Displacement of metal atoms from salts by hydrogen and the role of this reaction in chain processes

The interaction of hydrogen atoms with a variety of alkali metal and alkaline-earth metal salts results not only in the recombination of these atoms but also in the displacement, into the gas phase, of free radicals (CaCl^·(A ¹ P _1/2, B ² S ⁺) and CaF^·(A ² P)) and metal atoms, including their excited species, which are detected spectroscopically. Transmission spectra indicate that the NaCl surface undergoes metallization when treated with a high-frequency discharge and a rarefied hydrogen flame. Combustion is affected by the gas-phase hydrogen atoms involved in the chain reaction and by the varying composition and properties of the surface. The concentration of Na atoms over the NaCl surface at 770 K is 10⁹?10¹¹ cm^?3 in a stream of H atoms at 1 Torr and in the 2H₂ + O₂ flame at 4 Torr. The concentration of sodium atoms in the ² P _3/2 and ² P _1/2 excited states is ～5 × 10⁶?5 × 10⁸ cm^?3. The role of the discovered reactions in combustion, pyrolysis, and plasma chemistry is discussed.     

Spin dependence of low energy charge exchange between H 2 + and Na

The difference in charge exchange rate in collisions between spin oriented sodium atoms and H ₂ ⁺ ions has been measured at an energy of about 1 eV. H ₂ ⁺ was stored in a Penning trap and polarized by spin exchange with Na beam atoms from a hexapole magnet. The ion loss from the trap due to charge exchange was different as we depolarized the atomic beam. From the data we obtain a ratio of cross sections for singlet and triplet collisionsQ ₁/Q ₃=1.5±0.2 andQ ₃=1.2·10^?15 cm².     

The energy levels of hydrogen isotopes mesic molecular complexes

A method to calculate isotope effects in diatomic molecules is developed. The energy levels of hydrogen isotopes and mesic molecular complexes which are used for calculation ofddμ anddtμ mesic molecule resonant formation rates are found. The obtained values agree with the experimental data within ～ 5·10^?4 eV accuracy.     

Quasi-resonant energy transfer in collisions: Na2(A1Σ u + )+K(4S)

Cross sections for electron energy transfer from the initial rotational stateJ′of the two lowest vibrational levelsv′=0 andv′=1 of excited dimers Na₂(A) to potassium atoms as described by Na₂(A¹Σ _u ⁺ ,v′J′)+K(4S)→Na₂ (X¹Σ _g ⁺ ,v″J″)+K(4P)+ΔE have been examined by laser-induced fluorescence. A strong increase of the cross section by as much as an order of magnitude has been observed for those dimerv′J′-levels for which the dipole transitions are close to resonance of the 4S-4P transitions in the atom (ΔE<4 cm^?1). The absolute cross sections for energy transfer have been calculated by the Rabitz approximation of first-order perturbation theory. In the case of closest energy resonance (ΔE=0.9 cm^?1) the cross section is Q=7.8×10^?13 cm².     

CS(lav) and computational study of degeneracy averaged differerential cross sections and Δm-integral cross sections: Hez.sbndCO,HDz.sbnd

CC and l-average CS calculations of degeneracy averaged differential cross sections and Δm-integral cross sections have been performed for Hez.sbndCO at E = 60 cm^?1 and E = 80 cm^?1, for HDz.sbndNe at E = 254 cm^?1, and for Hez.sbndH₂ at E = 1520 cm^?1. The l_avz.sbndCS degeneracy averaged differential cross sections are generally in good agreement with the CC cross sections. The previously observed shifts in the diffraction oscillations for odd rotationally inelastic transitions for Hez.sbndCO and HDz.sbndNe do not occur due to proper phase choice and l? = l_av choice rather than l? = 1 or l′. The l_avz.sbndCS approximation gives reliable results for most Δm-integral cross sections except for those σ_cs(jm, jm′) cross sections for which the CC cross sections σ(jm;jm′) and σ(jm′;jm) differ by a large amount.     

