Endoergic Charge Transfer Research Articles

Velocity Map Imaging Study of Charge-Transfer and Proton-Transfer Reactions of CH3 Radicals with H3(.).

The velocity map imaging method has been applied to crossed beam studies of charge transfer and proton transfer between methyl (CH3) radicals formed by pyrolysis and H3(+) cations over the collision energy range from 1.2 to 3.4 eV. Vibrational excitation in the H3(+) reactants plays an important role both in promoting endoergic charge transfer and in supplying energy to the products of the proton-transfer reaction. Excited H3(+) reactants with vibrational energy in excess of the barrier lead to energy-resonant charge transfer via long-range collisions. A small fraction of collisions that take place at low impact parameters appear to form charge-transfer products with higher levels of internal excitation. The proton-transfer reaction exhibits direct, stripping-like dynamics. Consistent with the kinematics of proton transfer, incremental kinetic energy supplied to the reactants is strongly directed into product relative kinetic energy, as predicted by the concept of "induced repulsive energy release".

Elastic and charge transfer processes in H++CO collisions

Proton and hydrogen atom time-of-flight spectra in collision energy range of E(trans) = 9.5-30 eV show that the endoergic charge transfer process in the H+ + CO system is almost an order of magnitude less probable than the elastic scattering [G. Niedner-Schatteburg and J. P. Toennies, Adv. Chem. Phys. LXXXII, 553 (1992)]. Ab initio computations at the multireference configuration interaction level have been performed to obtain the ground- and several low-lying excited electronic state potential energy curves in three different molecular orientations namely, H+ approaching the O-end and the C-end (collinear), and H+ approaching the CO molecule in perpendicular configuration with fixed CO internuclear distance. Nonadiabatic coupling terms between the ground electronic state (H+ + CO) and the three low-lying excited electronic states (H + CO+) have been computed and the corresponding diabatic potentials have been obtained. A time-dependent wavepacket dynamics study is modeled first involving only the ground and the first excited states and then involving the ground and the three lowest excited states at the collision energy of 9.5 eV. The overall charge transfer probability have been found to be approximately 20%-30% which is in qualitative agreement with the experimental findings.

Vibrational mode and collision energy effects on reaction of H2CO+ with C2H2: Charge state competition and the role of Franck-Condon factors in endoergic charge transfer

The effects of collision energy (E(col)) and six different H(2)CO(+) vibrational states on the title reaction have been studied over the center-of-mass E(col) range from 0.1 to 2.6 eV, including measurements of product ion recoil velocity distributions. Ab initio and Rice-Ramsperger-Kassel-Marcus calculations were used to examine the properties of complexes and transition states that might be important in mediating the reaction. Reaction is largely direct, despite the presence of multiple deep wells on the potential surface. Five product channels are observed, with a total reaction cross section at the collision limit. The competition among the major H(2) (+) transfer, hydrogen transfer, and proton transfer channels is strongly affected by E(col) and H(2)CO(+) vibrational excitation, providing insight into the factors that control competition and charge state "unmixing" during product separation. One of the more interesting results is that endoergic charge transfer appears to be controlled by Franck-Condon factors, implying that it occurs at large inter-reactant separations, contrary to the expectation that endoergic reactions should require intimate collisions to drive the necessary energy conversion.

Anomalous line shapes caused by charge transfer in low-pressure discharges

Doppler profiles are calculated for initial velocity distributions after charge transfer collisions. Such profiles are applied to emissions from excited Fe + following Ar +-Fe collisions, assuming an energy-independent cross section. It is shown that exoergic charge transfer processes may give significantly non-Gaussian line shapes with lines that are flatter and wider than would be expected from a Maxwellian distribution. Endoergic charge transfer processes, however, will give Gaussian line shapes with a linewidth described by a temperature only slightly below the source temperature. For exoergic reactions, our results differ slightly from results obtained using an adaptation of the method developed by Biondi and coworkers for dissociative recombination.

