Capture Rate Constants Research Articles

Measuring Kinetics under Vibrational Strong Coupling: Testing for a Change in the Nucleophilicity of Water and Alcohols.

Vibrational Strong Coupling (VSC) has been reported to change the rate of organic reactions. However, a lack of convenient and reliable methods to measure reaction kinetics under VSC makes it challenging to obtain mechanistic insight into its influence, hindering progress in the field. Here, we use recently developed fixed-width optical cavities to obtain large kinetic datasets under VSC with small errors (± 1-5%) in an operationally simple manner using UV-vis spectroscopy. The setup is used to test whether VSC changes a fundamental kinetic property of polar reactions, nucleophilicity, for water and alcohols, species commonly used in VSC-modified chemistry. We determined the rate constants for nucleophilic capture with a library of benzhydrilium ions as reference electrophiles with and without strong coupling of the nucleophile's key vibrations. For all investigated combinations of electrophiles and nucleophiles, only minor changes in the observed rate constants of the reactions were observed, independently of the coupled bands. These results indicate that VSC does not substantially alter the nucleophilicity of water and alcohols, suggesting that polar reactions are modified through other, presently unknown mechanisms. Fixed-width cavities allow for convenient and reproducible UV-vis kinetics, facilitating mechanistic studies of VSC-modified chemistry.

Measuring Kinetics under Vibrational Strong Coupling: Testing for a Change in the Nucleophilicity of Water and Alcohols

Vibrational Strong Coupling (VSC) has been reported to change the rate of organic reactions. However, a lack of convenient and reliable methods to measure reaction kinetics under VSC makes it challenging to obtain mechanistic insight into its influence, hindering progress in the field. Here, we use recently developed fixed‐width optical cavities to obtain large kinetic datasets under VSC with small errors (± 1‐5%) in an operationally simple manner using UV‐vis spectroscopy. The setup is used to test whether VSC changes a fundamental kinetic property of polar reactions, nucleophilicity, for water and alcohols, species commonly used in VSC‐modified chemistry. We determined the rate constants for nucleophilic capture with a library of benzhydrilium ions as reference electrophiles with and without strong coupling of the nucleophile’s key vibrations. For all investigated combinations of electrophiles and nucleophiles, only minor changes in the observed rate constants of the reactions were observed, independently of the coupled bands. These results indicate that VSC does not substantially alter the nucleophilicity of water and alcohols, suggesting that polar reactions are modified through other, presently unknown mechanisms. Fixed‐width cavities allow for convenient and reproducible UV‐vis kinetics, facilitating mechanistic studies of VSC‐modified chemistry.

Kinetics of O3 with Ca+ and Its Higher Oxides CaOn+ (n = 1-3) and Updates to a Model of Meteoric Calcium in the Mesosphere and Lower Thermosphere.

The room-temperature rate constants and product branching fractions of CaOn+ (n = 0-3) + O3 are measured using a selected ion flow tube apparatus. Ca+ + O3 produces CaO+ + O2 with k = 9 ± 4 × 10-10 cm3 s-1, within uncertainty equal to the Langevin capture rate constant. This value is significantly larger than several literature values. Most likely, those values were underestimated due to the reformation of Ca+ from the sequential chemistry of higher calcium oxide cations with O3, as explored here. A rate constant of 8 ± 3 × 10-10 cm3 s-1 is recommended. Both CaO+ and CaO2+ react near the capture rate constant with ozone. The CaO+ reaction yields both CaO2+ + O2 (0.80 ± 0.15 branching) and Ca+ + 2O2. Similarly, the CaO2+ reaction yields both CaO3+ + O2 (0.85 ± 0.15 branching) and CaO+ + 2O2. CaO3+ + O3 yield CaO2+ + 2O2 at 2 ± 1 × 10-11 cm3 s-1, about 2% of the capture rate constant. The results are supported using density functional calculations and statistical modeling. In general, CaOn+ + O3 yield CaOn+1+ + O2, the expected oxidation. Some fraction of CaOn+1+ is produced with sufficient internal energy to further dissociate to CaOn-1+ + O2, yielding the same products as the oxidation of O3 by CaOn+. Mesospheric Ca and Ca+ concentrations are modeled as functions of day, latitude, and altitude using the Whole Atmosphere Community Climate Model (WACCM); incorporating the updated rate constants improves agreement with concentrations derived from lidar measurements.

