Dissociation Of F2 Research Articles

High-level coupled-cluster energetics by merging moment expansions with selected configuration interaction.

Inspired by our earlier semi-stochastic work aimed at converging high-level coupled-cluster (CC) energetics [J. E. Deustua, J. Shen, and P. Piecuch, Phys. Rev. Lett. 119, 223003 (2017) and J. E. Deustua, J. Shen, and P. Piecuch, J. Chem. Phys. 154, 124103 (2021)], we propose a novel form of the CC(P; Q) theory in which the stochastic Quantum Monte Carlo propagations, used to identify dominant higher-than-doubly excited determinants, are replaced by the selected configuration interaction (CI) approach using the perturbative selection made iteratively (CIPSI) algorithm. The advantages of the resulting CIPSI-driven CC(P; Q) methodology are illustrated by a few molecular examples, including the dissociation of F2 and the automerization of cyclobutadiene, where we recover the electronic energies corresponding to the CC calculations with a full treatment of singles, doubles, and triples based on the information extracted from compact CI wave functions originating from relatively inexpensive Hamiltonian diagonalizations.

Development and characterization of high-repetition-rate sources for supersonic beams of fluorine radicals.

We present and compare two high-pressure, high-repetition-rate electric-discharge sources for the generation of supersonic beams of fluorine radicals. The sources are based on dielectric-barrier-discharge (DBD) and plate-discharge units attached to a pulsed solenoid valve. The corrosion-resistant discharge sources were operated with fluorine gas seeded in helium up to backing pressures as high as 30 bars. We employed a (3 + 1) resonance-enhanced multiphoton ionization combined with velocity-map imaging for the optimization, characterization, and comparison of the fluorine beams. Additionally, universal femtosecond-laser-ionization detection was used for the characterization of the discharge sources at experimental repetition rates up to 200Hz. Our results show that the plate discharge is more efficient in F2 dissociation than the DBD by a factor between 8 and 9, whereas the DBD produces internally colder fluorine radicals.

W3X: A Cost-Effective Post-CCSD(T) Composite Procedure.

We have formulated the W3X procedure by incorporating cost-effective post-CCSD(T) components (up to the CCSDT(Q) level) into the W1X-1 protocol, the latter representing a recently reported economical yet accurate approximation to CCSD(T)/CBS. For medium-sized systems, W3X is moderately more computationally demanding than W1X-1, but it is significantly less costly than the W3.2lite and (especially) W3.2 procedures. Because of the use of the cost-effective W1X-1 method as the underlying CCSD(T) component, W3X is also less expensive than the W2.2 protocol, which does not incorporate post-CCSD(T) excitations. We find that, for single-reference systems (the G2/97 set and most of the W4-11 set), W3X is comparable in accuracy to the underlying W1X-1 protocol, as might have been expected. For the more challenging cases of the multireference systems within the W4-11 set, the dissociation of F2 and the automerization of cyclobutadiene, W3X provides improved performance compared with the CCSD(T)-based procedures (W1X-1 and W2.2). Highly multireference chromium oxides CrO, CrO2, and CrO3 are still somewhat challenging for W3X (and even for the higher-level W3.2lite and W3.2 procedures), but the inclusion of the economical post-CCSD(T) terms in W3X already leads to a significant improvement over W1X-1. Thus, W3X provides a cost-effective means for treating systems with significant (but perhaps not excessive) multireference character that are otherwise not well-described by CCSD(T)-based methods.

Numerical study of flow field characteristics for triplet impingement injector and combustor

The chemical reactions in combustor are described by integrating Eddy-Dissipation Model with Arrhenius rate coefficients. The cold and react flow fields of triplet impingement injector and combustor are simulated using the three-dimensional computational fluid dynamics method. Both helicity and mixing length are introduced to study the mixing mechanism and the effect of triplet impingement injector. The key characteristic parameters of combustor, e.g., spatial distributions of total temperature and pressure, residence time of gas flow in combustor, are obtained. The flow field characteristics of triplet impingement injector and combustor for Fuel-Oxidizer-Fuel (F-O-F) and Oxidizer-Fuel-Oxidizer (O-F-O) triplet arrangements are analyzed. The degree of F2 dissociation in combustor exit plane for O-F-O triplet arrangement is 13.5% higher than that for the F-O-F triplet arrangement under conditions of specified composition proportioning and combustor characteristic length. Experimental result demonstrates that the output power of laser is increased by 17%.

