Metal Atoms In Gas Phase Research Articles

Advanced techniques for quantum-state specific reaction dynamics of gas phase metal atoms

One of the themes of modern molecular reaction dynamics is to characterize elementary chemical reactions from “quantum state to quantum state”, and the study of molecular reaction dynamics in excited states can help test the validity of modern chemical theories and provide methods to control chemical reactions. The subject of this review is to describe the recent experimental techniques used to study the reaction dynamics of metal atoms in the gas phase. Through these techniques, information such as the internal energy distribution and angular distribution of the nascent products or the three-dimensional stereodynamic reactivity can be obtained. In addition, by preparing metal atoms with specific excited electronic states or orbital arrangements, information about the reactivity of the electronic states enriches the relevant understanding of the electron transfer mechanism in metal reaction dynamics.

Gas Phase Chemical Evolution of Uranium, Aluminum, and Iron Oxides

We use a recently developed plasma-flow reactor to experimentally investigate the formation of oxide nanoparticles from gas phase metal atoms during oxidation, homogeneous nucleation, condensation, and agglomeration processes. Gas phase uranium, aluminum, and iron atoms were cooled from 5000 K to 1000 K over short-time scales (∆t < 30 ms) at atmospheric pressures in the presence of excess oxygen. In-situ emission spectroscopy is used to measure the variation in monoxide/atomic emission intensity ratios as a function of temperature and oxygen fugacity. Condensed oxide nanoparticles are collected inside the reactor for ex-situ analyses using scanning and transmission electron microscopy (SEM, TEM) to determine their structural compositions and sizes. A chemical kinetics model is also developed to describe the gas phase reactions of iron and aluminum metals. The resulting sizes and forms of the crystalline nanoparticles (FeO-wustite, eta-Al2O3, UO2, and alpha-UO3) depend on the thermodynamic properties, kinetically-limited gas phase chemical reactions, and local redox conditions. This work shows the nucleation and growth of metal oxide particles in rapidly-cooling gas is closely coupled to the kinetically-controlled chemical pathways for vapor-phase oxide formation.

Kinetic Study of the Reactions of Ca(1S) and Sr(1S) Atoms with Cl2 in the Temperature Ranges from Respectively 303−1038 K and 303−991 K

A kinetic investigation of the second-order reactions Ca(1S) + Cl2(1 ) →(k1Ca) CaCl + Cl and Sr(1S) + Cl2(1 ) →(k1Sr) SrCl + Cl was carried out in a fast-flow reactor in the temperature ranges of respectively 303−1038 K and 303−991 K. The calcium and strontium atoms in the gas phase were generated by thermal evaporation of the solid metal pellets. The concentration of the gas-phase metal atoms was followed by means of atomic absorption spectroscopy (AAS) at wavelengths of 422.7 nm for calcium and 460.7 nm for strontium atoms. Both reactions show an Arrhenius behavior and the rate constants are given by k1Ca = [(6.0 ± 0.8) × 10-10]exp(−0.2 ± 0.5 kJ mol-1/RT)cm3 molecule-1 s-1 and k1Sr = [(7.3 ± 0.6) × 10-10]exp(−0.4 ± 0.4 kJ mol-1/RT)cm3 molecule-1 s-1. The results are interpreted in terms of the electron-jump mechanism. For both the Ca/Cl2 and the Sr/Cl2 reactions, the experimental rate constants are too high to be quantitatively explained by the classical electron-jump mechanism. However, the modified el...

Formation Process of Empty and Metal-Containing Fullerenes—Molecular Dynamics and FT-ICR Studies

The formation mechanism of empty and metal-containing fullerene was studied through MD (molecular dynamics) simulations and FT-ICR (Fourier transform ion cyclotron resonance) mass spectroscopy of laser vaporized carbon cluster. Multi-body classical potential functions for metal-carbon and metal-metal interactions were constructed based on DFT (density functional theory) calculations of various forms of small clusters MCn and Mn (M = La, Sc, Ni). Using the modified Brenner potential for carbon-carbon interaction, the clustering process starting from 500 isolated carbon atoms and 5 metal atoms in gas phase was simulated under the controlled temperature condition at 3000K. The difference of clustering process of La@Cn, Sc@Cn and NiCn were compared with empty fullerene formation simulation. FT-ICR mass spectrometer directly connected to the laser vaporization cluster beam source was implemented in order to experimentally study the clustering process. The increase of cluster nozzle pressure roughly corresponded to the later stage of the molecular dynamics simulation. The FT-ICR mass spectra of metal-carbon composite clusters were compared for various sample materials used for arc-discharge generation of metal-containing fullerene and SWNT (single-wall carbon nanotube); La, Y., Sc, Gd, Ce, Ca, and Ni-Y. Positive La-C, Y-C, Sc-C, Gd-C, Ce-C binary clusters commonly showed strong MC2n+ signal in the range of 36 < 2n with intense magic numbers at MC44 +, MC50 + and MC60 +. It was speculated that the even-numbered clusters corresponded to the annealed random caged clusters observed in the MD simulation.

