Gas-phase Atoms Research Articles

Direct Hydrogenation of Dinitrogen and Dioxygen via Eley-Rideal Reactions.

Most Eley-Rideal abstraction reactions involve an energetic gas-phase atom reacting directly with a surface adsorbate to form a molecular product. Molecular projectiles are generally less reactive, may dissociate upon collision with the surface, and thus more difficult to prove that they can participate intact in abstraction reactions. Here we provide experimental evidence for direct reactions occurring between molecular N2 (+) and O2 (+) projectiles and surface-adsorbed D atoms in two steps: first, the two atoms of the diatomic molecule undergo consecutive collisions with a metal surface atom without bond rupture; and second, the rebounding molecule abstracts a surface D atom to form N2 D and O2 D intermediates, respectively, detected as ions. The kinematics of the collisional interaction confirms product formation by an Eley-Rideal reaction mechanism and accounts for inelastic energy losses commensurate with surface re-ionization. Such energetic hydrogenation of dinitrogen may provide facile activation of its triple bond as a first step towards bond cleavage.

Complex Orbitals, Multiple Local Minima, and Symmetry Breaking in Perdew-Zunger Self-Interaction Corrected Density Functional Theory Calculations.

Implentation of seminumerical stability analysis for calculations using the Perdew-Zunger self-interaction correction is described. It is shown that real-valued solutions of the Perdew-Zunger equations for gas phase atoms are unstable with respect to imaginary orbital rotations, confirming that a proper implementation of the correction requires complex-valued orbitals. The orbital density dependence of the self-interaction corrected functional is found to lead to multiple local minima in the case of the acrylic acid, H6, and benzene molecules. In the case of benzene, symmetry breaking that results in incorrect ground state geometry is found to occur, erroneously leading to alternating bond lengths in the molecule.

Structural and theoretical study of six dinuclear Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Hg(II) complexes containing ON-NO donor azodye ligand, 4,4’-(1E,1’E)-(5,5’-(propane- 2,2-diyl)bis(2-hydroxy-5,1-phenylene)bis(diazene-2,1-diyl)dibenzenesulfonic a

Six dimeric transitional metal complexes with ON-NO donor bis-bidentate azodye ligand 4,4’-(1E,1’E)-(5,5’-(propane-2,2-diyl)bis(2-hydroxy-5,1-phenylene)bis(diazene-2,1-diyl)dibenzenesulfonic acid (PDHPDDDBSA) have been synthesised and characterized by elemental, analytical, conductivity measurement, magnetic moments, spectral, thermal, X-ray diffraction (powder pattern) and theoretical methods. The Co(II) and Ni(II) complexes are found to be octahedral, Cu(II) complex is distorted-octahedral and a tetrahedral stereochemistry has been suggested to Zn(II), Cd(II) and Hg(II) complexes. The thermal analysis data provided the kinetic parameters as order of decomposition reaction, activation energy and frequency factor. All theoretical calculations of the ligand and the Cu(II) and Zn(II) complexes were made using Gaussian 03 rev. A.01 programme package hybrid DFT/B3LYP hybrid functional level of theory with LANL2DZ basic set for copper and zinc atoms in gas phase and 6-31 G(d,p) basic set for other atoms. The DFT study of the ligand and the complexes is used to calculate the bond length, bond angle, HOMO, LUMO and dipole moment values. The calculated HOMO-LUMO energy gap shows that a charge transfer occurs within the molecule. The Cd(II) complex is found to have an orthorhombic crystal system. KEY WORDS: Azodye complexes, Thermogravimetric study, Theoretical study (DFT/B3LYP) Bull. Chem. Soc. Ethiop. 2016, 30(1), 87-100. DOI: http://dx.doi.org/10.4314/bcse.v30i1.8

