Hydridization and catalysis by lanthanide films

1984 ◽  
Vol 393 (1805) ◽  
pp. 257-276 ◽  
Keyword(s):  
Activation Energy ◽  
High Vacuum ◽  
Deuterium Atom ◽  
Ultra High Vacuum ◽  
Gas Uptake ◽  
Metal Sublattice ◽  
Hydrogen Deuterium ◽  
Higher Temperature ◽  
Temperature Programmed ◽  
Interstitial Hydrogen

Hydrogen absorption to give the dihydrides MH 2+1 containing interstitial hydrogen H i has been studied for the metals Gd, Dy, Er, Yb and Lu in the form of films deposited in ultra-high vacuum on glass. Film areas were determined by Kr adsorption, and hydrogen content, in particular inter­stitial hydrogen H i , characterized by gas uptake, temperature programmed desorption, electrical conductivity and work function measurements by the diode method. The catalytic activity of the dihydride films for the H 2 + D 2 → 2HD reaction was studied at a pressure of 1.1 Torr over 175-579 K, and at 273 K over 0.19-6.2 Torr. Arrhenius plots for the rate con­stant show a low temperature low activation energy region changing over at a temperature T c to a higher temperature higher activation energy régime, with T c on average for the five metals about 50 K below the tem­perature T max at which the interstitial hydrogen H i has disappeared. The suggested mechanisms are T < T c : D 2 + H i □ s → (D 2 H i )□ s → □ s D i + HD, (1) T > T c : D 2 + H 2 + 4□ s → 2(D i □ s ) (H i □ s ) → 4□ s + 2HD, (2) where H i □ s , D i □ s , denotes a hydrogen, deuterium, atom held on a surface octahedral site in the f. c. c. metal sublattice. These mechanisms agree with the observed approximate first-order pressure dependency down to 77 K. The rate constants at both 273 K (under T c ) and 573 K (over T c ) decrease over Gd, Dy, Er, to Yb, and rise again to Lu, and this is discussed in terms of the metal-hydrogen, H i □ s or D i □ s bond strength.

scholarly journals Nitrile versus isonitrile adsorption at interstellar grain surfaces

2017 ◽  
Vol 608 ◽  
pp. A50 ◽  
Author(s):  
M. Bertin ◽  
M. Doronin ◽  
X. Michaut ◽  
L. Philippe ◽  
A. Markovits ◽  
...  
Keyword(s):  
Density Functional ◽  
Surface Defects ◽  
High Vacuum ◽  
Solid Support ◽  
Ultra High Vacuum ◽  
Structural Defects ◽  
Adsorption Energies ◽  
The Difference ◽  
Temperature Programmed ◽  
Open Question

Context. Almost 20% of the ~200 different species detected in the interstellar and circumstellar media present a carbon atom linked to nitrogen by a triple bond. Of these 37 molecules, 30 are nitrile R-CN compounds, the remaining 7 belonging to the isonitrile R-NC family. How these species behave in their interactions with the grain surfaces is still an open question. Aims. In a previous work, we have investigated whether the difference between nitrile and isonitrile functional groups may induce differences in the adsorption energies of the related isomers at the surfaces of interstellar grains of various nature and morphologies. This study is a follow up of this work, where we focus on the adsorption on carbonaceous aromatic surfaces. Methods. The question is addressed by means of a concerted experimental and theoretical approach of the adsorption energies of CH3CN and CH3NC on the surface of graphite (with and without surface defects). The experimental determination of the molecule and surface interaction energies is carried out using temperature-programmed desorption in an ultra-high vacuum between 70 and 160 K. Theoretically, the question is addressed using first-principle periodic density functional theory to represent the organised solid support. Results. The adsorption energy of each compound is found to be very sensitive to the structural defects of the aromatic carbonaceous surface: these defects, expected to be present in a large numbers and great diversity on a realistic surface, significantly increase the average adsorption energies to more than 50% as compared to adsorption on perfect graphene planes. The most stable isomer (CH3CN) interacts more efficiently with the carbonaceous solid support than the higher energy isomer (CH3NC), however.

