Collision-induced dissociation of H 2 + ions into the fragmentations H+H(2p) and H++H(2p)

Using a photon-particle delayed coincidence method the energy distributions of H +H(2p) and H⁺+H(2p) fragment pairs have been measured arising from collisional dissociation of 10 keV H ₂ ⁺ ions incident on various target gases. H fragments in their 2p state are identified by the Lymanα radiation emitted. The distribution of H+H(2p) fragment pairs arising from dissociative charge exchange reveals a sharp increase below 0.2 eV in the center-of-mass frame of the H ₂ ⁺ ion. This is ascribed to predissociation of vibrational levels of higher H₂ Rydberg states close above then=2 dissociation limit by those H₂ Rydberg states which separate into H+H(2p) fragments. Only direct transitions into the continuum of theGK ¹ ∑ _g ⁺ state may compete. Some structure at 0.3–0.5 eV is attributed to the three statesI ¹ П _g,i ³ П _g, andh ³ bE _g ⁺ having potential barriers of this height. The distributions for H⁺+H(2p) have maxima at 3.4, 3.8, and 4.2 eV for a H₂, Ar, and He target, respectively, indicating that the 2pπ _u state as well as the 3dσ _g state ofH ₂ ⁺ is excited. The H+H(2p) process has a greater probability than the H⁺+H(2p) process for Ar and H₂ targets, though all electronic H₂ states under consideration are bound.     

Stoßdissoziation von H 2 + - und D 2 + -Ionen

The angular and energy distribution of protons produced by collision-induced dissociations of H ₂ ⁺ ions with energies of 10 and 20 keV were measured in a parabola spectrograph. From these measurements the velocity distribution of the protons in the center of mass system of the H ₂ ⁺ ion can be calculated. This gives information about the type, the abundance, and the anisotropy of the processes involved. The most frequent transitions leading to dissociations are the excitation of the 2pσ_u state, the ionisation of the H ₂ ⁺ ion, the transition into the vibrational continuum, and the electron capture into the 1³ σ _u ⁺ state of the hydrogen. It is shown that the cross section for an electronic transition depends on the velocity of the ion, the distance of the nuclei in the ion, the angle between the internuclear axis and the direction of the primary ion beam, and the excitation energy of the target. The fraction of protons produced by vibrational excitation increases with increasing atomic number of the target. Concerning electronic transitions D ₂ ⁺ ions equal H ₂ ⁺ ions of the same velocity.     

Collision-induced dissociation of H 3 + ions

For collision energies between 100 and 500 eV the collision induced dissociation of H ₃ ⁺ colliding with H₂, He and Kr gas targets was measured. We obtained total cross sections and angular distributions of the charged collision fragments for the main reaction channels. H ₃ ⁺ +H₂→H⁺+2H² and H ₃ ⁺ +H₂→H ₂ ⁺ +H₂+H. An analysis of the kinetics yields that the dissociation proceeds via vibrational-rotational excitation of H ₃ ⁺ by mutually induced dipolmoments.     

Ionization and dissociative ionization of methane molecules

A technique for mass-spectrometric investigation of the yield of positive ions produced by direct and electron-impact dissociative ionization of methane molecules is described, and respective experimental data are presented. Doubly charged C ₂ ⁺ , CH ₃ ²⁺ , and CH ₄ ²⁺ ions, as well as singly charged D ₂ ⁺ , CD ₃ ⁺ , and CD ₄ ⁺ ions, are detected in the mass spectrum of a methane molecule at electron energy U _e = 90 eV for the first time. From ionization efficiency curves, the ionization energy of the parent molecule and the appearance energy of fragment ions are determined. The ionization energy of the CH₄ molecule is found to be 12.62 ± 0.20 eV. Electron-molecular reactions that may take place when a low-energy electron beam interacts with a methane molecule are analyzed. The ionization process and the formation of methane molecule fragments are studied.     

Molecular compton profiles from inelastic atom-electron scattering

Recent measurements of the electron loss peak from H ₂ ⁺ ions impinging on Ar have been analyzed in terms of projectile Compton profiles (CP). It is shown that projected momentum distributions, i.e. CP's, can be determined for the molecular ion H ₂ ⁺ .     

