Dissociation Cross Sections Research Articles

Direct And Dissociative Ionization Cross Section Of Oxygen Molecule From Threshold To 10 Kev

Due to abundant applications of ionization in various fields of applied sciences, it’s desirable to calculate absolute ionization cross sections of various atoms or molecules. In this literature, we have calculated the absolute direct and dissociative ionization cross sections of the oxygen molecule from threshold energy to 10,000 eV by using revisited Jain-Khare semi-empirical approach. In this literature, the total direct and absolute ionization cross section data have compared with easily available experimental and/or theoretical data. The present results give a better account for the ionization cross sections up to higher incident electron energies.

Growth of cobalt films at room temperature using sequential exposures of cobalt tricarbonyl nitrosyl and low energy electrons

Cobalt thin films were grown at room temperature using sequential exposures of cobalt tricarbonyl nitrosyl (CTN, Co(CO)3NO) and low energy (75–175 eV) electrons. During this cyclic growth process, the CTN molecules were first adsorbed on the substrate. The electrons then induced the desorption of the carbonyl and nitrosyl ligands from the adsorbed CTN. The removal of CO and NO ligands produced new adsorption sites. Subsequent CTN exposures allowed CTN to react with these new adsorption sites on the substrate. In situ ellipsometry was utilized to monitor the film thickness during the electron enhanced growth. Co growth rates as high as 1.3 Å/cycle were observed by in situ ellipsometry depending on the reaction conditions. The in situ ellipsometry also observed the CTN adsorption and the removal of the carbonyl and nitrosyl ligands. Quadrupole mass spectrometer measurements confirmed the desorption of CO and NO during electron exposures. X-ray photoelectron spectroscopy (XPS) measured N XPS signals from the Co films deposited using electron exposures at 200 eV. The N/Co XPS signal ratio was consistent with the dissociation of 13% of the nitrosyl ligands on the CTN precursors that lead to Co deposition. In contrast, the negligible C XPS signals from the Co films indicated that the CO ligands were desorbed completely from CTN by the electron exposures at 200 eV. Under identical reaction conditions at lower incident electron currents, the maximum growth rate was obtained at an electron energy of 125 eV. Because the Co growth depends on the electron flux, the Co films were deposited only on the surface area irradiated by the electron beam. The spatial profile of the Co film deposited using long electron exposure times was mapped by ex situ spectroscopic ellipsometry. This spatial profile displayed a pronounced flat top that was consistent with the electron flux desorbing nearly all the CO and NO surface coverage in the central area of the electron beam during each reaction cycle. The spatial profile was used to calculate an electron induced desorption cross section of σ = 2 × 10−17 cm2 at 200 eV. This cross section was in approximate agreement with the cross sections for the electron impact dissociation of CTN in the gas phase.

Quasi-classical trajectory-based non-equilibrium chemical reaction models for hypersonic air flows

Phenomenological models, such as Park’s widely used two temperature model, overpredict the reaction rate coefficients at vibrationally cold conditions and underpredict it at vibrationally hot conditions. To this end, two new chemical reaction models, the nonequilibrium total temperature (NETT) and nonequilibrium piecewise interpolation models for the continuum framework are presented. The focus is on matching the reaction rate coefficients calculated using a quasiclassical trajectory based dissociation cross section database. The NETT model is an intuitive model based on physical understanding of the reaction at a molecular level. A new nonequilibrium parameter and the use of total temperature in the exponential term of the Arrhenius fit ensure the NETT model has a simple and straightforward implementation. The efficacy of the new model was investigated for several equilibrium and nonequilibrium conditions in the form of heat bath simulations. Additionally, two-dimensional hypersonic flows around a flat blunt-body were simulated by employing various chemical reaction models to validate the new models using experimental shock tube data. Park’s two temperature model predicted higher dissociation rates and a higher degree of dissociation leading to lower peak vibrational temperatures compared to those predicted by the new nonequilibrium models. Overall, the present work demonstrates that the new nonequilibrium models perform better than Park’s two temperature model, especially in simulations with a high degree of nonequilibrium, particularly as observed in re-entry flows.

