On the relation between the activation energy for electron attachment reactions and the size of their thermal rate coefficients

Formulation of the cosmic ray-driven electron-induced reaction mechanism for quantitative understanding of global ozone depletion

This paper formulates the cosmic ray-driven electron-induced reaction as a universal mechanism to provide a quantitative understanding of global ozone depletion. Based on a proposed electrostatic bonding mechanism for charge-induced adsorption of molecules on surfaces and on the measured dissociative electron transfer (DET) cross sections of ozone-depleting substances (ODSs) adsorbed on ice, an analytical equation is derived to give atmospheric chlorine atom concentration: [Formula: see text] where Φe is the prehydrated electron (epre-) flux produced by cosmic ray ionization on atmospheric particle surfaces, [Formula: see text] is the surface coverage of an ODS, and ki is the ODS's effective DET coefficient that is the product of the DET cross section, the lifetimes of surface-trapped epre- and Cl-, and the particle surface area density. With concentrations of ODSs as the sole variable, our calculated results of time-series ozone depletion rates in global regions in the 1960s, 1980s, and 2000s show generally good agreement with observations, particularly with ground-based ozonesonde data and satellite-measured data over Antarctica and with satellite data in a narrow altitude band at 13 to 20 km of the tropics. Good agreements with satellite data in the Arctic and midlatitudes are also found. A previously unreported effect of denitrification on ozone loss is found and expressed quantitatively. But this equation overestimates tropospheric ozone loss at northern midlatitudes and the Arctic, likely due to increased ozone production by the halogen chemistry in polluted regions. The results render confidence in applying the equation to achieve a quantitative understanding of global ozone depletion.

Low-energy electron interactions with chlorotrimethylsilane (Si(CH3 )3 Cl), dichlorodimethylsilane (Si(CH3 )2 Cl2 ) and chloromethyldimethylsilane (SiH(CH3 )2 (CH2 Cl))

RATIONALE

Silane derivatives are widely used in industrial plasmas for manufacturing lighting devices, solar cells, displays, etc. Models of technological plasmas require quantitative data. The rate coefficients (k) and the activation energies (E_a ) of thermal electron attachment for chlorotrimethylsilane (Si(CH₃ )₃ Cl), dichlorodimethylsilane (Si(CH₃ )₂ Cl₂ ) and chloromethyldimethylsilane (SiH(CH₃ )₂ (CH₂ Cl)) are reported. This is important for understanding the basic processes occurring in plasmas.

METHODS

The pulsed Townsend technique (known as the electron swarm method) has been applied for the measurements. In this technique, electrons generated by a laser, under a uniform electric field, traverse to an anode and induce a charge on it. In the buffer gas charge grows linearly, but in the presence of a scavenger, electrons are captured, and thus the rate of charge increase slows down with time. From the shape of the pulse, the kinetic parameters are determined.

RESULTS

Kinetic parameters from the study of thermal electron attachment by Si(CH₃ )₃ Cl, Si(CH₃ )₂ Cl₂ and SiH(CH₃ )₂ (CH₂ Cl) were determined for the first time. The corresponding rate coefficients at 298 K are equal to (9.56 ± 0.02) × 10^-11 , (6.62 ± 0.02) × 10^-11 and (1.24 ± 0.05) × 10^-11  cm³  s^-1 and E_a values are equal to 0.29 ± 0.01, 0.24 ± 0.01 and 0.31 ± 0.01 eV for Si(CH₃ )₃ Cl, Si(CH₃ )₂ Cl₂ and SiH(CH₃ )₂ (CH₂ Cl), respectively. The experiment was performed in the 298-378 K temperature range.

CONCLUSIONS

The presented results provide important new information about fundamental quantities such as rate coefficients and activation energies for thermal electron capture by chlorinated derivatives of silane. These data can further advance our understanding of thermal electron interactions with chlorosilanes that can be used to control the important species in the plasmas of many modern technologies.

Studies on the Processes of Electron Capture and Clustering of Benzyl Chloride by Ion Mobility Spectrometry

This paper presents the results of the study on the course of the benzyl chloride (BzCl) ionization process in a drift tube ion mobility spectrometer (DT IMS) in which nitrogen was used as the carrier gas. BzCl ionization follows the dissociative electron capture mechanism. The chloride ions produced in this process take part in the formation of cluster ions. Using DT IMS allows for estimation of the value of the electron attachment rate for BzCl and the equilibrium constant for the cluster ion formation. The basic experimental method used in this work was to analyze drift time spectra obtained for the introduction of the sample to the spectrometer with the drift gas. The theoretical interpretation of the results is based on the mathematical description of the ion transport. This description takes into account the phenomenon of diffusion, as well as the processes of formation and dissociation of ionic clusters occurring during the movement of ions in the drift section.

