Electron Attachment Rate Coefficients Research Articles

Dissociative electron attachment to CCl 4 molecules at low electron energies with meV resolution

Using the laser photoelectron attachment (LPA) method we investigated dissociative electron attachment to carbon tetrachloride [ e −( E) + CCl 4 → Cl − + CCl 3] in the electron energy range 0 < E ≤ 173 meV with very high resolution (energy width 1 meV). With reference to a reliable thermal electron attachment rate coefficient k e ( T = 300 K) absolute values for the attachment cross section σ e ( E) are derived. The σ e ( E) ∝ E −1/2 behaviour of the cross section for E → 0, theoretically predicted for s-wave attachment to molecules without permanent electric dipole moment, is approximately reached at very low energies ( E ≲ 0.3 meV). Toward higher energies the cross section decreases more rapidly. At thresholds for vibrational excitation of the neutral molecule, the cross section exhibits pronounced cusp structure of downward step character due to coupling of the attachment process with scattering channels. Comparisons are made with cross sections derived from previous photoelectron attachment work and from experiments involving electron beams, electron swarms and Rydberg atoms. Combination of our LPA cross section with beam data results in a recommended total DA cross section for CCl 4 over the energy range 0–2 eV. Based on this cross section, we calculate and report the energy dependence of the rate coefficient k e ( E) for monoenergetic free electron attachment and the electron temperature dependence of the rate coefficient k e ( T e ) for free electron attachment involving a Maxwellian electron ensemble and CCl 4 gas at room temperature ( T G = 300 K). The effects of electron energy resolution on measured low energy electron attachment yields, in particular on the effective location and energy width of the “zero energy peak,” are discussed in some detail.

Low-temperature electron attachment to CH3I

Absolute rate coefficients for electron attachment to methyl iodidehave been measured under thermal equilibrium conditions between 48 and300 K. The temperature dependence of our results at low temperatures isfound to differ from recent results taken using the laserphotoionization attachment (LPA) method. Tentative explanationsincluding possible effects due to electron attachment to clusters aresuggested.

Competition between nondissociative and dissociative electron attachment to halogenated cyclic alkenes in the gas phase

A flowing-afterglow Langmuir probe apparatus with mass spectral analysis was used to measure rate constants, identify reaction products, and measure branching fractions for electron attachment to 1,2-dichlorohexafluorocyclopentene ( c-C 5F 6Cl 2) and 1,2-dichlorooctafluorocyclohexene ( c-C 6F 8Cl 2) over the temperature range 295–555 K and buffer gas density range of 1.3–2.1 × 10 16 cm −3. Electron attachment to both of these compounds is efficient over this temperature range and the rate coefficient for attachment is relatively independent of temperature. At 295 K the electron attachment rate coefficient for c-C 5F 6Cl 2 is 2.5 ± 0.6 × 10 −7 cm 3 s −1 and for c-C 6F 8Cl 2 is 3.5 ± 0.9 × 10 −7 cm 3 s −1. At 300 K, electron attachment to both neutral reactants is predominantly nondissociative. At higher temperatures, a dissociative reaction channel forming Cl − is observed for both reaction systems. Significant branching fractions for Cl − (≥0.05) are observed at temperatures greater than 425 K for c-C 5F 6Cl 2 and at temperatures greater than 375 K for c-C 6F 8Cl 2. Over the temperature range of 300–550 K the branching fractions for formation of the nondissociative and dissociative products were not dependent on buffer gas density in the range of 1.3–2.1 × 10 16 cm −3.

An Automated System for Near-Real-Time Monitoring of Trace Atmospheric Halocarbons

A new gas chromatographic method developed to quantitatively determine important atmospheric halocarbons is described. Target compounds include replacement CFCs, chlorinated solvents, and biosynthesized (naturally produced) organohalogens, all trace gases in the atmosphere at concentrations ranging from 0.1 to 600 pptv (where pptv = 1 part in 10(-)(12) by volume). A combination of ultralow concentrations and relatively small electron attachment cross sections renders these compounds very difficult to routinely measure in the background air typical of remote atmospheric monitoring stations. Detection is achieved by preconcentration of a 200-mL air sample using an adsorbent-filled microtrap and enhancement of electron capture detector response by oxygen doping one of two detectors connected in series. Oxygen doping specifically targets halocarbons with relatively poor electron attachment rate coefficients. The work described here details construction of a novel analytical system, laboratory trials, and optimization followed by an extended field campaign at a remote atmospheric monitoring station, Mace Head, Ireland. A calibration standard or ambient air sample was acquired every hour using a cyclic, automated procedure without employing cryogenic preconcentration or refocusing. Overall precision of the analytical method for the target compounds is between 0.3 and 1.5%.

Experimental studies of cold electron attachment to SF6, CF3Br, and CCl2F2

A new technique has been used for the measurement of electron attachment rate coefficients for the molecules, SF6, CF3Br, and CCl2F2 at temperatures between 48 and 170 K. The results demonstrate very clearly the strong effect that internal vibrational energy of the molecules has on the attachment process.