Ionization and electron capture for H+ collisions at low keV energy with a water-vapor target

We have investigated the total relative ionization and dissociative electron capture cross section for proton collisions with water-vapor molecules within the energy range of 2.5–10 keV. Time of flight mass spectroscopy was employed in the measurements of the atomic and molecular singly charged particles H+, H2+, O+, OH+ and H2O+ as well as the doubly charged particles O++, H2O++, OH++, and the contribution of the different channels was evaluated. Ionization was found to be the dominant reaction channel as expected. The contribution of the singly charged particles was found to be one order of magnitude higher than that of the doubly charged particles.

Absolute ionization and dissociation cross sections of tetrahydrofuran: Fragmentation-ion production mechanisms

We report an experimental and theoretical investigation on the absolute ionization and fragmentation cross sections of tetrahydrofuran (THF, C4H8O), a base molecule of the DNA-backbone. The measurements enabled the identification of 50 ionic species produced by 12 eV up to 2000 eV electron impact which allowed us to make a close inspection of the mechanisms of both the primary vacancy production and the postcollisional fragmentation pathways which lead to the fragment ions. The experimental cross sections of the ionic species were examined in the framework of the fragmentation matrix model to attribute from which molecular orbitals (MOs) the electron is removed and find out the relative contribution of up to 15 outer and inner valence MOs in the fragment-ion production. A comparison between measured and calculated dissociative cross sections relative to the molecular parent ion cross section allowed us to clearly identify the prevalence of the single or double ionization in the fragment-ion production. Due to these different production mechanisms, the relative proportion of some ejected fragment-ions strongly depends on the impact energy. The single ionization of the MOs leading to selected fragment ions was compared to those measured at a low electron-impact energy using a reaction microscope with good agreement between the two techniques. The formation of the dications C4HnO2+, excluding the molecular parent dication, is also presented.

Electron-Impact Dissociation of Vibrationally-Excited Molecular Hydrogen into Neutral Fragments

We present convergent close-coupling (CCC) calculations of electron-impact dissociation of vibrationally-excited molecular hydrogen into neutral fragments. This work follows from our previous results for dissociation of molecular hydrogen in the ground vibrational level [Scarlett et al., Eur. Phys. J. D 72, 34 (2018)], which were obtained from calculations performed in a spherical coordinate system. The present calculations, performed utilizing a spheroidal formulation of the molecular CCC method, reproduce the previous dissociation cross sections for the ground vibrational level, while allowing the extension to scattering on excited levels.

Assessment of the self-consistency of electron-THF cross sections using electron swarm techniques: Mixtures of THF–Ar and THF–N2

The pulsed Townsend technique has been used to measure transport coefficients in mixtures of tetrahydrofuran (THF) with N2 and argon. These measurements are the first investigations of swarm transport in gas mixtures with THF, with the drift velocity and effective Townsend ionization coefficient reported for a range of reduced electric fields between 0.23 and 800 Td (1 Td = 10−21 V m2). These transport coefficients are compared with those calculated using a multiterm kinetic theory, using the cross section set developed in our previous studies [N. A. Garland et al., Phys. Rev. A 88, 062712 (2013) and M. J. E. Casey et al., J. Chem. Phys. 147, 195103 (2017)]. The swarm technique of iteratively adjusting cross sections to reproduce experimental transport measurements is subsequently utilized in this study to address the deficiencies in the earlier cross section sets, exposed by the gas mixture measurements. Refinement of the low-energy extrapolation of the quasielastic database cross section and the low-energy extrapolation and magnitude of the dissociative electron attachment cross section are detailed, as well as the adjustments to the two previously proposed neutral dissociation cross sections. These refinements were necessary in order to minimize differences between our measured and calculated transport coefficients.

