Product Branching Fractions Research Articles

Negative ion chemistry of SF4

A selected ion flow tube was used to conduct an extensive study of negative ion–molecule reactions of SF4 and SF−4. Rate constants and product ion branching fractions were measured for 56 reactions. The reactions bracket both the electron affinity of SF4 (1.5±0.2 eV or 34.6±4.6 kcal mol−1) and the fluoride affinity of SF3 (1.84±0.16 eV or 42.4±3.2 kcal mol−1). These results may be combined to give the neutral bond energy D(SF3–F)=3.74±0.34 eV or 86.2±7.8 kcal mol−1, independent of other thermochemical data except for the accurately known electron affinity of F. The heat of formation of SF−4 is derived from the electron affinity of SF4: ΔfH(SF−4)=−9.2±0.3 eV or −212.9±7.5 kcal mol−1. Lower limits to EA(SF2) and EA(SF3) are deduced from observation of SF−2(35%) and SF−3(65%) ion products of the reaction S−+SF4. Rapid fluoride transfer from both SF−2 and SF−3 to SF4 places upper limits on the electron affinities of SF2 and SF3. The combined results are 0.2 eV≤EA(SF2)≤1.6 eV and 2.0 eV≤EA(SF3)≤3.0 eV. We review the status of measurements of EA(SFn), n=1–7.

Product branching fractions in the reactions of NH(a 1Δ) and NH(X 3Σ−) with NO

Reactions of the imidogen radical NH with NO have been investigated in its electronically excited a 1Δ and ground X 3Σ− states, where UV- and VUV-LIF were used to monitor radical and atomic species in the 193 nm photolysis of HNCO/NO mixtures at 295±3 K in 20 Torr He. The rate constants for the reactions of NH(a 1Δ; v=0,1)+NO→products (1) and NH(X 3Σ−; v=0,1)+NO→products (2) were determined. No apparent effect on the rate constants of the vibrational excitation of NH was observed in either reaction. H and OH were observed as products in both reactions, whereas physical quenching of NH(a 1Δ) to NH(X 3Σ−) is dominant in Reaction (1). In addition, O(3P) was found to be produced in Reaction (1) for the first time. Product branching fractions in these reactions were determined using proper reference reactions, along with addition of Xe to control the relative contributions of Reactions (1) and (2). The branching fractions in Reaction (1) were: 0.57 for quenching (1a), 0.20 for H+N2O (1b), 0.14 for OH+N2 (1c), 0.08 for the sum of O+N2H (1d), and O+N2+H (1e), and in Reaction (2): 0.65 for H+N2O (2a), 0.30 for OH+N2 (2b). These results were found to be consistent with former theoretical predictions that Reactions (1) and (2) produced on the same doublet surfaces. It was elucidated that the observed delay of OH(v=0) production was due to the vibrational relaxation of OH(v≥1) produced in the reactions.

Negative ion reactions with PF5 and the electron affinity of PF5

Rate coefficients and product branching fractions have been determined for 31 ion–molecule reactions involving PF5 or PF−5. About half of the reactions studied show an ion–molecule association channel. NH−2 and OH− reaction with PF5 yields HF product. F− and electron transfer channels are also observed in many of the reactions studied. Consideration of the efficiency of the electron transfer channel in these reactions leads to the conclusion that the adiabatic electron affinity of PF5 is 0.75±0.15 eV.

