Electron Capture Rate Constants Research Articles

Measurement of ultrafast dynamics of photoexcited carriers in β-Ga2O3 by two-color optical pump-probe spectroscopy

We report results from ultrafast two-color optical pump-probe spectroscopy on bulk β-Ga2O3. A two-photon absorption scheme is used to photoexcite carriers with the pump pulse and free-carrier absorption of the probe pulse is used to record the subsequent dynamics of the photoexcited carriers. Our results are consistent with carrier recombination via defect-assisted processes. We also observe transient polarization-selective optical absorption of the probe pulse by defect states under nonequilibrium conditions. A rate equation model for electron and hole capture by defects is proposed and used to explain the data. Whereas the rate constants for electron capture by defects are found to be temperature-independent, they are measured to be strongly temperature-dependent for hole capture and point to a lattice deformation/relaxation process accompanying hole capture. Our results shed light on the mechanisms and rates associated with carrier capture by defects in β-Ga2O3.

On the use of electron capture rate constants to describe electron capture dissociation mass spectrometry of peptides.

Electron capture dissociation (ECD) tandem mass spectrometry (MS/MS) is a powerful analytical tool for peptide and protein structure analysis. The product ion abundance (PIA) distribution in ECD MS/MS is known to vary as a function of electron irradiation period. This variation complicates the development of a method of peptide identification by correlation of ECD MS/MS data with experimental and theoretical mass spectra. Here, we first develop a kinetic model to describe primary electron capture by peptide dications leading to product ion formation and secondary electron capture resulting in product ion neutralization. We apply the developed kinetic model to calculate product ion formation rate constants and electron capture rate constants of product ions from ECD mass spectra acquired using various electron irradiation periods. Contrary to ECD PIA distributions, the product ion formation rate constants are shown to be independent of electron irradiation period and, thus, may be employed to characterize ECD product ion formation more universally. The electron capture rate constants of product ions in ECD Fourier transform ion cyclotron resonance MS were found to correlate (with a correlation factor, R(2), of ca 0.9) with ion mobility cross sections of product ions in electron transfer dissociation. Finally, we demonstrate that the electron irradiation period influences the ratio of radical and even-electron c and z product ions.

A new apparatus for measuring rate constants and activation energies of thermal electron capture processes in the gas phase

The new apparatus for investigation of kinetics of a thermal electron capture process and its temperature dependence with pulsed Townsend technique has been built. This method gives straightforward results on electron drift velocities in any mixture and allows calculating the rate constants for electron capture as well as activation energies of this process. The thermal electron capture rate constants and activation energies for CH 3CCl 3, CHCl 2CH 2Cl and CH 3CHClCH 2CH 3 have been determined.

Low energy electron attachment by bromoalkanes

Thermal electron attachment processes in the mixtures of CH3CH2CH2Br, CH3CHBrCH3, CH2BrCH2Br, CH2FCH2Br and CF3CH2Br with carbon dioxide have been investigated using an electron swarm method. It has been found that all the investigated compounds attach electrons only in a two-body process. Corresponding rate constants have been determined to be equal to 1.1×10−11, 1.4×10−12, 1.8×10−8, 5×10−11 and 1.4×10−9cm3molecule−1s−1, respectively. The dependence of the electron capture rate constants on electronic polarizability of the accepting center of the molecule has been demonstrated. The average rate constant for CH3Br, CH3CH2Br and CH3CH2CH2Br equal to (6±2)×10−12cm3molecule−1s−1 is recommended.

Low-Energy Electron Attachment by Chloroalkanes

Thermal electron attachment rate constants for 1,1,2-trichloroethane, 1,1,2,2-tetrachloroethane, 1- and 2-chloropropanes, and 1,1-, 1,2-, 2,2-, and 1,3-dichloropropanes have been measured using an electron swarm method. It has been found that all the investigated compounds attach electrons only in a two-body process. Corresponding rate constants are equal to 1.4 × 10-10, 3.2 × 10-8, 2.7 × 10-13, 3.8 × 10-12, 5.7 × 10-11, 8.1 × 10-12, 6.3 × 10-12, and 1.2 × 10-11 cm3 molecule-1 s-1, respectively. The dependence of the electron capture rate constants on the electronic polarizability of the accepting center of the molecule and the vertical attachment energy has been demonstrated.