Symmetric charge-exchange reactions of N 2 + -ions investigated by laser-induced fluorescence

The symmetric charge-exchange reaction of fast N ₂ ⁺ ions (U _ion=800 eV) has been investigated by the method of laser-induced fluorescence. The distinction between the primary N ₂ ⁺ ion beam and the N ₂ ⁺ ions formed by charge-exchange was possible because of the different velocities of the two species which resulted in two spectral distributions separated by the Doppler-shift. Simulation techniques have been used in order to obtain rotation-vibration distributions for both the inelastically scattered fast ions and the charge-exchange products. The rotation distributions could be described by a temperature of 1,000 K for the inelastic and 300 K for the charge-exchange process. The latter process exhibits, however, a significant vibrational excitation which is attributed to the existence of a stable intermediate complex of N ₄ ⁺ . The preferred geometry for the intermediate collision complex is assumed to be non-planar.     

Radiative capture of deuterons in154Sm and238U

Angular — and velocity — distributions of electrons emitted near zero degree from projectile ions (H⁺, H ₂ ⁺ , H ₃ ⁺ , He⁺, He⁺⁺ at 1.7 MeV/u) traversing solid carbon foils (2 to 20) μg/cm²) have been measured. The data give clear evidence of production mechanisms which go beyond those reported elsewhere.     

Near resonance charge transfer from a laser excited donor state

A laser induced charge transfer from Na(3p) to hydrogen-ions at different kinetic energies has been studied. An enhancement factor of about 90% in the production rate of Lyman-α radiation has been demonstrated to occur only with H⁺ and Na(3p) atoms reaction; no enhancement was observed with either H ₂ ⁺ or H ₃ ⁺ ions. Absolute cross-section for the production of Lyman-α radiation during the collision of H⁺, H ₂ ⁺ and H ₃ ⁺ ions and Na(3p) and Na(3s) atoms have been measured in the energy range 1–600eV. The charge exchange reactions involving hydrogen-ions and Na(3p) atoms created by two different methods have also been compared.     

Dissociative excitation and ionisation in H 2 + — He collisions

Using a flight-time-difference method we measured the energy distributions of H-H⁺ and H⁺-H⁺ fragment pairs arising from collisional dissociation of 10keV H ₂ ⁺ ions incident on He. The distributions measured at an angle of 0.28° with respect to the beam direction have maxima at 4.3eV for H-H⁺ and at 8.7eV for H⁺-H⁺ in the center-of-mass frame. The sharp maximum of the angle distribution in the CM frame at right angles to the beam direction observed in experiments in which only protons are detected does not appear in this coincidence experiment. This supports an earlier assumption that this sharp maximum can be ascribed to dissociation by vibrational-rotational excitation.     

Features of the variation in reflection spectra of the F4MB/Al coating under bombardment by H+, H 2 + and H 3 + ions

Variations of reflection spectra for the F4MB/Al coating, based on fluoroplastic film, occurring under bombardment by H⁺, H ₂ ⁺ , and H ₃ ⁺ , ions with energy 15 keV are investigated. It is established that the variations in the reflection spectra and integrated absorption coefficient are not proportional to the mass of the hydrogen ion and obey the different regularities depending on the energy of the absorption band.     

Determination of the ground state energies of the H 2 + , D 2 + and H 2 + molecular ions taking into account relativistic corrections

On the basis of determination of the asymptotic behavior of correlation functions of the corresponding field currents with the corresponding quantum numbers an analytic method for determination of the energy spectrum of three-body Coulomb system is suggested. Our results show that the constituent masses of particles, which we have defined as masses of particles in a bound state, differ from masses of particles in a free-state. The constituent mass to the free state mass relation for the electron is greater than the same mass relation for the proton, deuteron and triton. It was also found that this constituent electron mass has different values in each systems, i.e. in H ₂ ⁺ , D ₂ ⁺ and T ₂ ⁺ hydrogen molecular ions. The contributions of exchange and self-energy diagrams were taken into account in the determination of the energy spectrum of the three-body Coulomb system. Our results show that the self-energy diagram contribution is inversely proportional to the square of the constituent mass of particles. This contribution is sufficient for the electron and is negligible for the proton, deuteron and triton. When defining the energy and the wave function (WF), it is necessary to take into account the contributions of both the exchange and self-energy diagrams.     