The formation and destruction of H3O−

We report the first measurements of rate constants for formation and reaction of the hydrated-hydride ion H3O−. We studied the Kleingeld–Nibbering reaction [Int. J. Mass Spectrom. Ion Phys. 49, 311 (1983)], namely, dehydrogenation of formaldehyde by hydroxide to form hydrated-hydride ion and carbon monoxide. The OD−+H2CO reaction is about 35% efficient at 298 K, with OD−/OH− exchange occurring in about half the reactions. H3O− was observed to undergo thermal dissociation in a helium carrier gas at room temperature with a rate constant of 1.6×10−12 cm3 s−1. We also studied a new reaction in which H3O− is formed: The association of OH− with H2 in a He carrier gas at low temperatures. The rate coefficient for this ternary reaction is 1×10−30 cm6 s−1 at 88 K. Rate coefficients and product branching fractions were determined for H3O− reactions with 19 neutral species at low temperatures (88–194 K) in an H2 carrier. The results of ion-beam studies, negative-ion photoelectron spectroscopy, and ion-molecule reaction data allow us to specify the hydride–water bond energy D0298(H−−H2O)=14.4±1.0 kcal mol−1 (0.62±0.04 eV). The heat of formation of H3O−, −37.5±1.0 kcal mol−1, and the proton affinity of H3O−, 386.0±1.0 kcal mol−1, are derived from these results. Dissociation of H3O− into OH− and H2 requires 4.5±1.0 kcal mol−1 energy.

Kinetics of the Reactions of F- with CF3Br and CF3I as a Function of Temperature, Kinetic Energy, Internal Temperature, and Pressure

Abstract : The reactions of F- with CF3Br and CF3I have been studied at several temperatures as functions of pressure and ion-neutral average center-of-mass kinetic energy ((KE sub cm)). Rate constants and product branching fractions were measured by using a variable temperature-selected ion flow drift tube apparatus. Both title reactions involve two product pathways: displacement and association, the latter being the major channel under most experimental conditions. The rate constants for displacement are governed by total energy, regardless of the relative contributions by translational and internal energy, indicating that the displacement reactions behave statistically. In contrast, the rate constants for association depend strongly on the internal energy of CF3X (rotations and vibrations) compared with the relatively weak effect of translational collision energy. For CF3Br the rate constants for the association channel increase with increasing pressure while those for displacement are independent of pressure, indicating that the additional reactivity leading to more association at higher pressure appears not to compete with the displacement pathway. This lack of competition may be the result of two different reaction intermediates. The more efficient CF3I reaction is at or near the high-pressure limit for association, showing no pressure dependence in both the association and displacement channels. Therefore, there appears to be no competition between these channels for CF3I as well, but the data are less definitive in this case.

Reactions of CF+, CF2+, and CF3+ with Perfluorobuta-1,3-diene

The gas-phase reactions of CF + , CF 2 + , and CF 3 + with perfluorobuta-1,3-diene (1,3-C 4 F 6 ) have been studied using a selected ion flow tube (SIFT) instrument. Reaction rate constants and product branching fractions were measured at 300 K. The reactions all proceed with unit efficiency. The reactions of CF + and CF 2 + with C 4 F 6 both proceed by two pathways: fluoride (F - ) transfer and charge transfer. The ion CF 3 + reacts with C 4 F 6 by fluoride transfer exclusively

Product branching fractions in the reaction of 3.sum.- NH (ND) with nitric oxide

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTProduct branching fractions in the reaction of 3.sum.- NH (ND) with nitric oxideJoseph L. Durant Jr.Cite this: J. Phys. Chem. 1994, 98, 2, 518–521Publication Date (Print):January 1, 1994Publication History Published online1 May 2002Published inissue 1 January 1994https://doi.org/10.1021/j100053a028RIGHTS & PERMISSIONSArticle Views49Altmetric-Citations39LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (493 KB) Get e-Alerts

Thermal energy reactions of OH − + Cl 2, Br 2: rate coefficients, product branching fractions and OH product vibrational populations

The rate coefficients, branching fractions, and OH product vibrational distributions are measured in a flowing afterglow apparatus for the OH − + Cl 2 and OH − + Br 2 reactions at thermal energy. Both reactions proceed at the Langevin collision rate, with k = 1.33(±0.13) × 10 −9 cm 3 molecule −1 s −1 for OH − Cl 2 and k = 1.60(±0.16) × 10 −9 cm 3 molecule −1 s −1 for OH − + Br 2. The branching fractions for OH − + Cl 2 are 0.75(±0.10) Cl − 2 + OH, 0.24(±0.10) Cl − + HOCl and a discernible ⩽0.01 HOCl − + Cl. For OH − + Br 2, the branching fractions are 0.75(±0.10) Br − 2 + OH, 0.15(±0.05) Br − + HOBr and 0.10(±0.05) HOBr − + Br. The vibrational populations of the OH product from the charge transfer channel are measured by laser-induced fluorescence detection and found to be OH( v = 1)/OH( v = 0) = 0.01(±0.01) for both reactions. While the OH( v) states are consistent with a Franck—Condon mechanism, arguments for a long-lived intermediate complex are made on the basis of substantial branching fractions into Cl − + HOCl (Br − + HOBr) and HOCl − + Cl (HOBr − + Br) channels and the extreme difference in bond length for Cl 2 and Cl − 2 (Br 2 and Br − 2).

