Charge Transfer Products Research Articles

Charge transfer and structured vibrational distributions in H++CH4 low-energy collisions

Inelastic and charge transfer collisions of protons with methane molecules have been investigated in a perpendicular-plane crossed beam experiment via the detection of the scattered protons and H atoms, respectively. Time-of-flight analysis of the protons and H atoms at scattering angles 0°≤θ≤10° and collision energies 10≤E≤30 eV provided information on internal energy distributions of the CH4 and CH+4 products. Excitation of the n(ν1 ,ν3) +m (ν2 ,ν4) type vibrations, with n,m=0, 1, 2,⋅⋅⋅was found to be the most probable assignment of the observed structured energy distributions of CH4 (1 A1 ) at θ≤4°. At θ>4°, the energy transfer increases steeply up to the dissociation limit while the vibrational structure was no longer resolved. In the case of charge transfer, the observed narrow internal energy distributions corresponding to a most probable average internal energy of CH+4 of about 0.95 eV was centered at the recombination energy of the proton indicative of quasiresonant charge transfer. In addition, fragmentation of CH+4 formed in charge transfer collisions of H+ with CH4 was investigated in an independent experiment using mass spectrometric analysis to identify the individual fragment species. The relative intensities of the parent and fragment ions (i.e., of CH+4, CH+3, and CH+2) were found to be in good agreement with the known values of the appearance potentials of the fragment ions and the distribution of the CH+4 internal energy as obtained from the differential cross sections. A mechanism is proposed to explain the experimental results based on vibronic symmetry correlation theory. This mechanism deals with vibronic interactions in the compound quasimolecule CH+5 and explains the origin of the unexpected excitation of infrared inactive modes [e.g., ν2 (E)] of the tetrahedral methane. The effects of Jahn-Teller distortions of the CH+4 charge transfer product are also discussed.

Gas-phase reactions of sulfides, mercaptans, and dimethyl methylphosphonate with ionic species derived from argon and water

Rate coefficients for gas-phase ion/molecule reactions involving sulfides, mercaptans of the form RSR′ (where R  H, CH 3, CH 3CH 2), and dimethyl methylphosphonate (DMMP) are measured using a selected ion flow tube (SIFT) technique. The ionic reactants include Ar +, Ar + 2, OH +, H 2O +, H 3O + (H 2O) n with n  0–3. All the rate coefficients indicate relatively fast, nearly collisional reactions. Ar + and Ar + 2 react with DMMP to produce the parent ion (charge transfer channel) as well as ionic fragments of DMMP. Reactions of DMMP and RSR′ with H 3O + result in proton transfer. Both proton and charge transfer products are observed when H 2O + reacts with DMMP. Hydration of H 3O + by up to three water molecules reduces the proton transfer reaction rate with DMMP by a factor of two.

A study of the symmetric charge transfer reaction H+2+H2 using the high resolution photoionization and crossed ion–neutral beam methods

A new ion–molecule reaction apparatus, which combines the crossed ion–neutral beam method, high resolution photoionization mass spectrometry, and charge transfer detection, has been developed. Using this apparatus, we have examined the relative total charge transfer cross sections of H+2+H2 as a function of the vibrational state of H+2, v′0 =0–4, at the center-of-mass collision energy (Ec.m.) range of 0.38–200 eV. The relative total charge transfer cross sections measured at Ec.m. =8, 16, 22.5, and 200 eV are in general agreement with a recent theoretical calculation based on the semiclassical energy conserving trajectory formulation. The vibrational energy effects on the cross sections for the charge transfer and the H+3+H channels at low collision energies (Ec.m. ≤1 eV) were directly observed. The rotational states, J=0, 1, and 2, of H+2(v0=0) were also selected in this experiment. Within experimental uncertainties, the rotational excitations of H+2(v′0 =0) have no effect on the relative total charge transfer cross sections at Ec.m. =2 and 4 eV. By calibrating the nominal relative total charge transfer cross sections obtained with an ionizing photon energy of 18 eV (688 Å) to absolute total charge transfer cross sections determined previously using low energy electron impact ionization, absolute total charge transfer cross sections for v0 =0 and 1 in the kinetic energy range of Ec.m. =8–200 eV were estimated. The absolute total charge transfer cross sections thus obtained for v′0=0 and 1 are lower than the theoretical values by approximately a factor of 2. However, the kinetic energy dependence of the total charge transfer cross section is in agreement with the theoretical calculation. The final vibrational state distributions of the charge transfer products H+2 from the reaction H+2(v0=0) +H2(v″0=0) → H2(v′)+H+2(v″) at Ec.m. =4, 8, and 16 eV have been probed by charge transfer reactions H+2(v″)+N2 and H+2(v″)+CO. The results are consistent with the theoretical prediction that approximately 92% and 85% of the product H+2 ions formed at Ec.m. =8 and 16 eV are in the v″=0 state, respectively.