Thermal Electron Capture in the Mixtures of Halocarbons and Environmental Gases

The electron capture processes in the mixtures containing halocarbons (CHF3 or CClF3) and nitrogen have been investigated. Second- and third-order kinetics were observed in both cases. The electron interaction with van der Waals complexes CHF3‚N2 and CClF3‚N2 was invoked to explain this behavior and the corresponding rate constants have been determined. The values for two-body processes (5 10 -14 cm 3 molecule -1 s -1 -CHF3 and 1.3 10 -13 cm 3 molecule -1 s -1 -CClF3) are fully consistent with the literature data. The estimated rate constants for electron capture by van der Waals complexes are equal to 1.2 10 -10 and 2.4 10 -11 cm 3 molecule -1 s -1 for CHF3‚N2 and CClF3‚N2, respectively.

Temperature-Dependent Electron Capture Detector Response to Common Alternative Fluorocarbons

The relative electron capture detector (ECD) response to alternative fluorocarbons (AFCs) using gas chromatography are found to be at least 1 order of magnitude lower than that for CFC-12. Detection limits for the chlorofluorocarbons CFC-12, HCFC-22, HCFC-123, and HCFC-124 are found to be 2.5, 90, 30, and 90 pg, respectively. Those for the hydrofluorocarbons are significantly poorer; 14 and 45 ng for HFC-125 and HFC-134a, respectively. HFC-152a was not detected using ECD. Since atmospheric concentrations of these compounds are in the low part-per-trillion level, GC-ECD is apparently not sensitive enough to be used for AFC analysis without substantial preconcentration. Two columns are evaluated for the AFC separation. The Poraplot Q WPLOT column showed good separation ability, though column bleed limits detection performance. A Carboxen 1004 packed column exhibits much lower interference. But separations are time consuming and peak broadening adversely affects limits of detection. Mechanisms for the ECD response are proposed based on thermodynamics and temperature-dependent ECD responses. CFC-12, HCFC-123, and HFC-125 apparently undergo ion-forming dissociative electron capture. The electron capture process for HCFC-22 and HFC-134a appear to form molecular ions. Both mechanisms appear to be operative for HCFC-124 electron capture. Dissociative electron capture rate constants for HCFC-123, HCFC-124, and HFC-125 are estimated to be 3.5 × 10(-)(10), 1.0 × 10(-)(10), and 5.6 × 10(-)(13) cm(3) s(-)(1), respectively at 300 °C.

Electron capture rate constants for solutes in alkane liquids measured by transient photoconductivity

Electron capture rate constants for several solutes in isooctane, cyclohexane and n-hexane are reported. The method of measurement, which allows determination of the rate constants with an accuracy estimated to be better than 5%, involves the use of low power pulses of 337 nm light from a nitrogen laser, and detection of the transient conductivity of escaped electrons.

Resonance Electron Capture Rate Constants for Substituted Nitrobenzenes

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTResonance Electron Capture Rate Constants for Substituted NitrobenzenesW. B. Knighton, R. S. Mock, D. S. McGrew, and E. P. GrimsrudCite this: J. Phys. Chem. 1994, 98, 14, 3770–3776Publication Date (Print):April 1, 1994Publication History Published online1 May 2002Published inissue 1 April 1994https://doi.org/10.1021/j100065a036RIGHTS & PERMISSIONSArticle Views53Altmetric-Citations18LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (841 KB) Get e-Alerts

Electron attachment reactions of perfluoroalkyl transition metal carbonyls: Rate constants and product analysis

Absolute rate constants for electron attachment reactions of CF3Mn(CO)5, CF3Co(CO)4, and C2F5Co(CO)4 have been measured in a gas flow system with electron cyclotron resonance (ECR) detection. Free electrons were generated by Penning ionization of argon gas by helium metastables produced in a low power radio-frequency discharge. The pressure was 2.3–2.5 Torr. The rate constants at 293 K in units of cm3 molecule−1 s−1 are (3.5±0.6)×10−7 for CF3Mn(CO)5; (2.0±0.4)×10−7 for CF3Co(CO)4; and (1.8±0.3)×10−7 for C2F5Co(CO)4. Negative ion mass spectra indicate that nondissociative attachment is important at thermal electron energy. The cross section for nondissociative capture falls rapidly with increasing electron energy. Dissociative product channels are also observed. Electron capture rate constants for several additional molecules are presented and compared with values from the literature. The rate constants in units of cm3 molecule−1 s−1 are 2.8×10−7 for SF6; 3.7×10−7 for CCl4; 3.1×10−7 for CFCl3; 1.8×10−9 for CF2Cl2; 1.5×10−8 for CF3Br; 3.8×10−9 for CHCl3; and 1.2×10−8 for C2F3Cl3.

