depending on the orientation of the parent ion, the Coulomb repulsion between the dissociation products has an effect either on the time-of-flight difference between the two ions (region II) or on the spacing between their impact on the detector (region UI). We show that this correlati.on plot can be used to extract complex information about the dissociation of molecular ions, like the parent ion lifetime, the molecule orientation when dissociation occurs, and the energy transferred by the dissociation (or kinetic energy release). Ab initio calculations are also performed to predict the stability of molecular ions in high field. For simplicity's sake, calculations are performed for the dissociation of Zn02+ ions (Figure 2). It is found that different dissociation paths can exist, with different barriers, depending on the molecule initial orientation. In particular, one orientation leads to the dissociation into neutral oxygen, which cannot be detected by atom probe and can cause important artifacts. This confirms the importance of a future study of dissociation phenomena in high field, both experimental and theoretical [5].

Probing high-lying N2++ and CO++ states via energy-selective fragment spectra

Dissociation of molecular ions from highly excited states is difficult to probe, so our knowledge of their dynamical evolution and the parameters governing the dissociation is limited. The main complication is due to the large density of high-lying states and crossing of states. The latter may change the kinematics of the fragments, but in general, the contributions from different states to the fragment kinetic energy distributions cannot be separated. Consequently, the exact nature of the evolution remains elusive. In the present work, we have performed kinematic analysis of the dissociation dynamics of di-cations of N2 and CO formed by photoionization, aiming to probe their highly excited states. Correlated fragment ion momenta are measured in coincidence with energy-analyzed ejected electrons, allowing us to estimate energy of the transient molecular ions. These measurements bring out the differences in kinematics of the fragmentation of transient molecular ions having different internal energies. Our analysis indicates that highly excited states decay primarily to their own asymptotic limits with only weak coupling to states decaying to lower asymptotes.

Behavior of molecules and molecular ions near a field emitter**This work is a partial contribution of the US Government and therefore is not subject to copyright in the United States.

The cold emission of particles from surfaces under intense electric fields is a process which underpins a variety of applications including atom probe tomography (APT), an analytical microscopy technique with near-atomic spatial resolution. Increasingly relying on fast laser pulsing to trigger the emission, APT experiments often incorporate the detection of molecular ions emitted from the specimen, in particular from covalently or ionically bonded materials. Notably, it has been proposed that neutral molecules can also be emitted during this process. However, this remains a contentious issue. To investigate the validity of this hypothesis, a careful review of the literature is combined with the development of new methods to treat experimental APT data, the modeling of ion trajectories, and the application of density-functional theory simulations to derive molecular ion energetics. It is shown that the direct thermal emission of neutral molecules is extremely unlikely. However, neutrals can still be formed in the course of an APT experiment by dissociation of metastable molecular ions.

Contribution of nitrogen atoms and ions to the luminescence emission during femotosecond filamentation in air

During femtosecond filamentation in air, nitrogen molecules and corresponding molecular ions undergo dissociation due to the high intensity of laser pulses, generating nitrogen atoms and atomic ions. The generated atoms and atomic ions emit luminescence in the UV range, which superposes on those emissions for the neutral and ionic nitrogen molecules. Here we report on a significant difference between the emission behavior of the 391-nm line and the other spectral lines under different pump laser polarizations. We attribute this difference to the contribution of the atomic ions to the luminescence emission around 391 nm. The difference becomes more evident in tightly focusing cases, providing an indirect but effective evidence for the dissociation of nitrogen molecular ions.

Effect of Nuclear Mass in Strong-Field Ionization of Hydrogen Molecules and Dissociation of Hydrogen Molecular Ions

SynopsisWe measure isotope effect in strong-field ionization and dissociation of molecular hydrogen using a mixed-gas target in a Reaction Microscope (REMI). We compare experimental results with theoretical predictions.