Some transport properties in plasmas containing argon and fluorine

In this paper some results of numerical evaluation of transport coefficients in plasmas in the mixtures of argon and fluorine are presented. These transport characteristics are given in the function of the temperature for low pressures ranging from 0,1 kPa to 1,0 kPa and for low temperatures between 500 K and 5 000 K in argon plasmas with 20% and 30% of the fluorine added. It is assumed that the system is kept under constant pressure and that a corresponding state of local thermodynamical equilibrium (LTE) is attained in it. The equilibrium plasma composition, necessary for the evaluations, was determined on the ground of the Saha equations for ionization processes and the law of mass action for the thermal dissociation of F2, combined with the charge conservation relation and the assumption that the pressure remained constant in the course of temperature variations. The ionization energy lowering, required in conjunction with the Saha equations, was obtained with the aid of a modified expression for the plasma Debye radius proposed previously. A previously derived expression for the modified Debye radius, offering the possibility to treat the plasmas considered as weakly non-ideal in the whole temperature range, is used. The cut-off at the Landau length rather than of the smallest of ionic radii is introduced. This alteration in the evaluation procedure brings different considerable changes in the final numerical results for the all relevant quantities.

Electron energy deposition in an electron-beam pumped KrF amplifier: Impact of the gas composition

Calculations for electron deposition in electron beam generated KrF laser at atmospheric pressure have been performed. The impact of the Ar/Kr/F2 gas mixture on the electron energy distribution function, electron density, and mean energy, energy per electron–ion pair, attachment, dissociation, excitation, and ionization rates have been investigated. The F2 abundance controls the low energy (≲9 eV) component of the distribution function, while both the fluorine and krypton mole fraction affect the distribution in the midenergy domain (9 to ∼25 eV). Consequently, the F2 attachment rate coefficient varies with the F2 mole fraction (xF2) such that the electron density scales as 1/xF20.7. The rate coefficient for direct dissociation of F2 is smaller than for attachment but the former contributes more to the total power dissipation (∼8% at xF2=0.01). The excitation-to-ionization ratio for Kr is not constant, as generally assumed, but increases by a factor of two with a decrease in either the Kr or F2 abundance. Combining the former and present investigations leads to a set of fitting formulas to be used in beam kinetics codes for various collision rates as a function of both the electron beam power density and the composition.

A Review of Recent Experiments and Calculations Relevant to the Kinetics of the HF Laser

A review of rate coefficients relevant to HF laser kinetics modeling is presented. The literature has been surveyed from the last published review in 1983 to the present. Updated HF Einstein emission coefficients are tabulated. Rate coefficients are categorized according to their role in the HF laser model: HF generation, reactive quenching, self-relaxation, V–V energy transfer, vibrational relaxation by atoms and molecules, F2 dissociation, and F atom recombination. In addition, a review of recent experiments and theoretical calculations relevant to the role of rotational nonequilibrium in HF lasers is presented. A list of recommended temperature dependent expressions for critical reaction rate coefficients is given.

Penetration depth of energetic F atoms from F2 dissociation in layered rare gas samples

Penetration depths of atoms with kinetic energy provided by photodissociation of parent molecules in the top layer of a multilayer sample are determined from the probability to cross a spacer layer of thickness d and to arrive at the interface to a substrate. Top layer growth up to a final thickness s corresponds to a continuous increase of the effective spacer layer thickness. Modeling of growth and comparison with sample-to-sample variation of d allows us to determine separately and in a consistent way the precursors’ dissociation cross section q⋅σ and the mean penetration depth d0 of the fragments together with elimination of contaminated samples. For F atoms with 4.3 eV kinetic energy from F2 dissociation values of q⋅σ=3×10−17 cm2 and d0=2.1 nm (8 to 9 monolayers) are derived for Ar spacers. A strong increase of d0 in the case of unintentional multistep excitation of F fragments is demonstrated.

A classical-path surface-hopping study of Mu, H and T hot-atom collisions with F2

The collision dynamics of processes pertinent to the hot-atom chemistry of the hydrogen isotopes Mu, H and T with F2 are explored using mixed quantum–classical techniques. We use classical-path surface-hopping and pure classical-path multi-surface trajectory methods with a set of 14 coupled potential energy surfaces (PESs) from a diatomics-in-molecules (DIM) model. We compare with results from the standard quasiclassical trajectory (QCT) method applied to the electronic ground state surface of the same DIM model. We compute reaction- and dissociation cross sections for centre-of-mass collision energies ranging from threshold to 20 eV. In order to quantify the notion of electronic non-adiabaticity we introduce various cross sections measuring electronically non-adiabatic processes. We find that non-adiabatic effects give rise to pronounced changes in the dynamics of the Mu+F2 system. In particular, collision-induced dissociation of F2 by Mu is an essentially electronically non-adiabatic process, proceeding efficiently by a mechanism involving transitions to excited repulsive PESs. In contrast, QCT calculations on the electronic ground state surface predict vanishingly small Mu+F2 dissociation cross sections. We also discuss some consequences of this behaviour for simulations of Mu hot-atom chemistry.

