Ilya I. Fabrikant

Absolute cross sections for dissociative electron attachment to condensed CH3Cl and CH3Br: Effects of potential energy curve crossing and capture probability

We report cross sections for the trapping of 0–10 eV electrons by CH3Cl and CH3Br physisorbed onto a Kr covered Pt substrate, measured as a function of Kr film thickness and methyl halide concentration. The molecules stabilize electrons incident at the surface by the dissociation of transient CH3Cl− and CH3Br− ions into an atomic anion and a neutral fragment [dissociative electron attachment DEA]. For CH3Cl, the condensed phase absolute DEA cross section at ≈0.5 eV, reaches 13×10−18 cm2±50%, which is 104–106 times larger than the gas phase cross section. At higher energies (5–10 eV) for CH3Cl, our measurements provide a lower limit for the DEA cross section. For CH3Br, the maximum DEA cross section occurs below the vacuum level; we measure an absolute magnitude of 3.0×10−16 cm2±50% near 0 eV, which is 100 times larger than the corresponding gas phase value. These enhancements in cross section arise from the lowering of the potential energy surfaces of intermediate anions due to polarization induced in the...

On the Validity of the Arrhenius Equation for Electron Attachment Rate Coefficients

The validity of the Arrhenius equation for dissociative electron attachment rate coefficients is investigated. A general analysis allows us to obtain estimates of the upper temperature bound for the range of validity of the Arrhenius equation in the endothermic case and both lower and upper bounds in the exothermic case with a reaction barrier. The results of the general discussion are illustrated by numerical examples whereby the rate coefficient, as a function of temperature for dissociative electron attachment, is calculated using the resonance R-matrix theory. In the endothermic case, the activation energy in the Arrhenius equation is close to the threshold energy, whereas in the case of exothermic reactions with an intermediate barrier, the activation energy is found to be substantially lower than the barrier height.

Dissociative attachment in hot CH3Cl: Experiment and theory

The dissociative attachment (DA) cross section of hot CH3Cl has been measured in a crossed electron–molecule beam apparatus at temperatures up to 750 K and electron energies from 0–0.5 eV. The results are compared to cross sections computed using a mixed ab initio‐semiempirical approach, treating CH3Cl as a quasidiatomic molecule. The theoretical treatment requires an anion potential curve in the stable region as a portion of the input data. Computations with three different basis sets show the results to be sensitive to the size of basis set from which the potential is determined. At high temperatures, the experimental DA cross sections are found to be in very good agreement with those derived from theory using the potential curve computed with the most flexible of the basis sets. At room temperature the theory suggests that the measured DA cross section is still limited by the presence of impurities.

Electron attachment to molecules in a cluster environment

Low-energy dissociative electron attachment (DEA) to the CF(2)Cl(2) and CF(3)Cl molecules in a water cluster environment is investigated theoretically. Calculations are performed for the water trimer and water hexamer. It is shown that the DEA cross section is strongly enhanced when the attaching molecule is embedded in a water cluster, and that this cross section grows as the number of water molecules in the cluster increases. This growth is explained by a trapping effect that is due to multiple scattering by water molecules while the electron is trapped in the cluster environment. The trapping increases the resonance lifetime and the negative ion survival probability. This confirms qualitatively existing experiments on electron attachment to the CF(2)Cl(2) molecule placed on the surface of H(2)O ice. The DEA cross sections are shown to be very sensitive to the position of the attaching molecule within the cluster and the orientation of the electron beam relative to the cluster.

Interaction of laser radiation with a negative ion in the presence of a strong static electric field

This paper provides a general theoretical description of a weakly bound atomic system (a negative ion) interacting simultaneously with two (generally strong) fields, a static electric field and a monochromatic laser field having an arbitrary elliptical polarization. The zero-range δ-potential is used to model the interaction of a bound electron in a negative ion as well as the interaction of a detached electron with the residual atom. Our treatment combines the quasistationary (complex energy) and quasienergy (Floquet) approaches. This quasistationary, quasienergy state (QQES) formalism is the most appropriate one for analysing a decaying quantum system under the influence of a periodic external perturbation. Existing QQES theory is reviewed and some new results are discussed: the Hellmann-Feynman theorem and the normalization procedure for QQES, and the definition of the dipole moment and the dynamic polarizability for a decaying atomic system (in strong static electric and/or laser fields). These results are illustrated using analytical formulae obtained from an exact solution of the QQES problem for a δ-model potential in two strong fields. Finally, from the imaginary part of the dynamic polarizability we obtain analytic results (to first order in the laser-field intensity) for the photodetachment cross section. Our results are then compared with those of previous theoretical studies.