Center-of-mass deflections in two processes: Collision-induced dissociations of 10 keV H 2 + ions in their electronic ground state 1sσ g and dissociative electron capture

By means of a flight-time difference apparatus the two collision induced processes of dissociative vibrational excitations $$H_2^ + (10keV,1s\sigma _g ) + target \to (H_2^ + )*(1s\sigma _g ) \to H^ + + H(1s)$$ and $$H_2^ + (10keV,1s\sigma _g ) + target \to (H_2 )*(unspecified) \to H + H$$ have been determined from an asymmetry in the lab-frame resulting from deflections of the centers-of-mass of the incident projectiles. H⁺+H velocity distributions from vibrational excitations of the electronic ground state of the H ₂ ⁺ ions will be presented which have been directly measured. Competition of any of the higher lying electronically excited states of the projectiles is estimated to be negligible. In the case of dissociative electron capture combined with simultaneous deflections of the centers-of-mass the electronic states of the intermediate H₂ molecules and of the H fragments could not be specified.     

The H—H+, H+—H+ and H—H fragmentation in the collision-induced dissociation of H 2 + ions

The collision-induced dissociation of 10 keV H ₂ ⁺ ions incident on H₂ has been studied in this paper. Total cross sections and angle distributions for the H⁺-H, H⁺-H⁺ and H-H fragmentations are given. Distributions over both angle and velocity have been measured for the H-H⁺ and H-H fragmentation.     

Mass spectrometer studies of the ion composition in the pink afterglow

The time dependences of the ion number densities in the pink afterglow of nitrogen, as represented by the ion wall currents, have been measured. The ions were extracted through an orifice from a flow system and analysed by a quadrupol mass spectrometer. It has been found thatN ₂ ⁺ ions are dominating in the early afterglow. With the beginning of the ionization processN ₃ ⁺ and at pressure >6 TorrN ₄ ⁺ become the majority ions. The ratio of the number densities(N ₃ ⁺ )/(N ₂ ⁺ ) reaches a maximum during the increase of the ionization processes before the maximum is reached. This behaviour suggestsN ₃ ⁺ ions to be the primary ions created by the ionization processes. The time dependences of the ion number densities (N ₂ ⁺ ), (N ₃ ⁺ ) and(N ₄ ⁺ ) are found to vary similar, showing that the ions are strongly coupled by conversion processes. The conversion processes are discussed. In the maximum of ionization at a total pressure of 4.4 Torr the ratios of the number densities of the afterglow ions (N ₂ ⁺ )∶(N ₃ ⁺ )∶(N ₄ ⁺) are 1∶1.9∶0.64.     

Relative Anfangsbesetzungszahlen der Zustände 2p und 3d folienangeregter Wasserstoffatome

The Lyman-α radiation of fast moving hydrogen atoms excited by a thin carbon foil is measured using a channel electron multiplier with a Mg F₂ filter. The photonintensity-versus-distance curve exhibits the 2p (1.596 ns) decay followed by a cascade tail originating mainly from the 3d level (15.47 ns). The data is analyzed by a computer program using the known mean lives of the 2p, 3d, and 3s states and yields the relative initial state populationY=N _2p ⁰ N _{3 d} ⁰ which depends on the kind of incident ions (H⁺ or H₂ ⁺). For H₂ ⁺ ions, excited with a single foil, there might be a slight dependence ofY on the beam velocity, but for H⁺ ions incident on the foil, no such dependence is found.     

Production and loss of N 2 + , Ne+ and Ne 2 + during the decay period of plasmas produced in neon-nitrogen mixtures

Studies of the time dependencies of the number density of N ₂ ⁺ , Ne⁺ and Ne ₂ ⁺ ions have been made during the decay period of plasmas produced in neon containing various concentrations of nitrogen molecules. Reaction rate constants were obtained for N ₂ ⁺ +N₂+Ne→N ₄ ⁺ +Ne((1.2±0.2)×10^?29 cm⁶ sec^?1) and Ne⁺+N₂→N ₂ ⁺ + Ne ((2.9±0.3) × 10^?12 cm³ sec^?1). The ambipolar diffusion coefficient of N ₂ ⁺ in neon was found to beD _a p _o =350±20 cm² sec^?1 Torr.     