HIGH-TEMPERATURE IONIZATION IN PROTOPLANETARY DISKS

We calculate the abundances of electrons and ions in the hot (> 500 K), dusty parts of protoplanetary disks, treating for the first time the effects of thermionic and ion emission from the dust grains. High-temperature ionization modeling has involved simply assuming that alkali elements such as potassium occur as gas-phase atoms and are collisionally ionized following the Saha equation. We show that the Saha equation often does not hold, because free charges are produced by thermionic and ion emission and destroyed when they stick to grain surfaces. This means the ionization state depends not on the first ionization potential of the alkali atoms, but rather on the grains' work functions. The charged species' abundances typically rise abruptly above about 800 K, with little qualitative dependence on the work function, gas density, or dust-to-gas mass ratio. Applying our results, we find that protoplanetary disks' dead zone, where high diffusivities stifle magnetorotational turbulence, has its inner edge located where the temperature exceeds a threshold value ~1000 K. The threshold is set by ambipolar diffusion except at the highest densities, where it is set by Ohmic resistivity. We find that the disk gas can be diffusively loaded onto the stellar magnetosphere at temperatures below a similar threshold. We investigate whether the "short-circuit" instability of current sheets can operate in disks and find that it cannot, or works only in a narrow range of conditions; it appears not to be the chondrule formation mechanism. We also suggest that thermionic emission is important for determining the rate of Ohmic heating in hot Jupiters.

The extreme nonlinear optics of gases and femtosecond optical filamentation

Under certain conditions, powerful ultrashort laser pulses can form greatly extended, propagating filaments of concentrated high intensity in gases, leaving behind a very long trail of plasma. Such filaments can be much longer than the longitudinal scale over which a laser beam typically diverges by diffraction, with possible applications ranging from laser-guided electrical discharges to high power laser propagation in the atmosphere. Understanding in detail the microscopic processes leading to filamentation requires ultrafast measurements of the strong field nonlinear response of gas phase atoms and molecules, including absolute measurements of nonlinear laser-induced polarization and high field ionization. Such measurements enable the assessment of filamentation models and make possible the design of experiments pursuing applications. In this paper, we review filamentation in gases and some applications, and discuss results from diagnostics developed at Maryland for ultrafast measurements of laser-gas interactions.

Optical response of gas-phase atoms at less than λ/80 from a dielectric surface.

We present experimental observations of atom-light interactions within tens of nanometers (down to 11 nm) of a sapphire surface. Using photon counting we detect the fluorescence from of order one thousand Rb or Cs atoms, confined in a vapor with thickness much less than the optical excitation wavelength. The asymmetry in the spectral line shape provides a direct readout of the atom-surface potential. A numerical fit indicates a power law -C(α)/r(α) with α = 3.02 ± 0.06 confirming that the van der Waals interaction dominates over other effects. The extreme sensitivity of our photon-counting technique may allow the search for atom-surface bound states.

Temporal changes in absolute atom densities in H2 and N2 mixture gas plasmas by surface modifications of reactor wall

Real time vacuum ultraviolet absorption spectroscopic (VUVAS) measurements revealed that temporal changes in absolute atom densities in H2 and N2 mixture gas (H2/N2) plasmas were affected by the conditions of the reactor inner wall-surface made of anodically-oxidized aluminum. Depending on preceding processes prior to the H2/N2 plasma process, N and H atom densities decreased for a nitrided wall surface, while no change occurred for a hydrogenated surface. Thus, the process history was revealed in the loss-properties of atoms on the reactor wall-surfaces. Suppressing the temporal variations in the gas-phase atom density is crucially important for realizing the precise control of the material processing with a high reproducibility.

Lattice Oxygen Activity in Pr- and La-Doped CeO2for Low-Temperature Soot Oxidation