An experimental and chemical kinetic modeling study of the role of potassium in the moist oxidation of CO

The effect of KCl on moist CO oxidation in a laminar flow quartz reactor was investigated. Experiments were conducted in the absence of O2 (gasification), as well as under reducing and fuel-lean conditions, in the temperature range 873–1573 K, and the results were interpreted in terms of a chemical kinetic model. The impact of reactions on the quartz surface of the reactor was carefully examined. Under the conditions of interest, KCl reacts with SiO2 to form potassium silicates, releasing HCl to the gas phase. This reaction alters the condition of the quartz surface, making it more active in catalyzing radical recombination, even in the presence of water vapor. Under gasification and reducing conditions, loss of hydrogen atoms on the wall, enhanced by the exposure to KCl, strongly inhibits CO oxidation, with gas-phase inhibition playing a minor role. Under fuel-lean conditions, the state of the surface does not affect the CO oxidation and the observed inhibition can be attributed to gas-phase reactions. The most important reaction for the inhibition is the chain terminating step KO2 + OH ⇆ KOH + O2. Assuming that this reaction proceeds without a barrier, the rate constant is controlled by a long-range dipole–dipole interaction. We calculate a capture rate constant of 2.5E15 T−0.163 cm3 mol−1 s−1. This value, which is significantly higher than earlier estimates used in modeling, allows a satisfactory prediction of the inhibiting effect of KCl on CO oxidation under oxidizing conditions.

Rotational Cooling Effect on the Rate Constant in the CH3F + Ca+ Reaction at Low Collision Energies.

The rotational cooling effect on the reaction rate constant of the gas-phase ion-polar-molecule reaction CH3F + Ca+ → CH3 + CaF+ was experimentally studied at low collision energies. Fluoromethane molecules showed higher reactivity as the rotational temperature decreased. The experimental rate constants were compared with the capture rate constants which were obtained by the Perturbed Rotational State (PRS) theory assuming the rotational level distribution corresponding to the experimental conditions. The PRS result shows a strong dependence of the capture rate constants on the rotational level distribution in accordance with the experimental findings. However, the PRS capture rate constants deviate from the measurement values as the average collision energy increases especially when the fluoromethane molecules are rotationally cooled far below room temperature. The present paper suggests that the rotational state distribution significantly affects the rate constants of ion-polar-molecule reactions and is one of the important issues to be considered in the study of molecular synthesis in the interstellar medium, where the thermal equilibrium is not necessarily established.

Effect of Intersystem Crossings on the Kinetics of Thermal Ion-Molecule Reactions: Ti+ + O2, CO2, and N2O.

A selected-ion flow tube apparatus has been used to measure rate constants and product branching fractions of 2Ti+ reacting with O2, CO2, and N2O over the range of 200-600 K. Ti+ + O2 proceeds at near the Langevin capture rate constant of 6-7 × 10-10 cm3 s-1 at all temperatures to yield 4TiO+ + O. Reactions initiated on doublet or quartet surfaces are formally spin-allowed; however, the 50% of reactions initiated on sextet surfaces must undergo an intersystem crossing (ISC). Statistical theory is used to calculate the energy and angular momentum dependences of the specific rate constants for the competing isomerization and dissociation channels. This acts as an internal clock on the lifetime to ISC, setting an upper limit on the order of τISC < 1e-11 s. 2Ti+ + CO2 produces 4TiO+ + CO less efficiently, with a rate constant fit as 5.5 ± 1.3 × 10-11 (T/300)-1.1±0.2 cm3 s-1. The reaction is formally spin-prohibited, and statistical modeling shows that ISC, not a submerged transition state, is rate-limiting, occurring with a lifetime on the order of 10-7 s. Ti+ + N2O proceeds at near the capture rate constant. In this case, both Ti+ON2 and Ti+N2O entrance channel complexes are formed and can interconvert over a barrier. The main product is >90% TiO+ + N2, and the remainder is TiN+ + NO. Both channels need to undergo ISC to form ground-state products but TiO+ can be formed in an excited state exothermically. Therefore, kinetic information is obtained only for the TiN+ channel, where ISC occurs with a lifetime on the order of 10-9 s. Statistical modeling indicates that the dipole-preferred Ti+ON2 complex is formed in ∼80% of collisions, and this value is reproduced using a capture model based on the generic ion-dipole + quadrupole long-range potential.