Spin exchange in low energy electron-ion collisions

The elastic scattering of an electron by a hydrogenic ion of charge (Z-1) is investigated. For scattering energiesE _kin in the eV region, where the Bohr-Sommerfeld parametery is large compared to one, the cross section for spin exchange of the electrons is found to be proportional toZ ^?2 andE ^?1 _kin. Spin exchange is dominated by radiative electron capture forZ>25.     

Dissociation of ethane by electron impact

The absolute total dissociation cross section for ethane is reported for electron energies between 10 and 600 eV. A maximum value of 7.6 × 10^?16 cm² occurs at 80 eV while the apparent threshold is ≈ 10 eV. Dissociative ionization is more probable than dissociation into neutral fragments at all energies except in the threshold region. The data indicates that fragmentation involving methane elimination (c^? + C₂ H₆ → e^? + CH₄ + CH₂) occurs in less than 2% of the dissociative events for 50 < E < 600 eV. Arguments are presented which suggest that some of the lower excited states of ethane are stable against dissociation.     

Fine structure transition cross sections for several alkali + rare gas systems

The energy dependence, E_c.m. </ 0.2 eV, of the inelastic total cross sections for the ²P_{$\text{1}\text{2}$} → ²P_{$\text{3}\text{2}$} fine structure transition of the lowest excited states of the alkali atoms are calculated for the following systems: Na, K, Rb + He, Ne, Ar and Cs + He. Encouraging agreement between theory and experiment is obtained.     

QCT study of isotopic effects in H + HBr abstraction and exchange reactions

The method of quasi-classical trajectories on an LEPS hypersurface was used for studying the influence of the exchange of one or both of the hydrogen atoms for deuterium in the reaction H¹ + H²Br. As expected, the reaction cross sections of the exchange and abstraction reactions were found to increase if H¹ was replaced by D and decrease if H² was replaced by deuterium. A similar change in the reaction cross sections have also been observed for vibrationally excited reactants. The distribution of vibrational (rotational) energy is related to the ω_e (B_e) values of the respective reactants and products.     

Electron and positron atomic elastic scattering cross sections

A method was developed to calculate the total and differential elastic-scattering cross sections for incident electrons and positrons in the energy range from $\text{0.01}\text{eV}$ to $\text{1}\text{MeV}$ for atoms of Z=1–100. For electrons, hydrogen, helium, nitrogen, oxygen, krypton, and xenon, and for positrons, helium, neon, and argon atoms were considered for comparison with experimental data.First, the variationally optimized atomic static potentials were calculated for each atom by solving the Dirac equations for bound electron states. Second, the Dirac equations for a free electron or positron are solved for an atom using the previously calculated static potential accomplished (in the case of electrons) by “adjusted” Hara's exchange potential for a free-state particle. Additional to the exchange effects, the charge cloud polarization effects are considered applying the correlation-polarization potential of O'Connell and Lane (with correction of Padial and Norcross) for incident electrons, and of Jain for incident positrons.The total, cutoff and differential elastic-scattering cross sections are calculated for incident electrons and positrons with the help of the relativistic partial wave analysis. The solid state effects for scattering in solids are described by means of a muffin-tin model, i.e. the potentials of neighboring atoms are superpositioned in such a way that the resulting potential and its derivative are zero in the middle distance between the atoms. The potential of isolated atom is calculated up to the radius at which the long-range polarization potential becomes a value of −10⁻⁸.     

Collision cross sections for excitation energy transfer in Na* (3P 1/2)+K(4S 1/2)⇔Na*(3P 3/2)+K(4S 1/2) processes

The cross sections for the excitation energy transfer between the 3² P _J states of sodium atoms by collisions with ground-state potassium atoms have been measured by resonant Doppler-free two-photon spectroscopy, where the population densities of directly pumped and collisionally excited Na(3P _J )(J=1/2, 3/2) levels were probed by counter-propagating Na(3P _J ) → Na(4D _{3/2, 5/2}) excitation and detected with the thermionic diode. Cross sections of σ(3P _1/2 → 3P _3/2)=190 Ų±20% and σ(3P _3/2 → 3P _1/2)=100 Ų±20% were found. The theoretical calculations taking into account the long-range interaction termsR ^?6,R ^?8 andR ^?10 yield a value σ(3P _1/2 → 3P _3/2)=165 Ų. On the basis of these long-range interaction potentials the differential cross section has been calculated and compared with recently published experimental data. Very good agreement between the theoretical and experimental data was found.     