Deposition of Iron on Si(111)-(7×7): Photo- and Electron-Assisted Decomposition of Fe(CO)5

1986 ◽  
Vol 75 ◽  
Author(s):  
J. R. Swanson ◽  
C. M. Friend ◽  
Y. J. Chabal
Keyword(s):  
Auger Electron ◽  
Silicon Crystal ◽  
High Vacuum ◽  
Low Frequency ◽  
Temperature Programmed Desorption ◽  
Thermal Reaction ◽  
Ultra High Vacuum ◽  
Temperature Programmed ◽  
Ultraviolet Photons ◽  
Induced Decomposition

AbstractLaser- and electron-assisted deposition of Fe on Si(111)-(7×7) surfaces using decomposition of Fe(CO)5 has been investigated with multiple internal reflection Fourier transform infrared, Auger electron and temperature programmed desorption spectroscopies and low energy electron diffraction under ultra-high vacuum conditions. No thermal reaction was apparent in temperature programmed desorption experiments: only molecular Fe(CO)5 desorption was observed at temperatures of 150 and 170 K, corresponding to desorption energies in the range of 7–10 kcal./mole. Fe(CO)5 decomposition could be induced using either incident 1.6 keV electrons or ultraviolet photons. Significant amounts of carbon were deposited from the electron induced decomposition, consistent with earlier reports on the Si(100) surface. In contrast, ultraviolet photolysis did not result in any detectable incorporation of carbon or oxygen into the iron deposits. No partially decarbonylated Fe(CO)x, x<5, fragments were detected subsequent to exposure to photons using infrared spectroscopy. However, a new, unresolved low frequency shoulder did appear in the infrared spectrum after exposing the Fe(CO)5 covered Si(111)-(7×7) crystal to the electron beam. Iron photodeposition was evident in the Auger electron spectra obtained subsequent to photolysis and annealing of the surface to either 300 K or 1000 K in order to desorb unreacted Fe(CO)5. These data suggest that there are no surface stable Fe(CO)x, x<5, species in the photodeposition process. Instead, photolysis yields Fe atoms directly, even at low temperatures. Annealing to temperatures on the order of 1000 K subsequent to iron deposition resulted in a significant decrease in the Fe:Si ratio as measured by Auger electron spectroscopy. In addition, CO could not be readsorbed on a surface where the Fe(CO)5 had been decomposed. This is attributed to dissolution of Fe into the bulk silicon crystal.

scholarly journals Elektronische Wechselwirkung zwischen aufgedampften Germaniumfilmen und Sauerstoff

1963 ◽  
Vol 18 (2) ◽  
pp. 119-125 ◽  
Author(s):  
R. Suhrmann ◽  
M. Kruel ◽  
G. Wedler
Keyword(s):  
Boundary Layer ◽  
Electron Acceptor ◽  
Boundary Layer Thickness ◽  
High Vacuum ◽  
Ultra High Vacuum ◽  
Band Model ◽  
Electric Resistance ◽  
Before And After ◽  
Higher Temperature ◽  
Ordered State

Ge films of thicknesses within the p-boundary layer thickness are precipitated under ultra high vacuum conditions, at 77°K. They are investigated either in the disordered state (before annealing at a higher temperature) or in the ordered state (after annealing). The electric resistance R and the photoelectric work function Φ are measured before and after the influence of known amounts of oxygen. R is plotted as a function of time and coverage n, Φ as a function of n. Small amounts of oxygen cause R to increase without changing Φ, large amounts result in a succeeding decrease of R and cause Φ to increase by 0.1 to 0.2 V. The results are discussed by means of a band model. At small coverages oxygen acts as an electron donator, at higher coverages as an electron acceptor.

Selective Oxidation of Ammonia on Ruthenium to Form p(2 x 2) Nitrogen Layer

2013 ◽  
Vol 6 (1) ◽  
Author(s):  
S. Balgooyen ◽  
I. Waluyo
Keyword(s):  
Low Temperature ◽  
High Vacuum ◽  
Hydrogenation Reaction ◽  
Ultra High Vacuum ◽  
Surface Chemical ◽  
Low Energy Electron Diffraction ◽  
Chemical Studies ◽  
Low Energy Electron ◽  
Temperature Programmed ◽  
Oxidation Of Ammonia

Oxidation of ammonia was used to prepare a p(2 x 2) nitrogen layer on the Ru(0001) surface as verified by temperature-programmed desorption (TPD) and low energy electron diffraction (LEED). The process takes place in an ultra-high vacuum (UHV) chamber. The surface is precovered with oxygen and then exposed to ammonia at low temperature. Upon heating, the ammonia is oxidized to form water, which desorbs at low temperature to leave a nitrogencovered surface. The resulting layer can be used in a variety of surface chemical studies, including a hydrogenation reaction, which is an important part in the study of the Haber-Bosch process, in which ruthenium is used as a catalyst.