The reaction SF +5 + H 2S at near thermal energies: competition between association and binary reactions

The reaction between SF + 5 has been investigated in the energy from 0.052 to 1.7 eV, KE cm, using a selected ion flow drift tube (SIFDT) operating with helium buffer gas. At low energies a slow binary reactions channel into SF 4HS + occurs in parallel with a fast association reaction. Both rate coefficients decline from 3×10 −11 cm 3 s −1 and 3×10 −27 cm 6 s −1 respectively at 0.052 eV to <5×10 −12 cm 3 s −1 and < 5×10 −28 cm 6 s −1 respectively at 0.3 eV, KE cm. Above 0.3 eV the endoergic charge transfer channel becomes dominant, the rate coefficient increasing to 9×10 −12 cm 3s −1 at 1.7 eV, KE cm. SF + 5·H 2S was found to react fast with H 2S, the rate coefficient being 1.4×10 −9 cm 3 s −1.

Intensity Control of Emission Spectra in the He-Cd Mixture for a White Light Laser

By means of a special auxiliary discharge, additional positive helium ions were injected into a traditional flute-type He-Cd hollow cathode discharge, so as to realize the endoergic charge transfer process [K. I. Fujii: Proc. 10th Symp. Ion Sources & Ion Assisted Technology, Tokyo, 1986. p. 77] within the cathode cylinder. Spectroscopic measurements of spontaneous emission showed that a considerable increase in the strength of the red lines (6355 Å, 6360 Å) and of the green lines (5337 Å, 5378 Å) was achieved. This method is expected to be useful in the design of color-controllable white light lasers of the He-Cd+ hollow cathode type.

Hollow cathode and high voltage plane cathode type discharges for charge transfer reactions

Decreasing the pressure in the hollow cathode discharge, simultaneously ensuring room for the expanding negative glow, allows the discharge to transit, from the hollow cathode type into a high voltage discharge where the cathode cavity is filled with the cathode glow. Such an arrangement seems to be very effective for endoergic charge transfer excitation as it was observed in a He-Cu system. Voltage-current characteristics of the transition region are also presented.

Endoergic and resonant charge transfer excitation in He-Cu discharge

The intensity of Cu-II lines with upper level energies near and above those of the He ion was measured as a function of He pressure in a Cu hollow cathode tube. In this tube at low pressures the negative glow could expand above the cathode. The maximum intensity of the Cu-II 493.1 nm line was found in the low voltage, high pressure hollow cathode discharge region in accordance with a resonant charge transfer excitation process. Enhancement of the intensity of the Cu-II 436.5 nm and 417.9 nm lines was observed in the cathode glow at low pressures. Excitation of these lines is attributed to endoergic charge transfer collisions between He ions accelerated by the 2 kV tube voltage and ground state Cu atoms. The cross-section for this reaction exciting the 436.5 nm line was estimated to be of the order of 10−17 cm2.

Electron attachment to volatile uranyl molecules

Negative ion production by endoergic charge transfer from fast alkali atoms to a crossed molecular beam of uranyl complexes has been studied for three systems, UO/sub 2/L/sub 2/, UO/sub 2/L/sub 2/.THF, and UO/sub 2/L/sub 2/.TMP, where L denotes hexafluoroacetyacetonate, THF tetrahydrofuran, and TMP trimethyl phosphate. Near the lowest threshold, only negative ions of the parent molecules appear. The corresponding nominal electron affinities are 1.9 +/- 0.3, 1.6 +/- 0.2, and 1.5 +/- 0.3 eV, respectively. At higher collision energies, smaller negative ions appear, all containing the L/sup -/ anion; the variation with collision energy and source temperature indicates these come from dissociative electron attachment to the parent molecules rather than attahcment to products of thermal unimolecular decomposition. Qualitative electronic structure arguments suggest the attaching electron first enters a uranium 5f orbital and then migrates to the lowest vacant ..pi..* orbital of the L ligand.

