Opto-galvanic spectroscopy of a neon discharge: Mechanism studies

Irradiation of gaseous discharges with a tunable laser produces easily observed voltage changes at wavelengths which correspond to electronic transitions for species in the discharge. This work reports observations made in the 572–654 nm region on a neon glow discharge, which is operated at 5 torr (670 Pa) pressure and 2–25 mA current. A wide variety of absorption transitions have been detected which originate in the 1s_n (³P_2,1,0, ¹P₁) and 2p_n excited states. The resulting voltage signals are both positive and negative, and some transitions exhibit sign changes as a function of discharge current. Some of the results can be interpreted in terms of processes which directly affect the neon metastable atom concentration (³P₂, ³P₀) and states also produce voltage changes which can be explained in terms of processes which either enhance or decrease ionization.     

Radiative and nonradiative lifetimes in excited states of Ar,Kr and Xe atoms in a neon matrix

Synchrotron radiation with its intense continuum and its excellent time structure has been exploited for time resolved luminescence spectroscopy in the solid state. By selective excitation of n = 1, n′ = 2 exciton states of Xe, Kr and Ar atoms in a neon matrix we were able to identify the emitting states involved. Lifetimes within the cascade of radiative and radiationless relaxation between excited states as well as the radiative lifetimes for transitions to the ground state have been derived from the decay curves. Energy positions and radiative lifetimes of the emitting states correspond quite well with those of the free atoms. Radiative and radiationless relaxation processes take place within the manifold of excited states of the guest atoms. The rate constants for radiationless decay confirm an energy gap law. The order of the radiationless processes reaches in some cases extremely high values. Selection rules for spin and angular momentum are essential to understand the observed radiationless transition rates.     

Many-orbital cluster expansion for the exchange-repulsion energy in the interaction of closed-shell systems

The energy of exchange repulsion between two closed-shell systems described by determinantal wave functions has been represented as a sum of contributions arising from the interaction of two, three and four orbitals at a time. These contributions have been calculated for the interaction of two neon atoms. It has been found that in the van der Waals minimum region the two-orbital components are of secondary importance and that about 90% of the total exchange energy originates from the three-orbital interactions ofL-shell electrons. The four-orbital as well as the double-exchange terms have been found negligible. The approximate algorithms for evaluation of the exchange repulsion energy have been tested and discussed.This work was partly supported by the Polish Academy of Sciences within the project MR-I.9.     

MO-radiation interference phenomena as a “clock” for nuclear reaction times

Molecular orbitalK x-rays emitted during Pb+Pb collisions are investigated. Two results are presented: A slight filling up of the minimum observed in the differential radiation cross-section is given by radiative transitions from theM-shell to theL-shell. The minima are shifted with increasing nuclear contact times to higher photon energies. The observation of the shift allows the estimation of the sticking time.     

Vibrational relaxation and collision-induced dissociation of xenon fluoride by neon

Rate coefficients were calculated for vibrational relaxation and collision-induced dissociation of ground state xenon fluoride in neon at temperatures between 300 and 1000 K for each of nine vibrational levels. These coefficients were calculated using a pairwise additive potential energy surface, which consists of a Morse function for the XeF interaction and Lennard–Jones functions for the NeXe and NeF interactions. Rate coefficients are provided for both temperature and v- dependences. The vibrational relaxation and dissociation processes occur by multiquanta transitions. Dissociation can take place from all v-levels provided that the internal energy of the XeF molecule is close to the rotationless dissociation limit. The order of increase effectiveness of the various forms of energy in promoting dissociation in XeF was found to be translation–rotation-vibration. At room temperature, neon atoms were found to be more efficient than helium atoms in the dissociation processes; helium atoms were found to be more efficient than neon atoms in the vibrational relaxation of XeF. Strong vibration–rotation coupling in both vibrational relaxation and in the dissociation processes is demonstrated.     

Infrared spectra of ClCN+, ClNC+, and BrCN+ trapped in solid neon

When a mixture of ClCN or BrCN with a large excess of neon is codeposited at 4.3 K with a beam of neon atoms that have been excited in a microwave discharge, the infrared spectrum of the resulting solid includes prominent absorptions of the uncharged isocyanide, ClNC or BrNC, and of the corresponding cation, ClCN+ or BrCN+. The NC-stretching fundamentals of the isocyanides trapped in solid neon lie close to the positions for their previously reported argon-matrix counterparts. The CN-stretching absorptions of ClCN+ and BrCN+ and the CCl-stretching absorption of ClCN+ appear very close to the gas-phase band centers. Absorptions of two overtones and one combination band of ClCN+ are identified. Reversible photoisomerization of ClCN+ to ClNC+ occurs. The two stretching vibrational fundamentals and several infrared and near infrared absorptions associated with electronic transitions of ClNC+ are observed. Minor infrared peaks are attributed to the vibrational fundamental absorptions of the CX and CX(+) species (X=Cl,Br).     