Mode-to-mode vibrational energy flow in S1 benzene

Resolved fluorescence spectra from low pressures of benzene with nine added gases have been used to follow mode-to-mode vibrational relaxation in the S₁ state of benzene under “single-collision” conditions. Cw pumping of the S₁ fundamental 6¹ (ν″₆ = 522 cm^?1) allows study of collisional vibrational energy flow into each of four channels. Two channels consist of flow into single levels, and the others represent flow into unresolved pairs of levels. The mode-to-mode cross sections are much larger than those usually observed in ground electronic states, being near gas kinetic even for partners transferring energy by VT, R processes alone. The mode-to-mode transfer has highly specific patterns, with roughly seventy percent of the transfer going into the four channels in spite of many other nearby levels. The largest cross sections are always to a level 237 cm^?1 above the initial level rather than to a level nearly resonant (ΔE = 7 cm^?1) with the initia l level. A common pattern of flow occurs for the four gases transferring energy by VT, R processes alone, and another common pattern is established for the five gases which can also use VV transfers. With the exception of one channel, VV resonances with vibrationally complex partners increase cross sections by less than a factor of two over that provided by the VT, R path. VV transfers have a similarly small effect on the overall vibrational relaxation rate out of the initial level 6¹. Both the flow patterns and the VV versus VT, R competitions are accounted for with an extremely simple and general set of propensity rules taken directly from SSH calculations made by others for vibrational relaxation in ground electronic states. The rules are based on the degeneracies of the final levels, the number of vibrational quantum changes, and the amount of energy exchanged between vibrational and translational/rotational degrees of freedom. The rules seem general to relaxation in both ground and excited electronic states, whereas large cross sections seem a special property of the excited state. The cross sections for collision partners SF₆ and perfluorohexane are small relative to those for other partners with similar vibrational complexity and mass.     

Energy transfer in the course of triplet state quenching of aromatic hydrocarbons by nitroxyl radicals

Quenching of triplet states of aromatic hydrocarbons by nitroxyl radicals has been investigated by the flash photolysis method. There are two different mechanisms of triplet quenching: quenching occurs via enhanced intersystem crossing on exchange interaction with the radical for the triplet states of aromatic hydrocarbons which have low triplet energy (E_T < 14700 cm^?1); for very high triplet energies, energy transfer from the triplet molecule to the nitroxyl radical occurs. The energy of the excited nitroxyl radical was estimated to be 18000 cm^?1.     

Excitation and charge transfer in He++H collisions

For the conflictive case of He⁺+H collisions, we present a norm-method optimization of the parameters included in the (often used) two-electron translation factor of Errea et al. As surmised in a previous publication, a strong cut-off is needed at short internuclear distances to prevent the translation factor from marring the properities of the molecular expansion there. With a basis of 16 molecular states, we present the first calculations including translation factors, of total and partial charge exchange and excitation cross sections in He⁺+H collisions, as well as the alignment parameter A₂₀ for hydrogen excitation. Good agreement with experiment is reached up to the energy range where ionization and charge exchange cross sections are comparable.     

Studies of oxygen transport mechanism in electrolytes based on doped lanthanum silicate with apatite structure using techniques of oxygen isotopic heteroexchange and impedance spectroscopy

The work presents the results of studying the mechanism of oxygen transport for a new promising class of oxygen-containing electrolytes based on lanthanum silicate with an apatite structure using impedance spectroscopy and isotopic oxygen heteroexchange. At 1000 K, in the case of samples with an optimum composition including codoped Fe and Al, σ ～ 3 × 10^?3 to 10^?2 S/cm and D* reaches ～10^?8 cm²/s, which is close to the values of YSZ and Ce_0.9Gd_0.1O_{2 ? δ} (GDC). Lower energies of conductivity activation and oxygen diffusion for doped apatites (～0.5–0.8 eV instead of ～1 eV for GDC) and also equivalence as regards exchange of all oxygen atoms within apatite agree with the model, in which oxygen mobility is determined by a nonlinear cooperative migration process of oxygen atoms with fast exchange between interstitial and regular sites.     