Rutherford Backscattering Spectrometry Analysis of Growth Rate and Activation Energy for Self-formed Ti-rich Interface Layers in Cu(Ti)/Low-k Samples

2009 ◽  
Vol 1156 ◽  
Author(s):  
Kazuyuki Kohama ◽  
Kazuhiro Ito ◽  
Kenichi Mori ◽  
Kazuyoshi Maekawa ◽  
Yasuharu Shirai ◽  
...  
Keyword(s):  
Activation Energy ◽  
Dielectric Layer ◽  
Rutherford Backscattering Spectrometry ◽  
High Vacuum ◽  
Formation Enthalpy ◽  
Interface Layer ◽  
Rutherford Backscattering ◽  
Ultra High Vacuum ◽  
Dielectric Layers ◽  
Interface Layers

AbstractA new fabrication technique to prepare ultra-thin barrier layers for nano-scale Cu wires was proposed in our previous studies. Ti-rich layers formed at the Cu(Ti)/dielectric-layer interfaces consisted of crystalline TiC or TiSi and amorphous Ti oxides. The primary control factor for Ti-rich interface layer composition was the C concentration in the dielectric layers rather than the formation enthalpy of the Ti compounds. To investigate Ti-rich interface layer growth in Cu(Ti)/dielectric-layer samples annealed in ultra high vacuum, Rutherford Backscattering Spectrometry (RBS) was employed in the present study. Ti peaks were obtained only at the interface for all the samples. Molar amounts of Ti atoms segregated to the interface (n) were estimated from Ti peak areas. The n value was defined by n = Z·exp(-E/RT) · tm, where Z is a preexponential factor and E the activation energy for the reaction. The Z, E, and m values were estimated from plots of log n vs log t and log n vs 1/T. The m values are similar in all the samples. The E values for Ti atoms reacting with the dielectric layers containing carbon (except SiO2) tended to decrease with decreasing C concentration (decreasing k), while reaction rate coefficients (Z·exp(-E/RT)) were insensitive to C concentration in the dielectric layers. These factors lead to conclusion that growth of the Ti-rich interface layers is controlled by chemical reactions of the Ti atoms with the dielectric layers represented by the Z and E values, rather than diffusion in the Ti-rich interface layers.

scholarly journals Reactivity of formic acid (HCOOH) with H atoms on cold surfaces of interstellar interest

2020 ◽  
Vol 636 ◽  
pp. A4
Author(s):  
Henda Chaabouni ◽  
Saoud Baouche ◽  
Stephan Diana ◽  
Marco Minissale
Keyword(s):  
Formic Acid ◽  
Pyrolytic Graphite ◽  
High Vacuum ◽  
Amorphous Solid ◽  
Product Formation ◽  
Ultra High Vacuum ◽  
Star Forming ◽  
Chemical Surface ◽  
Surface Temperatures ◽  
Temperature Programmed

Context. Formic acid (HCOOH) is the simplest organic carboxylic acid in chemical synthesis and the significant species in interstellar chemistry. HCOOH has been abundantly detected in interstellar ices, dense molecular clouds and star-forming regions. Aims. Laboratory hydrogenation experiments of HCOOH molecules with H atoms were performed with two cryogenic ultra-high vacuum devices on amorphous solid water ices, and highly oriented pyrolytic graphite surfaces. The aim of this work is to study the reactivity of HCOOH molecules with H atoms at low surface temperature 10 K, low surface coverage of one monolayer to three layers, and low H-atom flux of about 3.0 × 1012 molecule cm−2 s−1. Methods. HCOOH and H beams were deposited on cold surfaces held at 10 K, and the condensed films were analyzed by in-situ Reflection Absorption InfraRed Spectroscopy and temperature programmed desorption (TPD) mass spectrometry technique by heating the sample from 10 to 200 K. Results. Using the temperature programmed during exposure desorption technique, we highlight the possible dimerization of HCOOH molecules at low surface temperatures between 10 and 100 K. In our HCOOH+H experiments, we evaluated a consumption of 20–30% of formic acid by comparing the TPD curves at m/z 46 of pure and H-exposed HCOOH ice. Conclusions. The hydrogenation HCOOH+H reaction is efficient at low surface temperatures. The main products identified experimentally are carbon dioxide (CO2) and water (H2O) molecules. CO bearing species CH3OH, and H2CO are also detected mainly on graphite surfaces. A chemical surface reaction route for the HCOOH+H system is proposed to explain the product formation.

scholarly journals Implications of Ice Morphology for Comet Formation

2005 ◽  
Vol 13 ◽  
pp. 491-494 ◽  
Author(s):  
M. P. Collings ◽  
J. W. Dever ◽  
M. R. S. McCoustra ◽  
H. J. Fraser
Keyword(s):  
Surface Science ◽  
High Vacuum ◽  
Binding Energies ◽  
Ultra High Vacuum ◽  
Water Ice ◽  
Reflection Absorption Infrared Spectroscopy ◽  
Thermodynamic Conditions ◽  
Temperature Programmed ◽  
Additional Constraints ◽  
Ice Morphology

AbstractLaboratory surface science under ultra-high vacuum (UHV) conditions allows us to simulate the growth of ices in astrophysical environments. Using the techniques of temperature programmed desorption (TPD), reflection-absorption infrared spectroscopy (RAIRS) and micro-balance methods, we have studied binary ice systems consisting of water (H2O) and variety of other species including carbon monoxide (CO), at astrophysically relevant conditions of temperature and pressure. We present results that demonstrate that the morphology of water ice has an important influence on the behavior of such systems, by allowing processes such as diffusion and trapping that can not be understood through a knowledge of the binding energies of the species alone. Through an understanding of the implications of water ice morphology on the behavior of ice mixtures in the interstellar environment, additional constraints can be placed on the thermodynamic conditions and ice compositions during comet formation.