ChemInform Abstract: Infrared Spectroscopic and Theoretical Investigations of the OUF2 and OThF2 Molecules with Triple Oxo Bond Character.

The title molecules are prepared by reaction of OF₂ molecules with laser ablated uranium and thorium atoms in solid argon and neon.     

Neon‐Bearing Ammonium Metal Formates: Formation and Behaviour under Pressure

The incorporation of noble gas atoms, in particular neon, into the pores of network structures is very challenging due to the weak interactions they experience with the network solid. Using high‐pressure single‐crystal X‐ray diffraction, we demonstrate that neon atoms enter into the extended network of ammonium metal formates, thus forming compounds Ne_x[NH₄][M(HCOO)₃]. This phenomenon modifies the compressional and structural behaviours of the ammonium metal formates under pressure. The neon atoms can be clearly localised within the centre of [M(HCOO)₃]₅ cages and the total saturation of this site is achieved after ～1.5 GPa. We find that by using argon as the pressure‐transmitting medium, the inclusion inside [NH₄][M(HCOO)₃] is inhibited due to the larger size of the argon. This study illustrates the size selectivity of [NH₄][M(HCOO)₃] compounds between neon and argon insertion under pressure, and the effect of inclusion on the high‐pressure behaviour of neon‐bearing ammonium metal formates.     

Odd-even effect in melting properties of 12 alkane-α,ω-diamides

Fusion and solid-to-solid transitions of a homologous series of 12 linear alkane-α,ω-diamides H₂NCO-(CH₂)_(n−2)-CONH₂, where n = (2 to 12 and 14), were investigated by differential scanning calorimetry (d.s.c.). The temperatures of fusion of even terms decreased from T_fus ≈ 572 K to about 460 K, whereas those of odd terms remained substantially constant at about 450 K. Solid-to-solid transitions were also detected for oxamide, malonamide, succinamide, adipamide, suberamide, and dodecanediamide. Regular odd-even alternation was displayed by the temperature, enthalpy, and entropy of fusion values, terms with even number of carbon atoms showing higher values than odd terms.This behaviour was attributed to different crystal packing allowing consonance between hydrogen bonding and dispersive interaction in even terms, which are characterised by multilayer structure, whereas in odd terms a strained three-dimensional network results in looser packing. Parallel alternation of densities in solid alkane-α,ω-diamides supports this interpretation. Comparison was made with literature values for temperatures, enthalpies, and entropies of fusion of isoelectronic linear alkanes, dicarboxylic acids, and alkyldiamines.     

Dynamics of Electronically Excited Metal Atoms (Zn and Cd) in Rare Gas Matrices: Simulation of Multiple-band Emission using Molecular Dynamics with Quantum Transitions

Molecular dynamics with quantum transitions approach is employed to simulate the spectroscopic characteristics of the ¹P₁↔¹S₀ transitions in atomic zinc and cadmium in order to gain insight into the excited state behavior of these atoms isolated in solid rare gases neon, argon, and krypton. The absorption and emission spectra are simulated. Non-radiative processes play a fundamental role in the transfer of population among the three electronic states initially accessed in absorption. Three distinct relaxation pathways were identified. Two of these are related to the dynamical modes described in previous works [McCaffrey and Kerins, J. Chem. Phys. 106 , 7885 (1997); Kerins and McCaffrey, J. Chem. Phys. 109 , 3131 (1998)] in which the system evolves to form a square planar configuration around the metal atom. The third distinct pathway involves motion on a hexagonal close packed plane. The temperature dependence of complex formation was also determined for the three relaxation pathways.     

NDDO MO Calculations

Siegbahn's potential model as extended by Ellison et al. is used with density matrix elements calculated by the NDDO/2 procedure, to correlate the K-shell binding energy shifts of C, N and O atoms in a few molecules containing only the first-row atoms. The correlation is not superior to that obtained with the CNDO/2 method when only the monopole term is retained in calculating the Madelung potential energy. However, the results are in excellent agreement with experiment when the two-parameters model including the dipole and quadrupole terms is used.     

Interaction second virial coefficients at low temperatures for NeKr and NeO2 systems

Published results of vapour—liquid and vapour—solid equilibrium studies for neon—krypton and neon—oxygen mixtures have been analysed to give values for the interaction second virial coefficients. B₁₂. Comparisons are made with other results for the NeKr system and with the predictions of some proposed intermolecular potentials.     