Vibrationally resolved excitation, dissociation, and rotational cross sections of NH radical by electron-impact using the R-matrix method

The ab initio R-matrix method is employed to study the electronic excitation of low-lying target states, the dissociation cross sections, and the rotational cross sections of the NH radical by electron impact for energies up to 15 eV. We have included dynamic interaction in our scattering model that uses close coupling formalism. There are 24 target states whose vertical excitation energies are below 15 eV, which are included in the trial wavefunction of the entire scattering system. In our scattering model, two electrons are frozen in the 1σ2 configuration and the remaining six electrons are free to move in the molecular orbitals 2σ 3σ 4σ 5σ 6σ 1π and 2π. The cc-pVTZ Dunning basis sets are used to optimize the target structure and electron scattering. The vibrationally resolved cross sections for transition from the ground state X 3Σ− to excited state c 1Π are computed by scaling the corresponding electronic excitation cross section by Franck-Condon factors. These factors are evaluated by using accurate potential energy curves of the ground state and the c excited state. We have compared the excitation cross section for the optically allowed transition X 3Σ− to A 3Π computed using our scattering model with the corresponding cross section evaluated in Born approximation, and the agreement is quite good indicating that for a reasonably high transition moment and a low threshold of the excited state, the effects of correlation and polarization are not very significant. The dissociative excitation cross section by electron impact into atomic fragments N and H is also shown which are caused mainly by c 1Π and a purely repulsive state 1 5Σ−. We have also shown that the vibrational cross section evaluated in Born approximation for the fundamental transition 0–1 of the ground state is quite small as compared to the rotational cross sections. The rotational cross sections are computed for j = 0 to j′ = 0–5, and it is shown that the dipole induced transition is predominant with an order of magnitude higher than the pure rotational 0–0 cross section. We have detected many low-lying core-excited shape resonances and provided their resonance parameters which form an important input to estimate dissociative electron attachment cross sections.

Dissociative reactions induced by electron impact and electron transport in TEOS vapor

Reactions for dissociative ionization and neutral dissociation collisions between an electron and a tetraethoxysilane [Si(OC2H5)4, TEOS] molecule are reported, and neutral dissociation cross sections are calculated by applying the classical Rice–Ramsperger–Kassel theory. Detailed electron collision cross section set of TEOS vapor, including 18 neutral dissociation cross sections and 20 ionization cross sections, is constructed. Electron transport coefficients, such as the mean-arrival-time drift velocity, bulk drift velocity, longitudinal bulk diffusion coefficient, and ionization coefficient, and rate coefficients for both elastic and inelastic collisions in TEOS vapor are calculated from the present cross section set using a Monte Carlo method. The validity of the present cross section set is demonstrated by comparing the calculated electron transport coefficients with measured data. Furthermore, fragmentation of TEOS molecules by electron impact is simulated, and then the number of fragments produced in the simulation is compared with the measured mass spectra.

Dissociative Excitation of Odd Triplet Levels of a Nickel Atom by Collisions of Electrons with Nickel Dibromide Molecules

The dissociative excitation of the odd triplet levels of a nickel atom by the collisions of electrons with nickel dibromide molecules is investigated using extended crossing beams and recording the optical signal of the excited atoms. In all, 50 excitation cross-sections of Ni I spectral lines are measured at an electron energy of 100 eV. Nine optical excitation functions (OEFs) are recorded in a range of electron energies of 0–100 eV. The obtained data on the dissociative excitation cross-sections of the spectral lines are used for the calculation of the excitation cross-sections of the energy levels as well as the contribution from the cascade transitions. The possible channels of dissociative excitation which manifest themselves in the region of low electron energies (E < 22 eV) are discussed.

Photoprocessing of large PAH cations.

In cosmic environments, polycyclic aromatic hydrocarbons (PAHs) strongly interact with vacuum ultraviolet (VUV) photons emitted by young stars. Trapped PAH cations ranging in size from 30 to 48 carbon atoms were irradiated by tunable synchrotron light (DESIRS beamline at SOLEIL). Their ionization and dissociation cross sections were determined and compared with TD-DFT computed photoabsorption cross sections. Evidence for radiative cooling is reported.

N–O bearing molecules produced by radiolysis of N2O and N2O:CO2 ices

N2O and CO2 can be formed in the interstellar space and in ices on the surface of outer solar system bodies, such as Pluto and Triton. Energetic ions can simulate the energy transfer processes that occur by cosmic ray irradiation of interstellar ices, comets, and icy solar system bodies. Proceeding systematic research, pure nitrous oxide (N2O) ice, and nitrous oxide and carbon dioxide (CO2) ice mixture were irradiated at 11 K with MeV-nitrogen (N+) and -xenon (Xe23+) ion beams. The chemical and physical effects induced by ion irradiation on the N2O ice and the N2O:CO2 ice mixture are compared. The formation and dissociation cross sections scale with the electronic stopping power (Se) roughly as σ ∼ a Sen, where n ∼ 3/2. The n power law is helpful for predicting the N2O formation and dissociation cross-sections for other ion beam projectiles and energies; these predictions will allow estimating the effects of the entire cosmic ray radiation field.