Hydration Activation of MgO Pellets for CO2 Adsorption

Mesoporous MgO is a promising candidate for CO₂ capture. In this study, MgO pellets were activated via hydration and the effects of the process on the texture, structures, and CO₂ adsorption properties of the pelletized MgO adsorbents were systematically studied. Two adsorption kinetics models were used to investigate the CO₂ adsorption kinetics of the pelletized MgO adsorbents. Simultaneously, their kinetic parameters and the influence of adsorption temperatures were determined. It was found that a 3D, flaky, inter-connected network structure appeared after hydration. The hydration process significantly improved the pore structure of the adsorbent and thereby improved the CO₂ adsorption capacity and equilibrium rate constant. The increase in CO₂ adsorption capacity was attributed to the development of their structure rather than the reaction of Mg(OH)₂ with CO₂. The results of a kinetics study showed that a pseudo-second-order model was suitable for predicting the entire adsorption process. Meanwhile, the equilibrium kinetics rate constant of CO₂ capture increased with adsorption temperature, but the initial adsorption rate was mainly related to the texture and structures of the adsorbents. Consequently, hydration by steam or liquid water was deemed an effective method for improving the adsorption performance of MgO pellets, and the hydrated adsorbents were suitable for use in practical CO₂ capture.

Gas-Phase Anionic Metal Clusters are Model Systems for Surface Oxidation: Kinetics of the Reactions of Mn- with O2 (M = V, Cr, Co, Ni; n = 1-15).

The reactions of anionic metal clusters Mn- with O2 (M = V (n = 1-15), Cr (n = 1-15), Co (n = 1-12), and Ni (n = 1-14)) are investigated from 300 to 600 K using a selected-ion flow tube. All rate constants show a positive temperature dependence, well described by an Arrhenius equation. Rate constants exceed (or are extrapolated to exceed at higher temperatures) the Langevin-Gioumousis-Stevenson capture rate constant. Application of a capture model accounting for the finite size of the clusters reproduces the size-dependent trends in reactivity. The assumption that reactivity is further controlled by an energetic barrier early in the reaction coordinate is consistent with the experimental observations. An observed correlation of the derived barrier heights on the electron binding energy of Mn- suggests the barrier may be formed at an avoided crossing between electronic states correlating to Mn- + O2 and Mn + O2- reactants, analogous to that previously proposed for Aln- + O2 systems. The mechanism is analogous to that for reactions of O2 with neutral metal surfaces, indicating that gas-phase reactions of anionic metal clusters can be an appropriate model systems for surface oxidation.

Multipole-moment effects in ion-molecule reactions at low temperatures: part I - ion-dipole enhancement of the rate coefficients of the He+ + NH3 and He+ + ND3 reactions at collisional energies Ecoll/kB near 0 K.

The energy dependence of the rates of the reactions between He+ and ammonia (NY3, Y = {H,D}), forming NY2+, Y and He as well as NY+, Y2 and He, and the corresponding product branching ratios have been measured at low collision energies Ecoll between 0 and kB·40 K using a recently developed merged-beam technique [Allmendinger et al., ChemPhysChem, 2016, 17, 3596]. To avoid heating of the ions by stray electric fields, the reactions are observed within the large orbit of a highly excited Rydberg electron. A beam of He Rydberg atoms was merged with a supersonic beam of ammonia using a curved surface-electrode Rydberg–Stark deflector, which is also used for adjusting the final velocity of the He Rydberg atoms, and thus the collision energy. A collision-energy resolution of about 200 mK was reached at the lowest Ecoll values. The reaction rate coefficients exhibit a sharp increase at collision energies below ∼kB·5 K and pronounced deviations from Langevin-capture behaviour. The experimental results are interpreted in terms of an adiabatic capture model describing the rotational-state-dependent orientation of the ammonia molecules by the electric field of the He+ atom. The model faithfully describes the experimental observations and enables the identification of three classes of |JKMp〉 rotational states of the ammonia molecules showing different low-energy capture behaviour: (A) high-field-seeking states with |KM| ≥ 1 correlating to the lower component of the umbrella-motion tunnelling doublet at low fields. These states undergo a negative linear Stark shift, which leads to strongly enhanced rate coefficients; (B) high-field-seeking states subject to a quadratic Stark shift at low fields and which exhibit only weak rate enhancements; and (C) low-field-seeking states with |KM| ≥ 1. These states exhibit a positive Stark shift at low fields, which completely suppresses the reactions at low collision energies. Marked differences in the low-energy reactivity of NH3 and ND3—the rate enhancements in ND3 are more pronounced than in NH3—are quantitatively explained by the model. They result from the reduced magnitudes of the tunnelling splitting and rotational intervals in ND3 and the different occupations of the rotational levels in the supersonic beam caused by the different nuclear-spin statistical weights. Thermal capture rate constants are derived from the model for the temperature range between 0 and 10 K relevant for astrochemistry. Comparison of the calculated thermal capture rate coefficients with the absolute reaction rates measured above 27 K by Marquette et al. (Chem. Phys. Lett., 1985, 122, 431) suggests that only 40% of the close collisions are reactive.