Time resolved study of a reactive low pressure plasma: Determination of basic data on electron-fluorine collisions

Time resolved spectroscopy and electron concentration measurements were performed on an Ar–He–F2 mixture excited by an external rf antenna creating a plasma with features of a plasma source generated by a Helicon reactor commonly used in materials processing. The low pressure range and the simplified chemistry allow an appropriate modelling of some processes and lead to the determination of some fundamental data concerning atomic and molecular fluorine. We give a F2 direct dissociation coefficient by electron impact of about 2.0×10−9 cm3 s−1 for an electron temperature of 5 eV. Excitation rate coefficients of some fluorine levels (3p 2P3/2, 3p 4D7/2, and 3p′ 2F7/2) are given as well as rate coefficients of line emissions following F2 dissociation by electron impact.

Compact valence bond functions with breathing orbitals: Application to the bond dissociation energies of F2 and FH

An original computational method of ab initio valence bond type is proposed, aiming at yielding accurate dissociation energy curves, while dealing with wave functions being very compact and clearly interpretable in terms of Lewis structures. The basic principle is that the wave function is allowed to have different orbitals for different valence bond structures. Thus, throughout the dissociation process, the so-called ‘‘breathing orbitals’’ follow the instantaneous charge fluctuations of the bond being broken by undergoing changes in size, hybridization, and polarization. The method is applied to the dissociation of F2 and FH. For each molecule, a wave function involving only three valence bond configurations yields equilibrium bond lengths within 0.01 Å, and dissociation energies within about 2 kcal/mol of the results of estimated or true full configuration interaction in the same basis sets. The effect of dynamical electron correlation on calculated dissociation energies is analyzed. It is shown that restricting the correlation to its nondynamical part results in an improper treatment of ionic terms due to a mean-field compromise in the optimization of the orbitals.

Electron-molecule close coupling with correlated target wave functions: Application to impact dissociation of F2.

An analysis of the close-coupling method as it is traditionally formulated for electron-scattering problems shows that it can give rise to spurious resonances at intermediate energies when multiconfiguration target states are used. These resonances are associated with excited states of the target that are not explicitly included in the close-coupling expansion. We develop a modification of the close-coupling technique that allows one to isolate and systematically eliminate terms from the total wave function which are responsible for the spurious behavior. We have applied this technique to the study of electron-impact dissociation of ${\mathrm{F}}_{2}$ through its two lowest excited states. The complex Kohn variational calculations we have performed on this system incorporate accurate, multiconfiguration descriptions of the target wave functions.

Direct dissociation of F2 in electron beam pumped excimer lasers: The effect on electron density

The electron impact dissociation of molecular fluorine in XeF and KrF excimer lasers is examined. Two methods of dissociation are discussed: dissociative attachment and direct neutral dissociation by excitation to the dissociative electronic states a3Πu and A1Πu. Computer models for the kinetics of the lasers are parameterized, and predictions of electron density are compared to experimental results for electron beam pumped Ne/Xe/F2 gas mixtures [W. D. Kimura, D. R. Guyer, S. E. Moody, J. F. Seamans, and D. H. Ford, Appl. Phys. Lett. 50, 60 (1987)]. To obtain agreement with experiment, the ratio of direct dissociation to dissociative attachment must be ≥2. The implications of these results with respect to electron quenching and the validation of computer models are discussed.

The Hartree-Fock dissociation of F2

We examine the stability of the restricted Hartree-Fock (RHF) wave function for F2 in the vicinity of the equilibrium internuclear distance (R=R e ) and the shape of the unrestricted Hartree-Fock (UHF) potential energy curve for the same system. The results depend on the basis set: With a split valence plus polarization basis, 6-31G(d), the RHF wave function is unstable at R e , and the UHF potential curve is purely dissociative. When the basis is extended to 6-311+G(3d) or 6-311+G(3df), the RHF wave function becomes stable, and the UHF potential curve acquires a local maximum for R slightly (∼0.02 a) greater than R e . The local maximum, however, is only 0.1 kcal/mol higher than the local minimum at R=R e .