Distribution ofK α L n satellite yields for mediumZ elements bombarded with16O ions

K _α X-ray spectra were measured with a Du Mond type curves crystal spectrometer for₄₂Mo and₄₆Pd targets bombarded with oxygen ions at 5.5 MeV/u energy (i.e. at velocity closely matching those of the targetL-shell electrons). The distribution ofL-shell vacancies produced in collisions with simultaneousK-shell ionization is deduced from the measured yield distribution of theK _α X-ray satellites. The distribution shows deviations from the binomial ones. The deviations can be accounted for by assuming that theL-shell vacancy production is due to two uncorrelated processes: the direct ionization by impact and the electron capture from theL-shell of the target atom into theK-shell of the projectile, both occurring at nearly central collisions. The corresponding probability values are deduced using simple statistical considerations.     

Diorganotin halide carboxylates,thiocarboxylates and halide haloacetates

Twenty-eight diorganotin halide acetates, thioacetates, haloacetates and carboxylates, eighteen not previously reported, have been synthesized by the reaction between the diorganotin oxides and acid halides, and by transacyloxylation reactions between diorganotin halide acetates and carboxylic acids. Infrared in the 2000—200 cm^-1 range, proton and carbon-13 NMR and tin-119m Mössbauer data are interpreted in terms of acetoxy-bridged, trigonal bipyramidal tin in the solid phase with depolymerization occurring in solution where a dynamic equilibrium between diorganotin dihalide and dicarboxylate species is rapidly established. A new infrared absorption also appears at 100–125 cm^-1 to higher frequency of the v_asym(CO₂) which disappears on heating. A dimeric structure based upon four-membered Sn₂O₂ or Sn₂OX rings in which carboxylate group oxygen atoms bridge tin atoms of a second molecule leaving the CO group free is proposed. The halide thioacetates are associated in the solid state but are monomeric in solution.     

Bildungsenthalpien ternärer Lavesphasen vom MgCu2-Typ im System MgCu2MgZn2

The heats of formation of the ternary cubic Laves phases of the pseudobinary system MgCu₂MgZn₂ were determined by solution calorimetry methods. Replacing copper atoms with zinc atoms increases the negative heat of formation. This change is mainly due to the change of interatomic interactions within the copper sublattice. Relative minima appearing in the ΔH^B vs composition-curve are interpreted in terms of ordering of copper and zinc atoms on the copper sublattice.     

13C NMR spectra of some methylplatinum(IV) complexes

The ¹³C NMR spectra of some methylplatinum(IV) Schiff-base and related complexes have been recorded. The results are interpreted in terms of the differing effects of various donor atoms on a platinumcarbon bond in the trans position.     

Ab-initio-calculations of low-lying verticalK-shell excitations in silane and mono-fluoro-silane

Multi-reference configuration interaction calculations of low-lying resonantK-shell excitations in the energy range below theK-shell ionization threshold have been performed for silane and mono-fluoro-silane. The calculated vertical transition energies are compared to experimentalK-shell photoabsorption spectra recorded by use of synchrotron radiation. The general structure and the intensity ratios of the experimental spectra are reproduced quite well by the present calculations; an assignment of the most prominent spectral features is undertaken. Correlations between the lowest dipole-allowedK-shell excitations of silane and mono-fluorosilane can be recognized.     

Argon–water system at low temperatures

The molecular dynamics method is used to simulate argon solutions in water and a thin water film–argon system at low temperatures. The correlation in motions of two closely spaced argon atoms is of another nature than the correlation of two neon atoms in a neon solid solution in ice II. The structure of hydrate shells of argon atoms contains five-membered rings composed of water molecules. The solubility of argon in a water film at low temperatures is noticeably higher than at room temperature. If a water film is first cooled to the glassy state and then argon atoms are added to it, then approximately as many argon atoms are absorbed on the film surface as they are present in a cooled film in equilibrium with the argon atmosphere. Argon atoms migrate from one pit to another on the rough surface of a solid water film.     

On the nature of bulk and surface excitations in Argon clusters

Absorption spectra of Ar clusters containing between 10 and 10⁶ atoms are dominated by strong transitions into bulk and surface states. The intensity variation of bulk and surface excitations is analyzed within a model, which divides the cluster into a surface layer and into an interior part. The thickness of the surface layer is determined by the intensity ratio of bulk and surface excitations. For then=2, 2′ excitons a reasonable value ranging between the radius of then=2 exciton and the nearest neighbour distance is obtained. In case of then=1 excitons the thickness of the surface layer is much smaller than the nearest-neighbour distance indicating that then=1 surface excitons might be interpreted as excitations of atoms on the cluster surface.