Mass spectrometry of d-ribose molecules

The results of mass spectra of d-ribose molecular beam study by electron impact for the 20–70 eV energy range at the evaporation temperature 310–430 K are presented. The experiment was carried out in two stages: first the mass spectra were measured in the range of 0–200 Da at different ionization energies, then relative cross-sections of single ionization of fragment ions were investigated in the electron energy range of 5–60 eV. The measured temperature dependences show the competing character of water and oxonium cation formation processes. It is the first determination of the appearance energy (AE) for the d-ribose molecule from the threshold region of the energy dependence of the effective ionization cross section. For the most intense fragment ions in the mass spectrum, the potentials of appearance are determined at the threshold of the dissociative ionization cross sections. Schemes for the fragmentation of positive d-ribose ions are proposed.

Quarkonium dissociation in perturbative QCD

For weakly bound quarkonia, we rederive the next-to-leading order cross sections of quarkonium dissociation by partons that include the hard thermal loop (HTL) resummation. Our results calculated with an effective vertex from the Bethe-Salpeter amplitude reduce to those obtained by potential nonrelativistic QCD (pNRQCD) in the relevant kinematical limit, and they can be used in a wide temperature range applicable to heavy quark systems in heavy ion collisions. Based on the lattice computation of the temperature-dependent binding energy, our numerical analysis on $\Upsilon(1S)$ indicates that at high temperature the dominant mechanism for quarkonium dissociation is inelastic parton scattering as expected in the quasifree approximation, while it is the gluo-dissociation at low temperature. By comparing with the momentum diffusion coefficient of a heavy quark, we discuss possible O(g) corrections to the next-to-leading order thermal width.

Vibrationally resolved capture cross section in p + H2 collision

The electron-capture process is described using distorted-wave-Born models by considering the influence of the vibrational motions of H2 and within the framework of the Born–Oppenheimer approximation. Non-dissociative and dissociative capture cross sections are calculated for incident proton energies EC = 20–300 keV. The vibrational distribution of is compared with that of the Franck–Condon factor. They are in fairly good agreement for EC < 100 keV, and discrepancies arise for higher EC.

Absolute total ionization and dissociative ionization cross sections for H+ impact on N2O within the energy range of 2–10 keV

Measurements were made for the ionization and dissociative ionization of N2O in collisions with 2–10 keV protons. Absolute cross sections were obtained employing the time of flight mass spectroscopy for the slow target molecular ions and ionic fragments. Cross sections for the formation of single and doubly charged fragments are reported.

Vibrational state-specific model for dissociation and recombination of the O2( 3Σg -)+O( 3P) system in DSMC.

A vibrational state-specific model for dissociation and recombination reactions within the direct simulation Monte Carlo method is introduced to study the energy level dynamics of the O2 + O system. The state-resolved cross sections for vibrational relaxation and dissociation reactions are obtained from a rotationally averaged quasi-classical trajectory database based on the Varandas and Pais O2( 3Σg -)+O( 3P) potential energy surface. A two-step binary collision framework is outlined to characterize the vibrational state-resolved recombination probabilities, which are constrained by detailed balance for orbiting pair formation, and microscopic reversibility applied to the dissociation cross sections for orbiting pair stabilization. The vibrational state-to-state (STS) model is compared to the phenomenological total collision energy (TCE) and quantum kinetic (QK) models through a series of 0-d non-equilibrium relaxation calculations. A quasi-steady state (QSS) region is established in the vibrational temperature profiles of the TCE, QK, and STS models under non-equilibrium heating. This QSS region is a result of the competition between vibrational relaxation by vibrational-translational (VT) transitions and O2 dissociation. The duration of QSS predicted by the STS model is approximately ten and four times that of the TCE and QK model predictions, respectively, and the total time to reach equilibrium is approximately 3.5 times that of the TCE model and 1.5 times that of the QK model. A distinct QSS region is not observed in the non-equilibrium cooling case. This is attributed to the relatively rapid VT transitions that work to equilibrate the vibrational energy distribution upon recombination, which is comparatively slow. The total time to reach equilibrium by the STS model in the non-equilibrium cooling case is five times and three times greater than those of the QK and TCE models, respectively.