Chemistry of C −2 and HC −2 with nitrogen, oxygen and nitrogen oxides

We have investigated the reactions of C − 2 and HC − 2 with N 2, N 2O, NO, NO 2 and O 2 by using a selected-ion flow drift tube at 300 K. Rate coefficients and branching fractions were measured as a function of ion/neutral average center-of-mass kinetic energy (〈KE cm〉) over the range from about 0.04 to 0.25 eV. The reaction of C − 2 with NO forms CN − (about 11%), NCO − (about 6%), and reactive electron detachment products (about 83%). The rate coefficient at 300K is 1.2 (±0.3) × 10 −10 cm 3 s −1 and decreases with energy as 〈KE cm〉 −0.7, while the product branching fractions are energy independent. The reaction of C − 2 with NO 2 forms C 2O − (about 74%), CNO − (or NCO −) (about 14%) and CN − (about 12%). The rate coefficient for this reaction is 4.8 (±1.2) × 10 −10 cm 3 s −1 at 300K, and both the rate coefficient and branching fractions are approximately independent of kinetic energy over the range investigated. The reaction of C − 2 with O 2 forms C 2O − (about 70%) and reactive electron detachment products (about 30%). The rate coefficient for this reaction is 1.5 (±0.4) × 10 −11 cm 3 s −1 at 300K and increases as 〈KE cm〉 +0.7 over the energy range studied. C − 2 is unreactive towards N 2 and N 2O within our experimental sensitivity over the energy range studied. HC − 2 reacts with NO 2 to form HC 2O − with a rate coefficient of 1.8 × 10 −10 cm 3 s −1 at 300 K; the rate coefficient decreases with energy as 〈KE cm〉 −0.7. HC − 2 does not react with N 2, N 2O, NO or O 2 within our experimental sensitivity over the energy range studied. The measured parameters are discussed in terms of the characteristic reactivity of the anions and possible reaction mechanisms. Finally, CN − was found to be unreactive towards NO and O 2 within experimental uncertainty at 300 K and no applied electric field, and reacts inefficiently with NO 2 with a rate coefficient less than or equal to 6 × 10 −12 cm 3 s −1.

Gas-phase reactions of oxide and superoxide anions with CF4, CF3Cl, CF3Br, CF3I, and C2F4 at 298 and 500 K

Rate constants and product branching fractions have been measured for the gas-phase reactions of oxide (O−) and superoxide (O2−) anions with the halocarbons CF4, CF3Cl, CF3Br, CF3I, and C2F4 using a variable temperature–selected ion flow tube (VT–SIFT) instrument operated at 298 and 500 K. The reactions of O− with CF3X (X=Cl, Br, I) are fast and produce F−, XF−, and XO− for all X. For CF3Cl and CF3Br, X− is also formed. For CF3I, CF3− and IOF− are minor products. O− reacts rapidly with C2F4 producing F− as the major ionic product, along with contributions from reactive detachment and minor formation of FCO−, CF3−, and C2F3O−. The reaction of O2− with CF3Cl is slow, and both clustering and X− formation were observed. For CF3Br and CF3I, the reactions with O2− are fast, and nondissociative charge transfer was observed in addition to X− formation. O2− reacts rapidly with C2F4 by reactive detachment, in addition to producing F− as the major ionic product with smaller amounts of F2−, FCO−, FCO2−, CF3O−, and C2F4O−. O− and O2− were both found to be unreactive with CF4 at 298 and 500 K. The efficiencies of the reactions of both O− and O2− with CF3X are greater for the heavier halides at both 298 and 500 K. The rate constants for the reactions of O2− with CF3X appear to correlate both with the rates of thermal electron attachment to CF3X and with the electron affinities of CF3X, indicating that the O2−+CF3X reaction mechanism may involve initial electron transfer followed by dissociation. Thus the negative electron affinity of CF3Cl may explain the very slow rate for reaction with O2− despite the available exothermic pathways.