Comparison of electron spin polarization of P 870 + Qa a − observed in Zn2+-containing and Fe2+-depleted bacterial reaction centers

Recently, a general model has been developed to explain electron spin polarized (ESP) electron paramagnetic resonance (EPR) signals found in systems where radical pairs are formed sequentially. The photosynthetic bacterial reaction center (RC) is such a system in which we can experimentally vary parameters (lifetime, structure, and magnetic interactions in the sequentially formed radical pairs) that affect ESP development in order to test this model. In Fe²⁺-depleted transfer step from intermediate radical pair, P ₈₇₀ ⁺ Q _a ^? which is produced in an electron transfer step from intermediate radical pair, P ₈₇₀ ⁺ I^?. (P ₈₇₀ ⁺ is the oxidized primary donor, a special pair of bacteriochlorophyll molecules, I^? is the reduced bacteriopheophytin acceptor, and Q _a ^? is the reduced primary quinone acceptor.) The lifetime of P ₈₇₀ ⁺ I^? can be shortened relative to the lifetime of P ₈₇₀ ⁺ I^? in Fe²⁺-depleted RCs by substitution of Zn²⁺. We report the first observation of X-band and Q-band ESP EPR signals due to P ₈₇₀ ⁺ Q^? from bacterial reaction centers that contain Zn²⁺. Comparison of these signals to those observed from Fe²⁺-depleted bacterial reaction centers shows intensity differences and g-factor shifts. The results are discussed in terms of the general sequential radical pair model.     

Gyromagnetic ratios of low-lying rotational states in156, 158, 160Gd

Transient-field precessions were measured simultaneously for levels in the ground-state bands of^{156, 158, 160}Gd as ions of these nuclei traversed a thin polarized Fe foil. Relative g-factors of levels up to 6 ₁ ⁺ were deduced, those of the 4 ₁ ⁺ levels being determined with greatest precision. In contrast with the conclusions of the recent report by Alzner et al. [1], our results are consistent with g(4 ₁ ⁺ ) having the same value in all three isotopes and imply g(2 ₁ ⁺ )=g(4 ₁ ⁺ ) in¹⁵⁶Gd, consistent with nuclear structure models.     

Excitation of bands of the first negative system of the N 2 + ion in collisions of N+ and O+ ions with N2 molecules

Absolute values of the excitation cross sections of the (0,0) bands [for O⁺(⁴ S), O⁺(² P)-N₂ pairs] and the (0,0), (0,1), (1,2), and (2,3) bands [for N⁺(³ P)-N₂ pairs] of the first negative system of the N ₂ ⁺ ion have been measured in collisions with nitrogen molecules of nitrogen and oxygen ions in the ground state and in a metastable state in the interval of ion energies 1–10 keV. The process of excitation of the (0,0) band of the first negative system of the N ₂ ⁺ ion by oxygen ions in the metastable ² P state is of a quasi-resonant character. The presence in the beam of ions in metastable states was monitored by measuring the excitation efficiency of the (0,0) band λ3914 Å of the N ₂ ⁺ ion in different operating regimes of the highfrequency ion source. For N⁺ ions in the ³ P ground state, as the collision frequency is decreased the relative vibrational population of the v′=1 and v′=2 levels of the B ²Σ _u ⁺ state of the N ₂ ⁺ ion is observed to deviate strongly from the value calculated in the Franck-Condon model.     

Experimental study of the energy dependence of the reaction rate coefficients of the reaction CH 3 + with N2O and NH3

The kinetics of the reaction of CH ₃ ⁺ ions with N₂O and NH₃ has been investigated in He and Ar buffers using Selected Ion Flow Drift Tube technique (SIFDT). Both studied reactions proceed with nearly collisional rate at near thermal energies. The rate coefficient of the reaction of CH ₃ ⁺ with N₂O is decreasing more than one order of magnitude (from 1.2×10^?9cm³s^?1 up to 8×10^?11cm³s^?1) with reactant ion/reactant neutral average centre-of-mass kinetic energy (E _r) increasing from near thermal up to 2 eV. The dominant product of this reaction is the isomer HCO⁺ (≥94%) and the minor product is CH₃O⁺. The reaction of CH ₃ ⁺ with NH₃ has two binary channels with the dominant product ion CH₂NH ₂ ⁺ (>70%) and the minor product ion NH ₄ ⁺ (≈10%) and three body association channel with product CH₃NH ₃ ⁺ (≈20% at 0.32 Torr). The rate coefficient of this reaction is decreasing nearly by one order of magnitude with increasingE _r from near thermal up to 0.8eV, forE _r>0.8 eV the rate coefficient increases with increasingE _r. The experimental results are interpreted in terms of a simple model assuming the reactions to proceed via the formation of long-lived collision complexes.