Gas-Phase and Surface Decomposition of Tris-Dimethylamino Arsenic

1993 ◽  
Vol 334 ◽  
Author(s):  
Ming Xi ◽  
Sateria Salim ◽  
Klavs F. Jensen ◽  
David A. Bohling
Keyword(s):  
Gas Phase ◽  
Hydrogen Transfer ◽  
Reaction Pathway ◽  
High Vacuum ◽  
Ultra High Vacuum ◽  
Phase Reaction ◽  
Temperature Programmed ◽  
Temperature Programmed Reaction ◽  
Surface Decomposition ◽  
Nitrogen Bond

AbstractGas phase and surface decomposition reactions of a novel arsenic precursor tris- (dimethylamino) arsenic (DMAAs) have been studied. Optical fiber-based Fourier transform infrared spectroscopy was used to monitor, in situ, the gas-phase pyrolysis of DMAAs. Homolysis of the arsenic-nitrogen bond with formation of dimethylamine radicals was identified as the key gas-phase reaction pathway. Formation of methylmethyleneimine from reactions with the decomposition products was directly observed. Surface decomposition of DMAAs on GaAs(100) was investigated by low energy electron diffraction and temperature-programmed reaction under ultra-high vacuum conditions. DMAAs adsorbed onto GaAs(100)-(4x6) was found to decompose with 100% efficiency and two surface reaction pathways were identified. The first reaction channel was homolysis of the arsenic-nitrogen bond with formation of dimethylamine radicals, whereas the second pathway involved a β-hydrogen transfer.

Polymerization of C-Si Films on Metal Substrates: Potential Adhesion/Diffusion Barriers for Microelectronics

1998 ◽  
Vol 511 ◽  
Author(s):  
Li Chen ◽  
J. A. Kelber
Keyword(s):  
High Vacuum ◽  
Temperature Programmed Desorption ◽  
Diffusion Barriers ◽  
Ultra High Vacuum ◽  
Polymeric Films ◽  
Vinyl Silane ◽  
Cu Substrates ◽  
Auger Spectra ◽  
Temperature Programmed ◽  
Si Films

ABSTRACTCarbon-Silicon polymeric films have been formed by electron beam bombardment (500eV) of molecularly adsorbed vinyl silane precursors under ultra-high vacuum (UHV) conditions. Temperature programmed desorption (TPD) studies show that polymerization is occurring via the vinyl groups, while Auger spectra show that the polymerized films have compositions very similar to the starting precursors; vinyltrichlorosilane (VTCS) or vinyltrimethylsilane(VTMS). VTCSderived films ˜ 100 Å thick show no reaction with Cu substrates and no diffusion of Cu until temperatures greater than 700 K, while Cu deposited on VTMS films on Al substrates show no diffusion prior to Al reaction/decomposition at 600 K. Auger and TPD studies also show that fluorocarbon precursors, such as perfluorobenzene can be incorporated into the films by e-beaminduced reactions, a first step in the controlled growth of adherent polymer films on unreactive substrates such as Cu.

The Surface Chemistry of CdTe Mocvd

1993 ◽  
Vol 334 ◽  
Author(s):  
Wen-Shryang Liu ◽  
Gregory B. Aupp
Keyword(s):  
Surface Chemistry ◽  
Large Fraction ◽  
High Vacuum ◽  
Temperature Programmed Desorption ◽  
Ultra High Vacuum ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Temperature Programmed ◽  
Surface Decomposition ◽  
Rate Limiting

AbstractTemperature programmed desorption (TPD) studies in ultra high vacuum revealed that diethyltellurium (DETe) and dimethylcadmium (DMCd) adsorb weakly on clean Si(100) and desorb upon heating without decomposing. These precursors adsorb both weakly and strongly on CdTe(111)A, with DMCd exhibiting the stronger interaction with the surface than DETe. Dimethylcadmium partially decomposes to produce Cd adatoms; a large fraction of the excess Cd atoms desorb upon heating. In contrast, DETe desorbs without decomposing, suggesting that the rate limiting step in CdTe MOCVD on CdTe(111)A is surface decomposition of the tellurium alkyl.

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