Electronic-state-specific transition metal cation chemistry: Fe++C3H8 and n-C4H1

We present total reaction cross sections and product branching fractions for collisions of specific electronic states of Fe+ with the linear alkanes C3H8 and n-C4H10 at two collision energies each, 0.2 and 1.0 eV. Resonant two-photon ionization prepares specific electronic state distributions of Fe+, as described in the preceding paper. A crossed beam experiment using pulsed, time-of-flight mass spectrometry measures total reaction cross sections averaged over known state distributions, from which we extract state-specific cross sections. The three lowest energy electronic terms of Fe+, 3d64s(6D), 3d7(4F), and 3d64s(4D), show remarkably similar reactivity with both alkanes. The relative cross section varies only a factor of 4 in Fe++C3H8 and less than a factor of 2 in Fe++C4H10, in spite of sampling both sextet and quartet spins, 3d64s and 3d7 configurations, and a range of 1.1 eV electronic energy. Product branching between H2 and alkane elimination is insensitive to initial electronic state as well. All reactions are inefficient compared with the Langevin cross section. We propose a model of Fe++alkane chemistry that assumes that all those Fe+ that insert in a C–H bond of the alkane proceed to elimination products. Spin-changing surface hops between diabatic potential surfaces correlating to different reactant asymptotes (i.e., electronic quenching) competes with bond insertion, explaining the low reactivity of the 3d7(4F) term.

Study of the doubly radiative decay J/ psi --> gamma gamma rho 0.

We present a study of the decay sequence $\frac{J}{\ensuremath{\psi}}\ensuremath{\rightarrow}\ensuremath{\gamma}X$, $X\ensuremath{\rightarrow}\ensuremath{\gamma}{\ensuremath{\rho}}^{0}$, ${\ensuremath{\rho}}^{0}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$, based on 5.8 \ifmmode\times\else\texttimes\fi{} ${10}^{6}$ produced $\frac{J}{\ensuremath{\psi}}$ collected by the Mark III detector at the SLAC storage ring SPEAR. In the $\ensuremath{\gamma}{\ensuremath{\rho}}^{0}$ mass spectrum, we find two peaks with masses and widths consistent with the axial-vector mesons ${f}_{1}(1285)$ and ${f}_{1}(1420)$ that have recently been observed in two-photon collisions. Fits to the angular distributions confirm that the first peak is an axial-vector state, but are unable to distinguish between the ${f}_{1}(1420)$ or the pseudoscalar $\ensuremath{\eta}(1430)$ for the high-mass peak. Product branching fractions for both objects are presented.

Multicollision chemistry of gas-phase transition-metal ions with small alkanes: rate constants and product branching at 0.75 torr of helium

The authors have studied the reactions of the gas-phase first transition series metal ions Sc/sup +/ through Zn/sup +/ with CH/sub 4/, C/sub 2/H/sub 6/, and C/sub 3/H/sub 8/ in the multicollision environment of He buffer gas as 0.75 Torr. Laser vaporization of pure metal targets creates reactant metal ions M/sup +/ in a fast flow reactor. For each primary M/sup +/ + alkane reaction step they obtain quantitative effective bimolecular rate constants and product branching fractions. The multicollision rate constants provide a strikingly different view of M/sup +/-alkane interactions than previous single-collision studies.

Observation of the decay B-->FX.

