Home About us Contact
|
Home About us Contact | ||
|
|
||
Threshold Energy (threshold + energy)
Selected AbstractsThe reaction between ethyl and molecular oxygen II: Further analysisINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 11 2001James A. Miller The present investigation is a rather substantial extension and elaboration of our previous work on the same reaction. In this article we accomplish four primary objectives: 1.We show quantitatively how sensitive the high-temperature rate coefficient k(T) is to E02, the threshold energy of the transition state for direct molecular elimination of HO2 from ethylperoxy radical (C2H5O2), thus deducing a value of E02=,3.0 kcal/mol (measured from reactants). 2.We derive the result that k0(T) , k,,(T) in the high-temperature regime, where k0(T) is the zero-pressure rate coefficient, and k,,(T) is the infinite-pressure rate coefficient for the bimolecular channel. 3.Most importantly, we discuss the three different regimes of the reaction (low-temperature, transition, and high-temperature) in terms of the eigenvectors and eigenvalues of G, the transition matrix of the master equation. The transition regime is shown to be a region of avoided crossing between the two chemically significant eigenvalue curves in which the thermal rate coefficient k (T ,p) jumps from one eigenvalue to the other. This jump is accompanied by a "mixing" of the corresponding eigenvectors, through which both eigenvectors deplete the reactant. The onset of the high-temperature regime is triggered by reaching the "stabilization limit" of the ethylperoxy adduct, a limit that is induced by a shift in equilibrium of the stabilization reaction. Our identification of the prompt and secondary HO2 formed by the reaction with these eigenvalue/eigenvector pairs leads to good agreement between theory and the experiments of Clifford et al. (J Phys Chem A 2000, 104, 11549,11560). 4.Lastly, we describe the master equation results in terms of a set of elementary reactions and phenomenological rate coefficients. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 732,740, 2001 [source] Reactions of BBrn+ (n = 0,2) at fluorinated and hydrocarbon self-assembled monolayer surfaces: observations of chemical selectivity in ion,surface scatteringJOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 7 2001Nathan Wade Abstract Ion,surface reactions involving BBrn+ (n = 0,2) with a fluorinated self-assembled monolayer (F-SAM) surface were investigated using a multi-sector scattering mass spectrometer. Collisions of the B+ ion yield BF2+ at threshold energy with the simpler product ion BF+· appearing at higher collision energies and remaining of lower abundance than BF2+ at all energies examined. In addition, the reactively sputtered ion CF+ accompanies the formation of BF2+ at low collision energies. These results stand in contrast with previous data on the ion,surface reactions of atomic ions with the F-SAM surface in that the threshold and most abundant reaction products in those cases involved the abstraction of a single fluorine atom. Gas-phase enthalpy data are consistent with BF2+ being the thermodynamically favored product. The fact that the abundance of BF2+ is relatively low and relatively insensitive to changes in collision energy suggests that this reaction proceeds through an entropically demanding intermediate at the vacuum,surface interface, one which involves interaction of the B+ ion simultaneously with two fluorine atoms. By contrast with the reaction of B+, the odd-electron species BBr+· reacts with the F-SAM surface to yield an abundant single-fluorine abstraction product, BBrF+. Corresponding gas-phase ion,molecule experiments involving B+ and BBr+· with C6F14 also yield the products BF+· and BF2+, but only in extremely low abundances and with no preference for double fluorine abstraction. Ion,surface reactions were also investigated for BBrn+ (n = 0,2) with a hydrocarbon self-assembled monolayer (H-SAM) surface. Reaction of the B+ ion and dissociative reactions of BBr+· result in the formation of BH2+, while the thermodynamically less favorable product BH+· is not observed. Collisions of BBr2+ with the H-SAM surface yield the dissociative ion,surface reaction products, BBrH+ and BBrCH3+. Substitution of bromine atoms on the projectile by hydrogen or alkyl radicals suggests that Br atoms may be transferred to the surface in a Br-for-H or Br-for-CH3 transfer reaction in an analogous fashion to known transhalogenation reactions at the F-SAM surface. The results for the H-SAM surface stand in contrast to those for the F-SAM surface in that enhanced neutralization of the primary ions gives secondary ion signals one to two orders of magnitude smaller than those obtained when using the F-SAM surface, consistent with the relative ionization energies of the two materials. Copyright © 2001 John Wiley & Sons, Ltd. [source] Photopolymerization with microscale resolution: Influence of the physico-chemical and photonic parametersJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 11 2008Olivier Soppera Abstract This article is aimed at demonstrating that physicochemical parameters can be used to control the spatial extent (length, width, and shape) of polymer objects in view of micro- and nano-fabrication applications. In particular, we showed that oxygen quenching and internal filter effects could be turned to advantage to modulate the response of the material by controlling the threshold energy of polymerization and/or the shape of the light into the photopolymerizable medium. The experimental configuration used in this study is based on light-induced polymerization at the extremity of an optical fiber that produces polymer micro-objects after development. Spectroscopic approaches and polymerization threshold measurements were performed to quantitatively evaluate the influence of the physicochemical parameters on the micropatterning of photopolymerizable material. Interestingly, fluorescence that is usually regarded as a process competing with photopolymerization reaction, was used for controlling the fabrication process. By this means, it was possible to better understand the impact of a nonhomogeneous irradiation on photopolymerization process and thus, to tune the shape and the size of the final polymer objects. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3783,3794, 2008 [source] Negative electron affinity on hydrogen terminated diamondPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 11 2005D. Takeuchi Abstract Total photoyield spectroscopy (TPYS) is applied to characterize the surface electronic properties of p- and n-type chemical-vapour-deposited (111) homoepitaxial diamond films with hydrogen terminated (H-terminated) surfaces. All experiments on H-terminated high quality diamond show the same threshold energy of 4.4 eV and comparable spectra in the regime 4.4,5.5 eV. A H-terminated intrinsic IIa (100) sample also show the same result. The onset of electron emission at 4.4 eV on p-, intrinsic and n-type diamond indicates that Fermi levels do not affect the sub-band spectra. Obviously, direct excitation of valence-band electrons into the vacuum takes place in the close vicinity of the surface. If we assume a transition from valence band maximum the threshold energy of 4.4 eV gives a negative electron affinity (NEA) of ,1.07 eV (Eg, 5.47 eV). TPYS therefore allows a precise and direct detection of the NEA of H-terminated diamond. It is interesting that the expected rise of photoyield as NEA in the regime hv > 5.47 eV is missing in n-type diamond. We attribute this to a surface space charge region due to ionized phosphorus atoms. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Mode-Selective Electrical Excitation of a Molecular RotorCHEMISTRY - A EUROPEAN JOURNAL, Issue 38 2009Heather Batteries not included: The rotation of individual molecular rotors can be driven electrically using electrons from a scanning tunneling microscope tip (see picture). Above the threshold energy of the CH stretch excitation, tunneling electrons are effective at selectively driving rotation, but not translation of the thioether rotors. The ability to rotate individual molecules on command will permit the interrogation of many important effects like rotational energy transfer along 1D molecular chains and rotational orientation switching in ordered arrays. [source] | ||||||||||