A study of the translational temperature dependence of the reaction rate constant between CH3CN and Ne+ at low temperatures.

We have measured the translational temperature dependence of the reaction rate constant for CH3CN + Ne+ → products at low temperatures. A cold Ne+ ensemble was embedded in Ca+ Coulomb crystals by a sympathetic laser cooling technique, while cold acetonitrile (CH3CN) molecules were produced by two types of Stark velocity filters to widely change the translational temperatures. The measured reaction rate constant gradually increases with the decrease in the translational temperature of the velocity-selected CH3CN molecules from 60 K down to 2 K, and thereby, a steep increase was observed at temperatures lower than 5 K. A comparison between experimental rate constants and the ion-dipole capture rate constants by the Perturbed Rotational State (PRS) theory was performed. The PRS capture rate constant reproduces well the reaction rate constant at a few kelvin but not for temperatures higher than 5 K. The result indicates that the reaction probability is small compared to typical ion-polar molecule reactions at temperatures above 5 K.

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.

Experimental results of transient testing at the amine plant at Technology Centre Mongstad: Open-loop responses and performance of decentralized control structures for load changes

Flexible operation of combined cycle thermal power plants with chemical absorption post combustion CO2 capture is a key aspect for the development of the technology. Several studies have assessed the performance of decentralized control structures applied to the post combustion CO2 capture process via dynamic process simulation, however there is a lack of published data from demonstration or pilot plants. In this work, experiments on transient testing were conducted at the amine plant at Technology Centre Mongstad, for flue gas from a combined cycle combined heat and power plant (3.7–4.1 CO2 vol%). The experiments include six tests on open-loop responses and eight tests on transient performance of decentralized control structures for fast power plant load change scenarios.The transient response of key process variables to changes in flue gas volumetric flow rate, solvent flow rate and reboiler duty were analyzed. In general the process stabilizes within 1 h for 20% step changes in process inputs, being the absorber column absorption rates the slowest process variable to stabilize to changes in reboiler duty and solvent flow rate. Tests on fast load changes (10%/min) in flue gas flow rate representing realistic load changes in an upstream power plant showed that decentralized control structures could be employed in order to bring the process to desired off-design steady-state operating conditions within (<60 min). However, oscillations and instabilities in absorption and desorption rates driven by interactions of the capture rate and stripper temperature feedback control loops can occur when the rich solvent flow rate is changed significantly and fast as a control action to reject the flue gas volumetric flow rate disturbance and keeping liquid to gas ratio or capture rate constant.

Collision Frequency for Energy Transfer in Unimolecular Reactions.

Pressure dependence of unimolecular reaction rates is governed by the energy transfer in collisions of reactants with bath gas molecules. Pressure-dependent rate constants can be theoretically determined by solving master equations for unimolecular reactions. In general, master equation formulations describe energy transfer processes using a collision frequency and a probability distribution model of the energy transferred per collision. The present study proposes a novel method for determining the collision frequency from the results of classical trajectory calculations. Classical trajectories for collisions of several polyatomic molecules (ethane, methane, tetrafluoromethane, and cyclohexane) with monatomic colliders (Ar, Kr, and Xe) were calculated on potential energy surfaces described by the third-order density-functional tight-binding method in combination with simple pairwise interaction potentials. Low-order (including non-integer-order) moments of the energy transferred in deactivating collisions were extracted from the trajectories and compared with those derived using some probability distribution models. The comparison demonstrates the inadequacy of the conventional Lennard-Jones collision model for representing the collision frequency and suggests a robust method for evaluating the collision frequency that is consistent with a given probability distribution model, such as the exponential-down model. The resulting collision frequencies for the exponential-down model are substantially higher than the Lennard-Jones collision frequencies and are close to the (hypothetical) capture rate constants for dispersion interactions. The practical adequacy of the exponential-down model is also briefly discussed.