Hydration Energies of Iron Hydroxide Cation: A Guided Ion Beam and Theoretical Investigation.

We present experimental collision-induced dissociation (CID) cross sections as a function of kinetic energy for FeOH+(H2O) n, where n = 1-4, with xenon (Xe) obtained using a guided ion beam tandem mass spectrometer. Complexes with n = 2-4 are observed to undergo water loss, followed by sequential water loss at higher collision energies. In addition, we find that loss of the neutral hydroxide group is competitive with the primary water loss for n = 1. Bond dissociation energies (BDEs) at 0 K are derived through modeling of the experimental cross sections after accounting for multiple collisions, kinetic shifts, and reactant internal and kinetic energy distributions. Quantum chemical calculations include geometry optimizations performed at the B3LYP/6-311+G(d,p) level of theory and then used for single point calculations at B3LYP, B3P86, MP2, and CCSD(T) levels with a 6-311+G(2d,2p) basis set. Additional geometry optimizations at the cam-B3LYP/def2-TZVP were also performed as well as empirical dispersion corrections at all levels. The various structures for the FeOH+(H2O) n complexes and their relative energies are discussed in detail. We also derive experimental BDEs for the OH loss from FeOH+(H2O) n, with n = 2-4, using the experimental BDE of n = 1 in combination with literature data for water loss from Fe+(H2O) n species. Measurements of BDEs for hydroxide and water loss from FeOH+(H2O) n ( n = 1-4) are the first such experimental measurements. Theoretically calculated BDEs are in reasonable agreement for water loss from both FeOH+(H2O) n and Fe+(H2O) n complexes and for D0(Fe+-OH) but are too low for the loss of OH from the larger hydrated complexes.

Соударения медленных электронов с молекулами тимина

AbstractUsing a hypocycloidal electron spectrometer, the total scattering cross section of slow (0–9 eV) electrons and the dissociative electron attachment cross section for thymine molecules in the gas phase were measured. The ionization cross section for a thymine molecule was studied in the energy range of 9–32 eV. Some features were found in the scattering cross section, caused by the formation and decay of short-lived states of the molecular negative ion. Three of them ( E = 0.32, 1.71, and 4.03 eV) relate to shape resonances; the others, which are observed for the first time, refer to the Feshbach resonances (or core-excited resonances). In the total dissociative attachment cross section in the energy range of E &lt; 4 eV, a clear structure is observed due to the formation of a negative ion (T–H)^–, and a less intense structure associated with the total contribution of fragment thymine ions is found above 4 eV. The correlation of the features found in the total scattering cross section and in the dissociative attachment cross section is assessed. The absolute total scattering cross section was obtained by normalizing the measured curve to the theoretical calculation. In the total ionization cross section, features are observed that are associated both with the effect of the formation of fr-agment ions and with ionization due to the ejection of electrons from the orbitals of the outer shell of the molecule.

Calculations of cross-sections, dissociation rate constants and transport coefficients of Xe2+ colliding with Xe.

A quantum formalism and classical treatment have been used for electrons and nuclei, respectively, in a hybrid method in order to study the dynamics of electronically ground-state ionic xenon dimer, Xe2+, in its parent gas. A semiempirical Diatomics In Molecules approach has been used to model the effective electronic Hamiltonian with different sets of input diatomic potentials (ionic and neutral). Non-reactive scattering and collision induced dissociation cross-sections have first been calculated and then injected in a Monte Carlo code for the simulations of the transport coefficients and dissociation rate constant calculated at ambient temperature and atmospheric pressure. Selected transport coefficients, such as Xe2+ mobility for which experimental measurements are available, have been compared to experimental results while transversal and longitudinal diffusion coefficients are compared to pseudo-experimental data obtained from inverse method calculations. Investigation of rotational-vibrational effects and the influence of different sets of ionic and neutral diatomic potentials have been studied.