We present evidence for inclusive F-meson production in B-meson decay. The product branching fraction B(B→FX)B(F+→φπ+) is measured to be 0.0038±0.010. The F momentum spectrum indicates the presence of a large component of two-body final states in the decay B→FX.Received 7 April 1986DOI:https://doi.org/10.1103/PhysRevLett.56.2781©1986 American Physical Society

Monte Carlo simulation of RRKM unimolecular decomposition in molecular beam experiments. III. Application to angular distributions and branching fractions for the system K + CsF

The experiments on the reactive systems K + CsF and K + RbF by Stolte et al. are unique in that product angular distributions and branching fractions with variation of both translational and rotational energies have been obtained. The results for the reaction K + CsF with translational energy variation have now been re-evaluated using an algorithm recently published by us. A satisfactory fit to the experimental data is found by a long-lived collision complex model including strict angular momentum conservation in both the reactive and the non-reactive channels. No “osculating” behaviour has to be added to the model. Angular momentum limits have to be included in the model to reach agreement with the measured branching fractions and cross sections. This indicates that the internal dynamics of the complexes is important. The calculation shows that the reactants giving reactive complexes have large rotational and vibrational energies, while their translational energy is nearly average. The internal energy of reactive pairs of molecules decreases with increasing translational energy. The question how rapidly the branching fraction varies with translational energy is thus more complicated to answer than believed previously.

New mesonic states near 160 GeV

Recently reported data on monojet events and on massive eν-jet and jet-jet systems at the CERN SPS p̄p collider suggest a common origin which could be the production of new hadronic states with mass in the neighbourhood of 160 GeV. We explore the consequences of postulating that such states exist and are excited in qq̄ or gg subprocesses. To be specific, we identify the states as coloured mesonic systems and we then compute production cross sections and branching fractions into qq̄, gg, gZ 0, gW, and gγ, obtaining fair consistency with data. Implications and predictions are discussed.

Observation of the Decayηc→φφand Determination of theηcSpin and Parity

In a study of 2.7 million J/psi decays with the Mark III, a new magnetic detector at SPEAR, we have observed the sequential decay J/psi..--> gamma..eta/sub c/, eta/sub c/..-->..phiphi. The product branching fraction is (1.02 +- 0.25 +- 0.14) x 10/sup -4/, where the quoted errors are statistical and systematic, respectively. Analysis of the final-state angular distributions provides the first experimental determination that the spin and parity of the eta/sub c/ are 0/sup -/.

A search for the ρ(2.2) in the ϒ region

We have searched in the decays of the ϒ(1S), ϒ(2S) and B mesons for the narrow state, γ, seen by the Mark II group in radiative ψ decay. We find the product branching fractions B[ ϒ(1 S) → γγ]· B[ γ → K +K −], B[ ϒ(2 S) → γ γ]· B[ γ → K +K −] and B[B → γX]· B[ γ → K +K −] to be less than 2×10 −4, 9×10 −5 and 3×10 −3, respectively at 90% confidence level. These limits constrain theoretical models if the γ is interpreted as a Higgs boson.

Dissociative recombination in electron-beam excited argon at high pressures

We report studies of dissociative recombination in electron-beam excited argon at pressures from 200 to 12 000 Torr. In these experiments, we measure the decay in absolute intensity of the fluorescence from Ar (3p5 4p) dissociative products. By summing over all significant product fluorescence, we determine the total effective recombination rate for electrons, the electron density, and the branching fraction for production of several individual states by recombination. For pressures above three atmospheres, similar quantities are observed by monitoring the Ar*2 excimer emission. At low pressures, we find that the rates are in good agreement with previous workers. At high pressures, we observe a factor of 6 increase in the recombination coefficient over that measured at 200 Torr. At a constant electron beam current, the electron density rises as the argon density increases, reaches a maximum at ∼5000 Torr, and decreases after 7000 Torr. A discussion of possible models to explain the pressure dependence of the recombination rate is presented.

Observation of the radiative transition ψ → γE(1420)

We have observed a radiative transition from the ψ to a state decaying into K SK ±π ∓, with mass M = 1.44 −0.015 +0.01 GeV/ c 2 and width Γ = 0.05 −0.02 +0.03 GeV/ c 2. We tentatively identify this state as the E(1420). Assuming that this state is an isospin singlet, we have determined the branching fraction product B(ψ → γ E) × B( E × K K π) = (3.6 ± 1.4) × 10 −3 .