Picosecond excitation and selective intramolecular rates in supersonic molecular beams. III. Photochemistry and rates of a charge transfer reaction

The picosecond state-selective dynamics and photochemistry of the molecule A–(CH2)3–φ, where A and φ are aromatic chromophores, was studied under collision-free conditions in a supersonic beam. Time-resolved fluorescence measurements of the reactant and the charge transfer (exciplex) product were undertaken as a function of specific vibrational energy above the zero point level of S1. From these studies along with an analysis of the excitation spectra, dispersed flourescence, and quantum yields, the following results and conclusions were reached: (i) IVR is much faster than reaction at all excess energies studied. (ii) The energy threshold for product formation is E0≂900 cm−1 (2.6 kcal/mol). The analysis of the rates using an effective temperature model gives a frequency factor of A0≊1.2×1010 s−1. Four torsions were identified as critical to the reaction dynamics which were modeled according to a multidimensional reaction coordinate using an RRKM scheme. (iii) The thermodynamics of the isolated charge transfer product indicates strong stabilization ΔH=−9.2 kcal/mol and extensive charge transfer, the static dipole moment is 13 D, and the charge transfer contribution to the total electronic wave function ‖c2‖2 is 0.86. (iv) A comparison of the present work to solution phase studies of A–(CH2)3–φ indicates similar static properties but different dynamics. The calculated thermal (room temperature) reaction time for exciplex formation in the vapor (540 ps) was compared to the shortest observed value in solution (1.4 ns) to assess the role of the solvent on the chain motions which lead to product formation.

Energy disposal in charge transfer reactions producing NH+.3: Dependence of the NH+.3+H2O reaction on NH+.3 internal energy

The division of available energy between translational and internal modes has been determined for seven charge transfer reactions which produce NH+.3. The charge transfer reagent ions studied were Ar+., Kr+., Xe+., N+.2, O+.2, CO+., and CO+.2. The average kinetic energy released in the reaction was measured directly using an ion cyclotron resonance (ICR) technique. The internal energy deposited in the NH+.3 product was determined by further reacting the NH+.3 product with H2O in a tandem ICR spectrometer. The product distribution of this subsequent reaction is shown to depend strongly on the NH+.3 internal energy; and this dependence is quantitatively determined. The energy deposited in the neutral charge transfer product is calculated using energy balance. Comparisons are made between charge transfer reagent ions of similar recombination energies to show the effects of reagent ion internal modes on the division of available reaction energy.

Atomic Collision Experiments Utilizing Low-Velocity, Highly-Charged Ion Beams

Intense beams of highly-stripped ions are now routinely produced at low velocities using the Brookhaven dual MP-tandems in a unique four-stage accel/decel mode. This mode of operation combines three stages of acceleration, stripping at high energy, and one stage of deceleration to near-zero velocity. To date, experiments have used 10-100 nA beams of bare and few-electron heavy ions at energies as low as 0.2 MeV/amu, and upgrades of the facility should push the lower limit below 0.1 MeV/amu. Recent experiments, such as measurements of charge transfer and x-ray production for S6-16+ on He and Ar at 6-20 MeV and P(b) measurements for MO x-rays produced in Cl16+ + Ar collisions at 20, 10 and 5 MeV have demonstrated the usefulness of highly-stripped, low-velocity projectiles. These experiments and a few possibilities for future experiments are discussed.