Ion-molecule reactions in trifluoromethyl iodide and pentafluoroethyl iodide

Ion-molecule reactions in trifluoromethyl iodide and pentafluoroethyl iodide were studied using time-of-flight and ion cyclotron resonance mass spectrometry, respectively, to obtain information about reaction rates and mechanistic pathways. In the CF 3I system, CF 3 + reacts with parent via F − transfer (exoergic) or via charge exchange (strongly endoergic). The I + ion reacts by I atom abstraction (exoergic) giving I 2 + or by endoergic charge exchange. The parent ion CF 3I + can decay via collisionally induced dissociation, but also undergoes condensation reactions with neutral CF 3I yielding CF 3I 2 + and the dimer ion (CF 3I) 2 +; the ether-structure product (CF 3) 2I + was not seen. Many of the same reaction types are seen in the C 2F 5I system. The CF + ion undergoes I − and CF 3 − transfer reactions with parent, giving C 2F 5 + and CF 2I +, respectively. As in the CF 3I system, CF 3 + (and also CF 2I + and C 2F 5 +) react with substrate either via energetically favorable F − transfer or via strongly unfavorable charge transfer processes, giving C 2F 4I + and C 2F 4I +, respectively, with each reactant. Again, I − reacts by energetically favorable I atom abstraction or unfavorable charge transfer. Endoergic charge transfer also occurs between parent and C 2F 4 + ion. Two condensation products were seen which are analogous to the methyl case — namely C 2F 5I 2 + and (C 2F 5I) 2 +. In both the CF 3I and C 2F 5I systems, there is strong evidence for participation of ions with as much as 1.5 eV of excitation energy.

Luminescence from He +O 2 collisions at low energy

The emission spectrum resulting from He +O 2 collisions in the kinetic energy range 25–400 eV has been measured between 1940 and 8500 Å. The results show that highly endoergic charge transfer processes dominate the spectrum, although exoergic processes with excited O 2 + production also occur. The nearly resonant charge exchange process yielding ground state He atoms and O 2 +[c 4Σ u −(v=0)] does not occur with high probability.

Single-Configuration Wavefunctions and Potential Curves for Low-Lying States of He2+, Ne2+, Ar2+, F2−, Cl2− and the Ground State of Cl2

Wavefunctions, orbital energies, and potential curves for He2+, Ne2+, Ar2+, F2−, and Cl2− have been calculated in the molecular-orbital, self-consistent-field approximation over a range Re ≲ R &amp;lt; ∞ for the ground state and those excited states which dissociate into an atom and an ion in their ground states. The ground state potential curve for Cl2 has been calculated for R ∼ Re. Dissociation energies and other parameters obtained from the calculated potential curves are compared with corresponding parameters obtained from elastic differential scattering measurements and resonant electron capture in noble gases, measurements of afterglow line profiles in dissociative recombination radiation in neon and argon, optical absorption spectra of VK centers, and endoergic charge transfer studies of halogen ions and molecules. A formal analysis and discussion of the sources of correlation error in the calculated potential curves and a discussion of the expansion errors are also given.

Electron Affinities of Halogen Diatomic Molecules as Determined by Endoergic Charge Transfer

Ion-pair formation by photon absorption at threshold wavelengths has been used to prepare I−, Br−, and F− ions with approximately room-temperature thermal energies, as verified by retarding-potential measurements. The primary ions were accelerated and their reactions with halogen molecules studied at laboratory kinetic energies from 0.0 to about 4.0 eV. Thresholds were determined for endoergic reactions of the type X−+Y2→X+Y2 −, where X may be the same as Y. At least two reactions were used in determining each electron affinity. The agreement was good in all cases. The values of electron affinity obtained are 3.08±0.1 eV for F2, 2.38±0.1 eV for Cl2, 2.51±0.1 eV for Br2, and 2.58±0.1 eV for I2. Interhalogen molecular ions such as IBr− were also observed, and measurement of the threshold for formation gave the value 2.7±0.2 eV for the electron affinity of IBr. The retarding-potential measurements of F− from F2 strongly support a value for the dissociation energy of F2 in the neighborhood of 1.6 eV.