Gas-phase chemical ionization of 4-alkyl branched-chain carboxylic acids and 3-methylindole using H3 O+ , NO+ , and O2+ ions.

4-Methyloctanoic acid, 4-ethyloctanoic acid, 4-methylnonanoic acid, and 3-methylindole are primary contributors to the distinctive aroma and flavor of lamb meat. The reactions of H3 O+ , NO+ , and O2+ with these compounds, and identification of the product ions and their distribution, are fundamental to their characterization and rapid, real-time trace analysis using selected ion flow tube mass spectrometry (SIFT-MS). The chemical ionization of pure standards of 4-ethyloctanoic acid, 4-methyloctanoic acid, 4-ethylnonanoic acid, and 3-methylindole was carried out using the H3 O+ , NO+ , and O2+ reagent ions of a V200™ SIFT mass spectrometer. Kinetic data were calculated using the Langevin collision rate with parameterized trajectory equations. Identification of product ions, distribution, and interferences was performed by further evaluation of the pertinent ion-molecule reaction mechanisms, careful spectral analyses, and molecular mass-molecular structure pairing. The collisional capture rate constants of the reaction of the precursor ions H3 O+ , NO+ , and O2+ , their extended hydrates and the analytes, which were assumed to occur at or near the collisional rate, were all of the order of 10-9 cm3 molecule s-1 - typical for bimolecular ion-molecule reactions. Positive identification of the primary and secondary product ions, fragmented ionic species, and potential ion conflicts and interferences, from each reagent ion channel, was determined for each compound. We have established the ion chemistry involved in the ionization of the 4-alkyl branched-chain fatty acids and 3-methylindole using the precursor ions, H3 O+ , NO+ , and O2+ in SIFT-MS. The ion-molecular chemistry and the associated kinetics serve as a fundamental basis for the accurate characterization of these compounds by SIFT-MS.

Calculation of the sensitivity of proton-transfer-reaction mass spectrometry (PTR-MS) for organic trace gases using molecular properties

Proton-transfer-reaction mass spectrometry (PTR-MS) allows the detection of a large number of trace gases in air through proton-transfer reaction with H3O+ reagent ions and detection by a mass spectrometer. Measurement sensitivities can be experimentally determined using calibration gases or calculated using the rate constant for the proton-transfer reaction, but rate constants have only been measured for a subset of compounds. Numerous theoretical approaches that describe the ion-molecule collision processes have shown how to accurately calculate capture collision rate constants between an ion and neutral molecules using the polarizability and permanent dipole moment of the molecule. Here we show that polarizability, dipole moment, and resulting capture rate constants for proton-transfer reactions of H3O+ with various different volatile organic compounds (VOCs) can be obtained using the molecular mass, elemental composition, and functionality of VOCs. The polarizabilities of a class of VOCs possessing a specific number of electronegative atoms were linearly correlated with their molecular mass. The dipole moments in a series of VOCs, in which VOCs contain a specific functional group and arbitrary residual hydrocarbon parts, can be approximated as a constant value. The capture rate constants calculated using polarizability and dipole moment, as estimated from molecular mass, elemental composition, and functional group, agreed within 10% with measured values for most VOCs. Those capture rate constants were applied to the calculation of the sensitivities of VOCs detected by our PTR-MS, taking into account the ion transmission efficiency and the degree of fragmentation of protonated VOCs observed in that instrument as well as chemical properties of the VOCs. The resulting calculated sensitivities agreed within 20–50% of those measured by PTR-MS, but several notable exceptions exist. This result shows that the neutral concentration of a VOC detected as a protonated molecule in PTR-MS can be approximated using only molecular mass, elemental composition, and functionality of the VOC. The present study is useful for all PTR-MS instruments regardless of the type of mass analyzer; however, the identification of elemental composition by high mass resolution instrumentation is important.