Infrared chemiluminescence from vibrationally excited no +: product branching in the N + + O 2 ion-molecule reaction

Vibrational chemiluminescence is detected from the reaction N +( 3P) + O 2(X 3Σ − g) in a flowing afterglow. The atom-transfer product, NO +(X 1Σ +, υ), displays a bimodal vibrational distribution spanning the range υ = 1−14. Excited N( 2D) is found to be the major charge-transfer product by observation of its further reaction with O 2 to produce NO(X 2Π, υ = 1−12).

Charge transfer and addition products between alkylated phenazine donors and ethanetetracarbonitrile (TCNE): crystal and molecular structure of the dye 2,2'-(2,3,5,10-tetrahydro-5,10-dimethylphenazine-2,3-diylidene)bis(propanedinitrile) (TMPP)

Charge transfer and addition products between alkylated phenazine donors and ethanetetracarbonitrile (TCNE): crystal and molecular structure of the dye 2,2'-(2,3,5,10-tetrahydro-5,10-dimethylphenazine-2,3-diylidene)bis(propanedinitrile) (TMPP)

A study of the reaction of N2+ with SO2 and its dependence on internal energy

The reaction of N2+ with SO2 has been studied in a tandem-ion cyclotron resonance mass spectrometer at less than 0.1 eV translational energy as a function of ion source pressure and collision chamber pressure. The internal energy state distribution of the N2+ reactant ions relaxes with increasing N2+/N2 collisions, producing a corresponding change in the branching ratio of the charge-transfer product SO2+ and the dissociative charge-transfer product SO+. With increasing SO2 pressure in the ICR cell, the dissociative charge-transfer channel is quenched. The total rate constant for charge-transfer was determined to be 7.0×10−10 cm3 molecule−1 sec−1; within experimental error, the rate constant is independent of the vibrational excitation of the N2+ ion.

Contributions of multi-electron processes to inner-shell charge transfer and vacancy production: projectile charge dependence in collisions of bare nuclei with argon

Previous coupled-channels calculations of inner-shell charge transfer and vacancy production for p+Ar collisions are extended to bare projectiles with nuclear charges Zp=2 and 6, in the collision energy range from 1 to 9 MeV amu-1. An improved expression for the charge transfer amplitude is derived in a 'distorted-phase' formulation of the impact parameter method. The authors denote by sigma C,VK the inclusive cross section, which has been measured by coincidence experiments, for producing all final states in which at least one target electron has been transferred to the projectile and at least one vacancy has been produced in the target K shell. In the single-particle model sigma C,VK reduces to sigma SPC,VK, which contains only the exclusive single-electron process of direct transfer from the target K shell. The contributions of multi-electron processes to their calculated sigma IPMC,VK increase strongly with Zp. For Zp=6, sigma IPMC,VK differs by up to a factor of three from sigma SPC,VK. The charge transfer cross sections sigma IPMCT and sigma IPMC,VK show a Zp dependence less than the Z5p of the OBK approximation. The calculated sigma IPMVK and sigma SPI,VK for Zp=6 are compared with experimental K vacancy production cross sections for C6+ and C4+ projectiles, respectively, and good agreement is found.

Charge transfer reactions OF Kr 2+ and Ne 2+ions with several molecular gases at 300 k

Measurements are reported of the rate coefficients and production distributions for the reactions of Kr 2+ and Ne 2+ions with H 2, N 2,O 2, CO 1, C0 2 and CH 4 at 300 K. The data are obtained with a selected ion flow tube (SIFT). The reaction rates are generally fast, proceeding at or near the gas kinetic limit. In the Kr 2+ reactions the only mechanism observed is single charge transfer, whereas for the more energetic Ne2+ ions parallel single and double charge transfer product channels are evident, the double charge transfer channels usually being dominant. The general features of the data are discussed in relation to those previously obtained For the corresponding reactions of Ar 2+ and Xe 2+ ions.

Ionization of N2O by low-energy electrons and near-thermal rare-gas ions

Emission spectra between 185 and 600 nm have been investigated following near-thermal charge exchange between He+ and N2O and ≤ 100 eV electron impact on N2O. The charge exchange produces N2O+Ã→X˜and N2+ B → X emission, but the two band systems account for at most 5% of all charge transfer products. These results and literature data on Ar+/N2O are discussed in the light of Franck—Condon and energy resonance criteria as applied to low-energy charge exchange. The electron-impact experiments revealed a weak (≈ 10−3) long-lived (≈ 50 × 10−6 s) component in the N2O+Ã→X˜emission.