Effects of Pr- and La-doped CeO2 on desorption of active lattice oxygen to soot oxidation were studied. Thermogravimetric analysis under N2 flow indicated that, despite the decrease in reducible Ce content, the amount of oxygen deficiency increases with increasing La content (x) to 15 mol % in Ce0.80–xPr0.20LaxO2. In inert atmosphere, the introduction of oxygen vacancies by doping La increased the available lattice oxygen for desorption. In the oxygen isotopic exchange, under 18O2 flow, desorption of oxygen molecules containing lattice oxygen atom (16O) was favored because of Pr and La doping in CeO2. These dopants increase the exchange rate constant between gas-phase and lattice oxygen atoms. The apparent activation energy, derived from an Arrhenius plot, for CeO2, Ce0.80Pr0.20O2, and Ce0.65 Pr0.20La0.15O2 were 311, 230, and 135 kJ/mol, respectively. The values indicated that Pr and La doping causes facile oxygen exchange between the gas phase and the solid. The comparison of the rate constants for 16O18O formation under 18O2 flow and 18O2/16O2 mixture flow showed that lattice oxygen predominantly contributes to the exchange reaction. Surface exchange coefficient (k) and diffusion coefficient (D) were obtained by secondary ion mass spectrometry (SIMS) depth analysis of the oxides after diffusion annealing in 18O2. The values of both k and D increase by doping Pr and La, and the surface exchange coefficient value, k, especially increases significantly. Hence, Pr and La doping enhances the oxygen exchange with lattice oxygen, and so high soot oxidation activity at low temperature could be related to the increased contribution of lattice oxygen.

Geometries and stabilities of l-ascorbic acid dimer and its derivatives: a computational investigation by density functional methods

Geometries, relative stabilities, and hydrogen bonds of l-ascorbic acid (LAA) and d-Erythroascorbate (DEAA) dimers as well as their S- and Se-substituted isomers in gas phase and water solvent are studied using density functional method. Furthermore, the hydrogen bond lengths in LAA and DEAA dimers are generally increased along with the binding dissociation energy of the dimers being decreased as apex O atoms in the five-membered C5 rings of LAA and DEAA dimers are substituted by S and Se atoms in gas phase and water solvent. Interestingly, one LAA dimer and its S- or Se-substituted isomer with four hydrogen bonds in gas and water solvent are the three-centers structures. In addition, the chemical bonding and charge distributions of all the dimers are discussed. A good agreement with available experimental results is reached.

Squeezed light from a silicon micromechanical resonator

Monitoring a mechanical object's motion, even with the gentle touch of light, fundamentally alters its dynamics. The experimental manifestation of this basic principle of quantum mechanics, its link to the quantum nature of light and the extension of quantum measurement to the macroscopic realm have all received extensive attention over the past half-century. The use of squeezed light, with quantum fluctuations below that of the vacuum field, was proposed nearly three decades ago as a means of reducing the optical read-out noise in precision force measurements. Conversely, it has also been proposed that a continuous measurement of a mirror's position with light may itself give rise to squeezed light. Such squeezed-light generation has recently been demonstrated in a system of ultracold gas-phase atoms whose centre-of-mass motion is analogous to the motion of a mirror. Here we describe the continuous position measurement of a solid-state, optomechanical system fabricated from a silicon microchip and comprising a micromechanical resonator coupled to a nanophotonic cavity. Laser light sent into the cavity is used to measure the fluctuations in the position of the mechanical resonator at a measurement rate comparable to its resonance frequency and greater than its thermal decoherence rate. Despite the mechanical resonator's highly excited thermal state (10(4) phonons), we observe, through homodyne detection, squeezing of the reflected light's fluctuation spectrum at a level 4.5 ± 0.2 per cent below that of vacuum noise over a bandwidth of a few megahertz around the mechanical resonance frequency of 28 megahertz. With further device improvements, on-chip squeezing at significant levels should be possible, making such integrated microscale devices well suited for precision metrology applications.

The road to hot electron photochemistry at surfaces: A personal recollection

A very important part of contemporary fs-laser surface photochemistry (SPC) is based on a proposed mechanism in which a laser pulse incident upon an adsorbate-covered surface photoexcites substrate electrons which in turn inelastically scatter from atoms and molecules (chemists may call them "reactants") in or on the surface. The present narrative outlines my own very personal SPC saga that began with early exposure to the wonders of and fascination with inelastic resonant electron scattering from gas phase atoms and molecules that dominated the Atomic and Electron Physics activities at NBS (now NIST) in 1968 when I arrived. How this lead to a fundamental understanding of important aspects of SPC is the focus of this essay.