Kinetics of the F,F2/Gd(s,v) reactions

The reaction of gadolinium with fluorine was studied in a low pressure, transonic flow reactor. Atomic fluorine reactant was generated by microwave discharge dissociation of F2 in argon. Kinetic measurements included the rate of change of the electrical resistance of isothermal gadolinium filaments due to reaction with atomic and molecular fluorine and the intensity of chemiluminescence from the vapor phase Gd/F2 reactionvs time, temperature, and F2 partial pressure. The surfaces of reacted Gd specimens were examined by scanning electron microscopy, polished specimen cross sections were viewed under the optical microscope, and the Gd/F2 chemiluminescent emission spectrum was obtained. A nonpassivating, metal rich, high melting and involatile fluoride, GdFx, forms on the metal at temperatures above 1180 K. An orthorhombic, passivating GdF3 forms at lower temperatures. The surface Gd activity remains high at the GdFx/fluorine interface, and F-atom reaction is first order with a reaction probability approximately 0.28. Rapid reaction atT < 1180 K is followed by an abrupt transition to passivated behavior. This change in reactivity is attributed to a phase transition from GdFx to orthorhombic GdF3 that occurs as the oxygen impurity concentration of the reacting metal decreases from its value near the original specimen surface to that of the bulk material.

Crossed molecular beam studies on the interaction potential for F(2P)+Xe(1S)

For the evaluation of the ground state XeF interaction potential, angular distributions of F atoms scattered off Xe were measured in crossed molecular beam experiments at collision energies of 2.11, 10.5, and 13.9 kcal/mole. F atoms, produced by thermal dissociation of F2 at 700 °C in a supersonic expansion using rare gas carriers, contain ∼78% F (2P3/2) ground state and ∼22% F (2P1/2) spin–orbit excited state. Consequently, three electronic states X1/2, I3/2 emerging from the 2P3/2 +1S0 asymptote, and II 1/2 from the 2P1/2+1S0 asymptote are involved in the scattering. A simple elastic approximation, neglecting interstate coupling, is used for the calculation of differential scattering cross sections in the evaluation of interaction potentials. Experimental results are found corroborating the spectroscopically derived potential of Tellinghuisen et al. (ε=3.359 kcal/mole, rm=2.293Å) for VX1/2(r), and the conclusion that VI3/2(r) and VII1/2(r) are in close resemblance to the ground state Ne+Xe interaction potential.

F2 absorption measurement in a combustion-driven chemical-laser nozzle

A sensitive F2 absorption diagnostic technique was used to determine F2 densities in the flow of a 10.2-cm-long combustion-driven chemical-laser nozzle. The diagnostic device was demonstrated to be readily transportable and operable in a combustion-driven chemical-laser field environment. Cold- and hot-flow measurements up to 970 K plenum temperature (i.e., no F2 dissociation cases) were in agreement; hence, the measurement technique appears to be reliable. The use of C2H4 as a combustor fuel did not result in particulate absorption or scattering of sufficient magnitude to affect the measurement. The flow dissociation level (α) was ≳0.8 for this nozzle. The measurement sensitivity achieved in these tests was ΔI/I0=6×10−4, which corresponds to an F2 density of 6.76×10−6 mole/l (i.e., 0.127 Torr F2 at 300 K) for a 10.2-cm path length.

Influence of molecular dissociation and degree of ionization on rare gas–halide laser properties

The influence of F2 dissociation and the accompanying increase in fractional ionization is examined for conditions typical of electron-beam-sustained KrF* lasers. It is found that for electron-density–F2-density ratios greater than about 10−3, rare-gas metastable loss due to electron excitation of higher levels begins to compete significantly with metastable-F2 reactions, thereby leading to a substantial reduction in rare gas–halide production efficiency.

Dissociation of fluorine by electron impact

Using a previously developed F-atom diagnostic technique, the efficiency of F2 dissociation by an electron beam or an electron-beam-sustained discharge was systematically studied. By monitoring the transient concentration of ions generated during the electron-beam-sustained discharge, we have determined the F−+F+2 ion-ion recombination and F2 dissociative attachment rate constants at various E/P conditions. The dissociative attachment rate constant of F2 at an average electron energy of 1 eV was found to be about (2.3±0.3) ×10−9 cm3/molecule sec−1 and increased with decreasing electron energy. The F−+F+2→3F recombination rate constant at an E/P of 18 kV/cm/atm was found to be about (1.5±0.2) ×10−8 cm3/molecule sec−1 and also increases with decreasing E/P values. The sustained-discharge enhancement of F-atom production from these experiments is F2 is small, ∼1.7.

Chemiluminescent titration of F(g) with Cl2(g) and microwave production of atomic fluorine

Chemiluminescent titration of atomic fluorine with Cl2 in a low pressure (133 Pa) flow apparatus has been investigated to establish the accuracy of this titration method and the efficiency of F atom production in a microwave discharge. Up to 90 % dissociation of F2(0.5–10 % in Ar) is achieved by a 100 W Evenson-type cavity 2450 MHz discharge when the microwave power exceeds 2500 kJ mol–1 F and discharge residence time exceeds 0.7 ms. Heterogeneous F atom loss on the fluorine-passivated Al2O3 discharge tube walls occurs with a recombination probability, γF≈ 2 × 10–4.