The determination of electron capture rate constants by the study of photocurrent systems in the non-steady state

The variations with elapsed time of heating and electron emission components of the coulostatic photopotential at a mercury electrode produced by light from a filtered xenon spark source have been studied for wavelengths close to 360 nm and 700 nm using 1 M KCl solution saturated with argon or various N2O/argon gas mixtures. After correcting for heating effects the photopotential component due to electron emission into pure KCl solution during the light pulse, and the relatively slow subsequent return of the deposited (and hydrated) electrons to the electrode by diffusion followed by desolvation, becomes apparent. A comparison of this potential waveform with the waveform observed with N2O present at low concentration gives a value for the bimolecular rate constant for electron capture by N2O of 3.9×109 M−1 s−1, showing that electrons are mainly hydrated prior to homogeneous capture by N2O. The concentration of hydrated electrons close to the electrode falls with increasing temperature in the absence of an added electron acceptor, due partly to the dependence of the diffusion coefficient of eaq− on temperature, and partly to enhanced scattering of the free photoelectrons in the solution phase prior to hydration.

The determination of electron capture rate constants by the study of photocurrent systems in the non-steady state

Summary The variations with elapsed time of heating and electron emission components of the coulostatic photopotential at a mercury electrode produced by light from a filtered xenon spark source have been studied for wavelengths close to 360 nm and 700 nm using 1 M KCl solution saturated with argon or various N2O/argon gas mixtures. After correcting for heating effects the photopotential component due to electron emission into pure KCl solution during the light pulse, and the relatively slow subsequent return of the deposited (and hydrated) electrons to the electrode by diffusion followed by desolvation, becomes apparent. A comparison of this potential waveform with the waveform observed with N2O present at low concentration gives a value for the bimolecular rate constant for electron capture by N2O of 3.9×109 M−1 s−1, showing that electrons are mainly hydrated prior to homogeneous capture by N2O. The concentration of hydrated electrons close to the electrode falls with increasing temperature in the absence of an added electron acceptor, due partly to the dependence of the diffusion coefficient of eaq− on temperature, and partly to enhanced scattering of the free photoelectrons in the solution phase prior to hydration.

Peaкции эaxbata и peкomбиhaции элeкtpohob b жидкom тиклoгeкcahe

The electron-pulse radiation-induced conductivity of aromatic hydrocarbon solutions in cyclohexane and its dependence on the concentration and time are investigated. A current I e due to the electron conductivity is revealed at a concentration of solute [ A] less than 10 −5 mol dm −3. The dependence of I e on [ A] and known values of electron and ion mobilities in cyclohexane lead to rate constants for electron capture by A of (2,1 ± 0,4) × 10 12 and (2,3 ± 0,4) × 10 12 dm 3 mol −1 s −1 for pyrene and diphenyl respectively. The electron and positive ion recombination rate constants are shown to be close to theoretical values for diffusion-controlled recombination.

Pulse-Radiolysis Study of the Formation of Benzyl Radicals from Benzyl Chloride in Cyclohexane in the Gas Phase

The yield of benzyl radical (Bz) formed by electron-beam pulse radiolysis of benzyl chloride (BzCl) in cyclohexane vapor (1.3 atm/120°) has been studied by optical absorption measurements. Electron scavengers lower the benzyl radical (Bz) absorption by competing with dissociative electron capture by BzCl. A study of this competition gave the relative rate constants for electron capture at 120°C (SF6 = 1) : BzCl = 1.6 × 10−3, CCl4 = 0.3, CH3Br = 5 × 10−5, C6H5NO2 ∼ 2 × 10−2. The maximum depression of the absorption, Δ(Bz)max, which was independent of the electron scavenger used and of the BzCl concentration (0.2 to at least 4 mole %), is believed to correspond to the yield of Bz from dissociative capture. With benzyl bromide, acetate, and formate in cyclohexane, Δ(Bz)max was the same as with benzyl chloride. Bz is also formed by processes other than dissociative capture which are not influenced by electron scavengers. The yield from these processes increases with increasing BzCl concentration. Using electron beam radiolysis, the decomposition of BzCl due to dissociative capture, measured by chemical analysis, G(− BzCl)e = 2.6 ± 0.4 molecules per 100 eV. 60Co γ-radiolysis gave G(− BzCl)e = 4.3 ± 0.1, which is close to the value expected if all of the electrons react by dissociative capture. Possible reasons for the low value with electron radiolysis are discussed.