METAL ADSORPTION CALORIMETRY

How strongly metals bond to materials is a basic issue in materials science that's important to many fields. The bond strength determines how efficiently oxide particles are coated with precious metals to form catalysts, for example. And it affects the performance of microelectronic devices by controlling electrical contacts and interface properties. Yet despite the need for a quantitative description of this materials mating process, reliable and widely applicable methods for measuring metal-surface bond strengths have long eluded scientists. But a new sensitive calorimetry technique that enables the strength of chemical bonds formed between gas-phase metal atoms and well-characterized solid surfaces to be determined on a per-atom basis offers a solution to this problem [ J. Chem. Phys. , 107 , 5547 (1997)]. Recent experiments were conducted at the University of Washington, Seattle, by postdoctoral researchers J. Todd Stuckless and Daniel J. Bald, graduate student David E. Starr, and professor of chemist...

Bare transition metal atoms in the gas phase: reactions of M, M+, and M2+ with hydrocarbons

This Account focuses on gas-phase measurements and high quality ab initio calculations that are beginning to explain how metal atom electronic structure determines chemical reactivity. The authors have an enormous body of basic chemical reactivity data for M[sup +] and a growing body of data for M[sup 2+] and neutral M. Sophisticated experiments can control the kinetic energy and the electronic state of M[sup +] reactants. One can study the reactivity of Fe[sup +] in the 3d[sup 6]4s, high-spin ground state, the 3d[sup 7], high-spin first excited state, or the 3d[sup 6]4s, low-spin second excited state. The authors have learned to follow the elimination of H[sub 2] and C[sub 2]H[sub 6] from bimolecular Ni[sup +](n-butane) complexes in real time, on a 50-ns time scale. In M[sup +] reactions, the authors can control the kinetic energy and the electronic state of the reactants. A key advantage in interpreting these results is that one understands the electronic structure of the bare metal atom reactants very well. Solution-phase chemists might well question the relevance of atomic species with genuine 1+ or 2+ charges and no ligands or solvent to the [open quotes]real world[close quotes] of organometallic chemistry. Yet connections surely exist, as witnessedmore » by the fact that Rh and Ir atoms are unusually reactive in all phases. Theoretical chemists are beginning to provide a conceptual framework that will unify seemingly diverse branches of experimental chemistry. Of necessity, the ab initio quantum chemist works on model transition metal systems, draws experimental evidence from all available sources, and tries to abstract from the calculations what is robust and common to all phases. Gas-phase metal atoms are idealized model systems well matched to the capabilities of modern theory. Many new conceptual insights in the next 10 years will come from careful analysis of ab initio wave functions informed by incisive gas-phase experiments. 30 refs., 5 figs., 1 tab.« less

Electron paramagnetic resonance spectra of coinage metal atoms trapped in evaporated alkali-metal chloride matrices at low temperatures

Copper, silver and gold atoms have been isolated in vapour-deposited alkali-metal chlorides in a rotating cryostat at 77 K. Their EPR spectra show much lower hyperfine interactions than gas-phase metal atoms or atoms prepared in alkali-metal chloride single crystals from metal ions in cationic substitutional sites by electron capture. The difference is thought to arise because the metal atoms in the rotating-cryostat experiments are located in asymmetric trapping sites at the interface between layers of the alkali-metal chloride, and resemble species trapped at surfaces and grain boundaries rather than those in the bulk solid.

Hollow-cathode sputtering source for the production of gas-phase metal atoms of the refractory elements

A simple, versatile hollow-cathode sputtering system for the production of gas-phase metal atoms has been developed. The source has been used to produce gas-phase metal-containing diatomic and polyatomic molecules. Chemiluminscence and laser-induced fluorescence studied have been used to characterize the source and show that it produces high densities (10 14/cm 3) of atoms at a low temperature (340 K).

A Mechanism for Non-Flame Atomization in Atomic Absorption Spectroscopy

A mechanism for atom formation in non-flame atomizers, used in atomic absorption spectrophotometry, is presented. The model assumes that gas-phase metal atoms are produced by the direct thermal reduction of metallic oxides, produced as intermediates in the carbon furnace. This mechanism does not assume that carbon is involved in the reduction step. The minimum appearance temperatures for 15 elements are calculated from equilibrium atom concentrations and free energy data. The calculated appearance temperatures are found to correlate with the experimentally determined values for these elements.