Measurement of the Abraham Force and Its Predicted QED Corrections in Crossed Electric and Magnetic Fields

We report the observation by a new method of mechanical momentum transferred to gas phase atoms and molecules upon application of crossed oscillating electric and static magnetic fields. We identify this momentum as the microscopic analogue of the classical Abraham force. Two QED predictions of additional magnetoelectrically induced mechanical momentum are addressed. One of them is experimentally refuted; the other is found to be currently below our experimental detection.

Generation, characterisation, and applications of atomic and molecular alignment and orientation

The gas phase is generally defined as a state of matter in which atoms or molecules are in constant, rapid, random Brownian motion. However, a range of techniques exist for preparing distributions of gas phase atoms and molecules whose motion is far from random, and whose orientation in space is well defined. In this Perspective, we will explore the nature of atomic and molecular alignment and orientation, the various techniques by which samples of spatially oriented species may be prepared and characterised, and some of the ways in which oriented molecules are being exploited to further our knowledge of molecular structure and dynamics.

Electric dipole moments and disalignment of interstellar dust grains

The degree to which interstellar grains align with respect to the interstellar magnetic field depends on disaligning as well as aligning mechanisms. For decades, it was assumed that disalignment was due primarily to the random angular impulses a grain receives when colliding with gas-phase atoms. Recently, a new disalignment mechanism has been considered, which may be very potent for a grain that has a time-varying electric dipole moment and drifts across the magnetic field. We provide quantitative estimates of the disalignment times for silicate grains with size > approximately 0.1 micron. These appear to be shorter than the time-scale for alignment by radiative torques, unless the grains contain superparamagnetic inclusions.

Photoelectric emission from the alkali metal doped vacuum-ice interface

The photoelectron photoemission spectra and thresholds for low coverages of Li and K adsorbed on water-ice have been measured, compared with photoionization spectra of the gas-phase atoms, and modeled by quantum chemical calculations. For both alkali metals the threshold for photoemission is dramatically decreased and the cross section increased on adsorption to the water-ice surface. Quantum chemical calculations suggest that the initial state is formed by the metal atoms adsorbed into the water-ice surface, forming a state with a delocalized electron distribution. This state is metastable and decays on the hundreds of seconds time scale at 92 K. The decay is markedly faster for Li than for K, probably due to diffusion into the ice film.

An evaluation of exchange-correlation functionals for the calculations of the ionization energies for atoms and molecules

In this paper, ionization energies of gas-phase atoms and molecules are calculated by energy-difference method and by approximate transition-state models with density functional theory (DFT). To determine the best functionals for ionization energies, we first study the H to Ar atoms. An approximation is used in which the electron density is first obtained from Kohn–Sham computations with an exchange-correlation potential V xc known as statistical average of orbital potentials (SAOP), after which the energy is computed from that density with 59 different exchange-correlation energy functionals E xc. For the 18 atoms, the best E xc functional providing an average absolute deviation (AAD) of only 0.110 eV is one known as the Krieger–Chen–Iafrate–Savin functional modified by Krieger, Chen, Iafrate, and Kurth, if one uses the spin-polarized spherical atom description. On the other hand, if one imposes the condition of integer-electrons, the best functional is the Becke 1997 functional modified by Wilson, Bradley, and Tozer, with an AAD of 0.107 eV, while several other functionals perform almost as well. For molecules, we can achieve an accuracy of AAD = 0.21 eV for valence VIPs of nonperhalo molecules with Δ E( V xc = SAOP;PBE0) using integer-electron description. For perhalo molecules our best approach is Δ E( V xc from either E xc or SAOP;mPW1PW) with full symmetry to obtain an AAD = 0.24 eV.