Rate Coefficients for Dissociative Electron Attachment by Halomethane Compounds between 300 and 800 K

The temperature dependence of the rate coefficients for thermal energy dissociative electron attachment reactions has been measured for nine halomethane compounds. Rate coefficients for CCl4, CFCl3, CF2Cl2, CF3Cl, CHCl3, CH2Cl2, CHFCl2, CH3I, and CH2Br2 were determined between 300 and 800 K from relative rate measurements by using SF6 as a reference compound. A flowing afterglow−electron cyclotron resonance technique was employed. Mass spectral analysis of product negative ions confirmed a dissociative attachment mechanism. Semiempirical quantum mechanical calculations of the geometry change associated with negative ion formation were carried out.

Comparison of rate coefficients for rydberg electron and free electron attachment

New experimental results on attachment reactions involving free electrons at sub-meV resolution allow for the first time a conclusive comparison of measured rate coefficients for Rydberg electron attachment with those calculated from the measured free electron cross sections on the basis of the quasi-free electron model for Rydberg electron collisions. Using classical velocity distributions for the highn Rydberg electrons and our measured free electron attachment cross sections, we calculate Rydberg electron attachment rate coefficientsknl for the two cases SF6 and HI for Rydberg binding energies |En| of 0.1–40 meV. We find a significant increase inknl towards lower binding energies, especially for HI, which is due to the deviation of the free electron cross section from the limitings-wave behaviour σ0∼E−1/2. The increase at |En|≲2 meV is in qualitative agreement with our highn Rydberg data (n≳80) if l-mixing due to residual electric fields is taken into account. For low l, Rydberg rate coefficientsknl(|En|) are significantly larger than free electron rate coefficientske(E=|En|), while for circular orbits (l=n−1) they agree. On average, attachment reactions of Rydberg electrons in low l orbits proceed with an effective collision energy substantially smaller than the binding energy |En|.

A study of electron attachment to C 70 using the FALP technique

The absolute reaction rate coefficients for electron attachment to C 70 have been measured in the electron temperature, T e, range from 300 to 4500 K using the flowing-afterglow/Langmuir probe technique. A detailed comparison of the electron attaching properties of C 70 with C 60 is made. Thus, whilst electron capture to C 60 occurs only in the p-wave channel at these T e, it is tentatively reasoned that electron capture by C 70 proceeds preferentially in the s-wave channel below a T e of about 1000 K and in the p-wave channel and/or the s-wave channel at the higher T e.

Thermal electron attachment to SF4 and SF6

Rate coefficients for electron attachment to SF4 and SF6 have been measured over the temperature range 300–550 K using a flowing-afterglow Langmuir-probe apparatus. The 300 K rate coefficient for SF4 is 2.5±0.6×10−8 cm3 s−1, a value 11 times smaller than the corresponding rate for SF6. The attachment rate coefficients for both SF4 and SF6 are nearly independent of temperature up to 500 K, and decline somewhat at still higher temperatures. SF−4 is the only ionic product of attachment to SF4 observed over the entire temperature range. SF−6 and SF−5 are products of attachment to SF6; an ‘‘activation energy’’ of 0.42±0.02 eV is inferred for SF−5 production.

SURVEY OF PREBREAKDOWN PARAMETERS OF NF3

Using the pulsed drift-tube-mass spectrometer system, we measured the prebreakdown pa-rameters of NF3 mixtures used in excimer lasers or produced in discharges of SF6 mixtures, including electron attachment coefficients, attachment rate coefficients, ionic mobilities and kinds of negative ions. The relations between electron attachment rate coefficients and mean electron energies are measured as well.

Calculation of time-dependent electron velocity distributions and rate coefficients for electron attachment to chlorofluoroethanes

For electron attachment to 1,1,1-C 2Cl 3F 3, 1,1,2-C 2Cl 3F 3, 1,1-C 2Cl 2F 4, and 1,2-C 2Cl 2F 4 (carrier gas Ar) the time-dependent electron velocity distribution functions are calculated via the Boltzmann equation, resulting in time-dependent attachment rate coefficients smaller than the equilibrium rate coefficients.

Improvements in the design of type IRM-O 2 device for investigations of the micro-sample extension at high temperatures in vacuum : D V Lebedev and B M Ovsyannikov,Zavodsk lab, 33 (3), 1967, 3748–375 ( in Russian)

The absolute reaction rate coefficients for electron attachment to C70 have been measured in the electron temperature, Te, range from 300 to 4500 K using the flowing-afterglow/Langmuir probe technique. A detailed comparison of the electron attaching properties of C70 with C60 is made. Thus, whilst electron capture to C60 occurs only in the p-wave channel at these Te, it is tentatively reasoned that electron capture by C70 proceeds preferentially in the s-wave channel below a Te of about 1000 K and in the p-wave channel and/or the s-wave channel at the higher Te.