Molecular charge transfer reactions of tritium

Charge transfer cross sections for the T+2 (X 2Σ+g, v′0) + T2 (X 1Σ+g, v″0) reactions have been computed using multistate eikonal and orbital treatments. Differential cross sections for excitation of individual charge transfer as well as the direct scattering channels were computed as a function of scattering angle and integral cross sections have been determined for incident ions with 0.01 to 5.0 keV kinetic energies. Converged cross sections require the inclusion of a large number of product vibrational states in the wavefunction expansion of the system, with the number increasing to over 100 different vibrational channels as the ion kinetic energy is increased to 5 keV. Differential cross sections for the formation of fast neutral products become more intense and concentrated in the forward direction as the vibrational quantum number of either the reactant ion or target molecule is increased. Integral cross sections involving vibrationally excited neutral target molecules and/or incident ions are larger than those for ground state reactants at low kinetic energies where both energy defects and vibrational overlaps exert a strong influence on the reaction mechanism. Large cross sections involving vibrationally excited species at low kinetic energies is a consequence of the larger number of product channels that are energetically accessible as the reactant vibrational state is increased. At high kinetic energies the vibrational overlaps exert a controlling influence on both tthe magnitude of the charge transfer cross sections and product vibrational state distributions.

Dissociative charge-transfer reactions of acetylene part 2 Xe +/C 2H 2, Kr +/C 2H 2, and Ne +/C 2H 2

The three dissociative charge-transfer reactions, Xe +/C 2H 2, Kr +/C 2H 2, and Ne +/C 2H 2, have been investigated over the reactant-ion energy range 100–2000 eV, using the same techniques as described previously for the Ar +/C 2H 2 reaction [1]. In contrast to the Ar +/C 2H 2 reaction, these reactions are dominated by momentum-transfer collisions, although long-range charge-transfer products were observed in the Xe +/C 2H 2, and Ne +/C 2H 2 systems. It is argued that the possibility of long-range interaction for these systems is severely restricted by Franck-Condon-like considerations, and that even if a long-range charge transfer were possible, the symmetry of the molecular energy states involved could restrict the chances of a successful long-range collision. Evidence was found for the possible involvement of autoionising levels of C 2H 2 in the long-range interaction between Xe + and C 2H 2. An attempt has been made to assess the dissociation pathways of the excited C 2H 2 + ion in the different systems by examination and comparison of translational-energy (TE) curves and delay curves. It is possible that induced polarisation of the target C 2H 2 molecule occurs and increases as the collision energy decreases towards 100 eV. This could account for some features displayed by the fragment-ion TE curves at the lower end of the energy scale. The possible excited states of C 2H 2 +, formed in the initial charge transfer prior to fragmentation, are discussed for each system. There is evidence that the C 2 + product ion in the Ne +/C 2H 2 reaction is formed from C 2H + in a consecutive mechanism occurring over a time period of ca. 1 μs.

Comparison of the energy dependences and branching ratios of the reactions of Ar+ and N2+ with SO2

The reactions of Ar+ and N2+ with SO2, which have similar energetics due to the nearly equal ionization potentials of Ar and N2, have been studied in a flow–drift tube from 0.04 to 3 eV relative kinetic energy. The energy dependences and product branching ratios of the two reactions are found to be almost identical. With increasing relative kinetic energy, the rate constants show a rather steep initial decline, followed by an equally steep increase. At the lower energies, both reactions yield only the charge-transfer product SO2+, and at the higher energies, both reactions yield predominately the dissociative charge-transfer product SO+.