Semiempirical R-matrix theory of low energy electron-CF3Cl inelastic scattering

We apply a semiempirical R-matrix theory to calculations of vibrational excitation and dissociative attachment in the molecule for electron energies below about 3 eV. We employ two sets of model parameters corresponding to two different forms of the potential curve. We find that our present, ab initio calculated anion curve gives vibrational excitation and dissociative attachment cross sections in good agreement with experimental measurements. We also compare the results of our theory with those of a recently published classical theory.

Vibrational Feshbach resonances in electron attachment to nitrous oxide clusters: decay into heterogeneous and homogeneous cluster anions

Abstract Using a high-resolution (ΔE≈1 meV) laser photoelectron attachment method, we have studied cluster anion formation in collisions of low-energy electrons (1–180 meV) with (N2O)N clusters. We show that formation of both heterogeneous cluster anions (N2O)qO− (q

Semiempirical calculations of inelastic electron-methylchloride scattering

Resonant R-matrix theory is used to describe inelastic electron-methylchloride scattering in a semiempirical way. Parameters of the theory are obtained by fitting differential vibrational excitation cross sections to experimental ones. Some specific features of the vibrational excitation cross sections are discussed, particularly the competition between the dipole and resonant contributions. The obtained parameters are used for calculations of the dissociative attachment from CH3Cl and CD3Cl with production of the Cl- negative ions. The results for a low-energy peak agree satisfactorily with the swarm analysis of Datskos et al. (1990) at temperatures of about 700-800 K. At lower temperatures the experimental cross sections in all measurements strongly exceed the theoretical ones.

The dependence of low-energy electron attachment to CF3Br on electron and vibrational energy

In a joint experimental and theoretical effort, we have studied dissociative electron attachment (DEA) to the CF3Br molecule at electron energies below 2 eV. Using two variants of the laser photoelectron attachment method with a thermal gas target (T(G) = 300 K), we measured the energy dependent yield for Br- formation over the range E = 3-1200 meV with resolutions of about 3 meV (E < 200 meV) and 35 meV. At the onsets for excitation of one and two quanta for the C-Br stretching mode nu3, downward cusps are detected. With reference to the recommended thermal (300 K) attachment rate coefficient k(A)(CF3Br) = 1.4 x 10(-8) cm3 s(-1), absolute cross sections have been determined for Br- formation. In addition, we studied Br- and (CF3Br)Br- formations with a seeded supersonic target beam (10% CF3Br in helium carrier gas, with a stagnation pressure of 1-4 bars and nozzle temperatures of 300 and 600 K) and found prominent structure in the anion yields due to cluster formation. Using the microwave pulse radiolysis swarm technique, allowing for controlled variation of the electron temperature by microwave heating, we studied the dependence of the absolute DEA rate coefficient on the mean electron energy E over the range of 0.04-2 eV at gas temperatures T(G) ranging from 173 to 600 K. For comparison with the experimental results, semiempirical resonance R-matrix calculations have been carried out. The input for the theory includes the known energetic and structural parameters of the neutral molecule and its anion; the parameters of the resonant anion curves are chosen with reference to the known thermal rate coefficient for the DEA process. For the gas temperature T(G) = 300 K, good overall agreement of the theoretical DEA cross section with the experimental results is observed; moreover, rate coefficients for Br- formation due to Rydberg electron transfer, calculated with both the experimental and the theoretical DEA cross sections, are found to agree with the previously reported absolute experimental values. At T(G) = 300 K, satisfactory agreement is also found between the calculated and experimental attachment rate coefficients for mean electron energies E = 0.04-2 eV. The strong increase of the measured rate coefficients with rising gas temperature, however, could be only partially recovered by the R-matrix results. The differences may result from the influence of thermal excitations of other vibrational modes not included in the theory.

A model describing inelastic processes in low-energy electron collisions with methyl chloride

Dissociative attachment and vibrational excitation processes in low-energy electron collisions with methyl chloride are considered. A theory based on a combination of the resonance R-matrix approach and the non-local-complex-potential approach is applied for the calculation of the cross sections. For the input data of the theory the author uses Falcetta and Jordans calculation of the adiabatic potential curve for the CH3Cl- anion. The calculated dissociative attachment cross sections averaged over the thermal vibrational distribution are about three orders of magnitude less than those obtained in a beam experiment of Chu et al. (1990). Possible reasons for this discrepancy are analysed. Vibrational excitation cross sections exhibit a sharp threshold peak and a broad resonance with the position corresponding to the vertical attachment energy. The author also investigates the sensitivity of the results to the input parameters.