Nitric Oxide Production from Surface Recombination of Oxygen and Nitrogen Atoms

ठExperimental results are presented that support the surface-catalyzed production of nitric oxide from the recombination of oxygen and nitrogen atoms on quartz. The experiments employ two-photon laser induced fluorescence detection of atomic oxygen and atomic nitrogen to characterize changes in gas-phase atom concentrations as the ratio of O to N atoms is varied at the opening of a diffusion-tube side-arm reactor. The measurements verify a correlation between decreased O-atom loss and enhanced N-atom loss in N2/N/O mixtures. Computational simulations of the side-arm reactor with a multi-species reaction-diffusion model strengthen the case for NO surface formation, reproducing observed changes in O- and N-atom concentration profiles with varying O/N ratios at the side-arm entrance when surface-catalyzed NO production is included in the boundary conditions.

Density functional study of double ionization energies

In this paper, double ionization energies (DIEs) of gas-phase atoms and molecules are calculated by energy difference method with density functional theory. To determine the best functional for double ionization energies, we first study 24 main group atoms in the second, third, and fourth periods. An approximation is used in which the electron density is first obtained from a density functional computation with the exchange-correlation potential V xc known as statistical average of orbital potentials, after which the energy is computed from that density with 59 different exchange-correlation energy functionals E xc. For the 24 atoms, the two best E xc functional providing DIEs with average absolute deviation (AAD) of only 0.25 eV are the Perdew-Burke-Ernzerhof functional modified by Hammer et al. [Phys. Rev. B 59, 6413 (1999)] and one known as the Krieger-Chen-Iafrate-Savin functional modified by Krieger et al. (unpublished). Surprisingly, none of the 20 available hybrid functionals is among the top 15 functionals for the DIEs of the 24 atoms. A similar procedure is then applied to molecules, with opposite results: Only hybrid functionals are among the top 15 functionals for a selection of 29 molecules. The best E xc functional for the 29 molecules is found to be the Becke 1997 functional modified by Wilson et al. [J. Chem. Phys. 115, 9233 (2001)]. With that functional, the AAD from experiment for DIEs of 29 molecules is just under 0.5 eV. If the two suspected values for C2H2 and Fe(CO)5 are excluded, the AAD improves to 0.3(2) eV. Many other hybrid functionals perform almost as well.

Angular momentum effects in electron scattering from atoms

This paper concerns angular momentum-dependent phenomena in excited gas-phase atoms using incident photons or electrons in scattering experiments. A brief overview indicates the main capabilities of experimental techniques and the information which can be deduced about atomic structure and dynamics from conservation of momenta with measurement of polarization and detection of the number of emerging electrons, photons and ions. Maximum information may be obtained when the incident particles and the targets are state-selected both before and after scattering. The fundamental scattering amplitudes and their relative phases, and consequently derived quantities such as the parameters describing the electron charge cloud of the atomic target, have enabled significant advances of understanding of collision mechanisms.The angular momentum-dependent scattering probabilities change when, for example, the spin-orbit interaction for the target electrons becomes large compared with the Coulomb electron-electron interactions and also when electron exchange and the relative orientation of the electron spins change. Several examples are discussed to indicate significant principles and recent advances. Major contributions to this field from the technology associated with electron spin production and detection time, as well as time-coincidence detection, are discussed. New results from the authors' laboratory are presented.

Dynamics of interfacial reactions between O(3P) atoms and long-chain liquid hydrocarbons

Recent progress that has been made towards understanding the dynamics of collisions at the gas–liquid interface is summarized briefly. We describe in this context a promising new approach to the experimental study of gas–liquid interfacial reactions that we have introduced. This is based on laser-photolytic production of reactive gas-phase atoms above the liquid surface and laser-spectroscopic probing of the resulting nascent products. This technique is illustrated for reaction of O(3P) atoms at the surface of the long-chain liquid hydrocarbon squalane (2,6,10,15,19,23-hexamethyltetracosane). Laser-induced fluorescence detection of the nascent OH has revealed mechanistically diagnostic correlations between its internal and translational energy distributions. Vibrationally excited OH molecules are able to escape the surface. At least two contributions to the product rotational distributions are identified, confirming and extending previous hypotheses of the participation of both direct and trapping-desorption mechanisms. We speculate briefly on future experimental and theoretical developments that might be necessary to address the many currently unanswered mechanistic questions for this, and other, classes of gas–liquid interfacial reaction.