Scattering of low energy He 2+ ions by Ne, Ar, and Kr atoms

Experimental studies of collisions of He 2+ ions with Ne, Ar, and Kr atoms have been carried out at laboratory kinetic energies in the range 8 ⩽ E 1 ⩽ 10 eV. For each collision pair, relative differential cross sections for elastic scattering, and for the formation of He + by single charge transfer [e.g., He 2+ + R = He + + (R +) *] were measured. Information concerning the initial states of the charge transfer products was also obtained, from measurements of the kinetic energy distributions of the He + + He = Ne +(2s 2p 6 2S) ± He +( 2S), whereas for the other systems, transfer proceeds via a number of channels. The He +-ion kinetic energy measurements indicated that for He 2+. Ar both Ar + both Ar + and Ar 2+ are formed in transfer, and that for He 2+, Kr only Kr 2+ (and no Kr +) was formed. The differential elastic scattering patterns were analyzed by means of cross section calculations based on an approximate form of the optical model. These calculations indicated that the pronounced shoulders observed in the σ el(θ) versus θ curves arose from scattering from an attractive potential well, in the presence of concurrent inelastic scattering. Using parametrized Morse potentials to represent the ground electronic states of (HeNe) 2+, (HeAr) 2+, and (HeKr) 2+, the corresponding well-depth are estimated to be, respectively: 1.0 eV, 2.1 eV and 2.6 eV.

Linear sweep voltammetry. Ohmic drop and chemical polarization

The effect of the uncompensated cell resistance and of double layer charging on the current-potential curves is analyzed in the case where kinetic control is not only by diffusion but also by secondary chemical reactions. The charge transfer is supposed to be nernstian and the simultaneous interference of diffusion polarization and chemical polarization is assumed to result in stationary state conditions. Four basic reaction schemes are considered: first order chemical reaction consecutive to charge transfer, first order e.c.e. reaction scheme, dimerization and disproportionation of the charge transfer product. Tables are given which provide the peak height, the peak potential and the peak width as functions of the various experimental factors (uncompensated cell resistance, double layer differential capacity, initial concentration, sweep rate…) grouped together in two dimensionless parameters. Correction procedures are deduced from the theoretical analysis. For small values of the ohmic drop, an approximate but simple procedure is suggested for the correction of peak potentials.

Disproportionation and ECE mechanisms—I. Theoretical analysis. Relationships for linear sweep voltammetry

A general mechanism involving two successive charge-transfer steps coupled with acid-base reactions is considered. It is shown that ECE reaction paths as well as disproportionation reactions must be taken into account in the general case. According to the value of the experimental parameters two limiting behaviours are to be considered (i) ECE mechanism without any kinetic interference of disproportionation reactions (ii) apparent disproportionation of the first charge-transfer product, the rate constant being dependent on the acidity of the medium. Theoretical relationships allowing the kinetic analysis of such reaction schemes by linear sweep voltammetry are established. Diagnostic criteria are developed whereby variation of peak current and peak potential with sweep rate and initial concentration can be used to characterize the mechanism. Rate-constant determination is discussed.

Potential-sweep voltammetry: Theoretical analysis of monomerization and dimerization mechanisms

On the basis of a previously reported general theory of chemical polarization under the conditions of potential-sweep voltammetry, current/potential curves have been calculated for electrode processes involving dimerization of the charge-transfer product or formation of the depolarizer by dissociation of an electro-inactive species (monomerization). Very fast chemical reactions, very much displaced at the equilibrium towards the electro-inactive species have been considered. Diagnostic criteria characterizing these mechanisms have been derived and compared with those for other types of electrochemical reactions.

Radiolysis of Hexafluoroethane

The radiolysis of C2F6 at 3-atm pressure has been examined. The products and 100 eV yields are CF4 (1.6), cyclo-C3F6 (0.30), C3F8 (0.21), C4H10 (0.14), and C2F2 (0.03). Good material balance is obtained; the F/C ratio in the products is 3.0. Experiments using radical scavengers indicate that 50% of the CF4 comes from radical reactions and 50% from nonradical reactions, that C3F8 and C4H10 are entirely formed by radical reactions, and that C2F2 and cyclo-C3F6 are probably formed by ionic reactions. In Ar–C2F6 mixtures energy transfer, thought to be charge transfer, is observed and ionic production of C3F8 is seen. C2F6 is about one-fifth as sensitive to radiation decomposition as is C2H6. It is concluded that excited molecule decompositions, particularly those giving molecular fluorine, are relatively unimportant in the radiolysis of perfluoroalkanes.