Effect of microsolvation on the OH−(H2O)n + CH3I rate constant. comparison of experiment and calculations for OH−(H2O)2 + CH3I

The rate constant for OH−(H2O)2+CH3I reaction was determined by selected ion flow tube (SIFT) experiments for temperatures in the range of 298–398K. It is found to be an order of magnitude smaller than the collision capture rate constant, a result substantially different than found previously for the OH−+CH3I and OH−(H2O)+CH3I reactions. The rate constants for these reactions are only ∼25% and ∼two times smaller, respectively, than their collision capture rate constants. Only two product ions are observed experimentally, i.e. I− and I−(H2O), and their respective percentage yields are 90:10 and 83:17 at 298 and 348K. The kinetics for the OH−(H2O)2+CH3I reaction were also studied by direct dynamics simulations using the DFT/B97-1/ECP/d electronic structure theory, the same theory used in previous direct dynamics simulations of the OH−+CH3I and OH−(H2O)+CH3I reactions. Simulations for OH−(H2O)2+CH3I at 387K give respective percentage yields of 91:9 for I− and I−(H2O), in good agreement with the experimental results. For both the experiments and simulations, the microsolvated ion I−(H2O)2 is not formed and the formation of I− dominates I−(H2O). For the OH−+CH3I and OH−(H2O)+CH3I reactions the experimental and direct dynamics simulation rate constants agree. However, this is not the case for OH−(H2O)2+CH3I, for which the simulation rate constant is 8–9 times larger than the experimental value. Comparisons of the experimental, simulation, and collision capture rate constants for the OH−(H2O)2+CH3I reaction indicate the height of the submerged SN2 barrier for the reaction is an important feature of its potential energy surface. The actual barrier is expected to be higher than the value given by the DFT/B97-1 calculations. In future work it will be important to perform higher level electronic structure calculations and establish an accurate value for this barrier. Preliminary calculations reported here indicate the barrier height is sensitive to the electronic structure method.

Protonated 1,4-difluorobenzene C6H5F2+: A promising precursor for proton-transfer chemical ionization

The reactivity of protonated 1,4-difluorobenzene C6H5F2+ as a Proton Transfer Reaction (PTR) precursor in a compact Fourier Transform Ion Cyclotron Resonance (FTICR) mass spectrometer has been evaluated in comparison with H3O+ precursor. The C6H5F2+ ion reacts with unsaturated and oxygenated VOCs by proton transfer. Kinetic studies have shown that this precursor induces less fragmentation than H3O+, so that the protonated molecule is detected in cases where H3O+ leads to total fragmentation following proton transfer. For the analytes considered the reaction rate constants are ca 80% of the capture rate constants. Use of C6H5F2+ in trace analysis conditions has been tested on a standard mixture of VOCs diluted in air. The protonation rates measured in this way using both precursors depend linearly on concentration, allowing quantitative analysis. Their ratio to theoretical capture rates is generally slightly higher with C6H5F2+ than with H3O+.

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.

Ultrafast dynamics of defect-assisted electron-hole recombination in monolayer MoS2.

In this Letter, we present nondegenerate ultrafast optical pump-probe studies of the carrier recombination dynamics in MoS2 monolayers. By tuning the probe to wavelengths much longer than the exciton line, we make the probe transmission sensitive to the total population of photoexcited electrons and holes. Our measurement reveals two distinct time scales over which the photoexcited electrons and holes recombine; a fast time scale that lasts ∼ 2 ps and a slow time scale that lasts longer than ∼ 100 ps. The temperature and the pump fluence dependence of the observed carrier dynamics are consistent with defect-assisted recombination as being the dominant mechanism for electron-hole recombination in which the electrons and holes are captured by defects via Auger processes. Strong Coulomb interactions in two-dimensional atomic materials, together with strong electron and hole correlations in two-dimensional metal dichalcogenides, make Auger processes particularly effective for carrier capture by defects. We present a model for carrier recombination dynamics that quantitatively explains all features of our data for different temperatures and pump fluences. The theoretical estimates for the rate constants for Auger carrier capture are in good agreement with the experimentally determined values. Our results underscore the important role played by Auger processes in two-dimensional atomic materials.

Capture Collisions of Polyynide Anions with Hydrogen Atoms: Effect of the Ion Dipole, Quadrupole, and Anisotropic Polarizability

Classical capture rate constants are calculated for a molecular ion with a dipole moment, quadrupole moment, and anisotropic polarizability colliding with a neutral polarizable atom. The free-energy effective potential method is used to average the potential energy over the interaction angle. Numerical methods are used to calculate the effective potential from the full anisotropic induction potential energy function, to solve for the position of the centrifugal barrier, and to determine the thermal rate constants. The capture collision model is applied to the reactions of polyynide anions, HC4−, HC6−, and HC8−, with atomic hydrogen, which are of interest in astrophysical environments, using calculated multipole moments and anisotropic polarizabilities. The dipole moment, quadrupole moment, and polarizability of the ion each contribute significantly toward the total predicted classical capture collision rates for these systems.