Contrast between the mechanisms for dissociative electron attachment to CH3SCN and CH3NCS

The kinetics of thermal electron attachment to methyl thiocyanate (CH₃SCN), methyl isothiocyanate (CH₃NCS), and ethyl thiocyanate (C₂H₅SCN) were measured using flowing afterglow-Langmuir probe apparatuses at temperatures between 300 and 1000 K. CH₃SCN and C₂H₅SCN undergo inefficient dissociative attachment to yield primarily SCN^- at 300 K (k = 2 × 10^-10 cm³ s^-1), with increasing efficiency as temperature increases. The increase is well described by activation energies of 0.17 eV (CH₃SCN) and 0.14 eV (C₂H₅SCN). CN^- product is formed at <1% branching at 300 K, increasing to ∼30% branching at 1000 K. Attachment to CH₃NCS yields exclusively SCN^- ionic product but at a rate at 300 K that is below our detection threshold (k < 10^-12 cm³ s^-1). The rate coefficient increases rapidly with increasing temperature (k = 6 × 10^-11 cm³ s^-1 at 600 K), in a manner well described by an activation energy of 0.51 eV. Calculations at the B3LYP/def2-TZVPPD level suggest that attachment to CH₃SCN proceeds through a dissociative state of CH₃SCN^-, while attachment to CH₃NCS initially forms a weakly bound transient anion CH₃NCS^-* that isomerizes over an energetic barrier to yield SCN^-. Kinetic modeling of the two systems is performed in an attempt to identify a kinetic signature differentiating the two mechanisms. The kinetic modeling reproduces the CH₃NCS data only if dissociation through the transient anion is considered.

Electron attachment to CF3 and CF3Br at temperatures up to 890 K: experimental test of the kinetic modeling approach

Thermal rate constants and product branching fractions for electron attachment to CF3Br and the CF3 radical have been measured over the temperature range 300-890 K, the upper limit being restricted by thermal decomposition of CF3Br. Both measurements were made in Flowing Afterglow Langmuir Probe apparatuses; the CF3Br measurement was made using standard techniques, and the CF3 measurement using the Variable Electron and Neutral Density Attachment Mass Spectrometry technique. Attachment to CF3Br proceeds exclusively by the dissociative channel yielding Br(-), with a rate constant increasing from 1.1 × 10(-8) cm(3) s(-1) at 300 K to 5.3 × 10(-8) cm(3) s(-1) at 890 K, somewhat lower than previous data at temperatures up to 777 K. CF3 attachment proceeds through competition between associative attachment yielding CF3 (-) and dissociative attachment yielding F(-). Prior data up to 600 K showed the rate constant monotonically increasing, with the partial rate constant of the dissociative channel following Arrhenius behavior; however, extrapolation of the data using a recently proposed kinetic modeling approach predicted the rate constant to turn over at higher temperatures, despite being only ~5% of the collision rate. The current data agree well with the previous kinetic modeling extrapolation, providing a demonstration of the predictive capabilities of the approach.

Pressure and temperature dependence of dissociative and non-dissociative electron attachment to CF3: experiments and kinetic modeling

The kinetics of electron attachment to CF(3) as a function of temperature (300-600 K) and pressure (0.75-2.5 Torr) were studied by variable electron and neutral density attachment mass spectrometry exploiting dissociative electron attachment to CF(3)Br as a radical source. Attachment occurs through competing dissociative (CF(3) + e(-) → CF(2) + F(-)) and non-dissociative channels (CF(3) + e(-) → CF(3)(-)). The rate constant of the dissociative channel increases strongly with temperature, while that of the non-dissociative channel decreases. The rate constant of the non-dissociative channel increases strongly with pressure, while that of the dissociative channel shows little dependence. The total rate constant of electron attachment increases with temperature and with pressure. The system is analyzed by kinetic modeling in terms of statistical theory in order to understand its properties and to extrapolate to conditions beyond those accessible in the experiment.