A study of the XeF molecule by atomic beam scattering with magnetic analysis

Absolute total cross sections for scattering of fluorine atoms by xenon have been measured in the 0.9–2.0 km s^? velocity range. Information is obtained on the long-range interaction and, using a technique for magnetic analysis of substates of F atoms, a characterization is given for the bonding in the two lowest excited states of XeF.     

Excimer formation mechanism in gaseous krypton and Kr/N2 mixtures

A new approach for the understanding of the energy relaxation dynamics of excited atoms involving a long-lived molecular precursor is presented here for krypton. Excitation of the gas close to the 5s[3/2]₂ metastable atomic level (E _at. ?E _exc.<kT) is achieved with an intense VUV laser source (I ≈ 10¹² photon/pulse) realized by resonantly enhanced 4-wave mixing (2ω₁ + ω₂) in room temperature mercury vapor (N _Hg ≈ 10¹³ at./cm³). The decay of the II. continuum luminescence (145 nm) is studied. In the pressure range 200–500 mbar, decay rates depend linearly on pressure but have a negative zero-pressure intersect. We show here that this result can be understood as an effect of the exchange of energy between two different “reservoirs” of atomic (5s[3/2]₂) and molecular (1_g) nature, and can be an inherent peculiarity of the recombination kinetics of excited atoms with several product channels. The efficiency of the model is checked for the Kr/N₂ system. Rate constants for relaxation processes are determined in pure krypton and in Kr/N₂ mixtures.     

Collision spectroscopy with aligned and oriented atoms

Neutralisation processes in 0.15–1.5 keV collisions of H^? with Na atoms in the 3s ground state or in the excited 3p state have been investigated by means of time-of-flight analysis of the neutral H atoms produced. The H^? - Na(3p) system, investigated here for the first time, is particularly interesting since the entrance channel is embedded in the [H - Na(3s)] +e ^? continuum, enabling Penning detachment to occur. The measured relative neutralisation cross section ratios σ(3p)/σ(3s) decrease from 3 to 1.6 with increasing energy. Based on earlier published results for σ(3s), σ(3p) total cross sections exceeding 100 Ų are estimated.     

Collisional ionization of highly excited neon atoms by nonpolar hydrocarbons

When benzene, acetylene and ethylene were allowed to collide with Ne Rydberg atoms (17?n?40), significant Ne⁺ ions were observed. Their cross sections were estimated, with reference to the ionization by H₂O, to be as large as 10^?14–10^?15 cm². However, no Ne⁺ signals were observed when ethane, methane and N₂ were used as targets. Theoretical estimates of the cross sections for ionization by interaction of the quadrupole moment and polarizability of the first three molecules are several orders of magnitude smaller than the experimental cross sections given above. A similarity of this phenomenon to the scattering of a thermal electron by benzene etc., observed by Christophorou et al., is suggested.     

Charge transfer in 1.4 MeV/u Ni q+→Kr collisions,q=16−22

Cross sections for the electron transfer from neutral Kr atoms to highly charged Ni ^q+ projectiles were measured at 1.4 MeV/u. Estimates on the contributions of single and double capture into the Ni-L shell and into excited states are extracted from measured cross sections for the Ni-L x-ray emission for incoming projectileL vacancies (q≧19). The role of metastable states produced in the capture process is analysed. We find that most of the capture directly populates the Ni-L shell, and the capture into excited states seems to be dominated by two-electron capture. The electrons involved in double transfer processes seem to be strongly correlated. An increase in the low-probability non-correlated double transfer with increasing number of incomingL-vacancies is indicated by the data.