Publisher Summary This chapter discusses resonance and threshold phenomena in low-energy electron collisions with molecules and clusters. Low-energy collisions of electrons with atoms and molecules are among the most important elementary processes in gaseous environments such as discharges, arcs, gas lasers, gaseous dielectrics and the earth's atmosphere. The dynamics behavior of low-energy electron-molecule collisions is discussed. The dynamical behavior of slow electrons traversing gases is to a large extent determined by two effects: the energy dependent evolution of the scattering phases for the relevant partial waves and the influence of temporary negative ion states (resonances). Some aspects of resonance and threshold phenomena are discussed. The theoretical description of electron-molecule collisions generally requires an adequate description of electronic, vibrational and rotational degrees of freedom. However, if the typical collision time is short compared to the rotational period, the molecule can be treated as having a fixed orientation during the collision process, and the result for the cross-section can be averaged over orientations. Treatment of vibrational dynamics is usually more important and more challenging to the theory. In the electron energy region important for applications, many inelastic processes such as vibrational excitation and dissociative electron attachment are driven by negative-ion resonances. The theoretical description of vibrational dynamics in these cases is usually based on the nonlocal complex potential describing the nuclear motion in the intermediate negative-ion state.

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Resonance and Threshold Phenomena in Low-Energy Electron Collisions with Molecules and Clusters

Dissociative electron attachment (DEA) processes occur in many important applied contexts, particularly gas discharges, plasmas, biological systems, and astrophysical environments. In this review, we survey the basic physics of DEA and the progress that has been made during past 14 years since the last important review on DEA (Hotop et al., Adv. At. Mol. Opt. Phys. 49, 86). This progress includes studies of DEA to simple diatomic and polyatomic molecules with high energy resolution revealing vibrational Feshbach resonances and threshold structures, studies of angular distribution of the fragmentation products allowing analysis of the symmetries of the resonances involved, and theoretical developments in investigating the dynamics of nuclear motion in DEA processes. Particular attention is paid to recent advances in DEA to biological molecules as the process is important for understanding radiation damage. Recent progress in understanding electron attachment to van der Waals clusters and the influence of cluster environments on DEA is also reviewed. The review concludes with a forward look and suggestions for new research directions.

Recent Progress in Dissociative Electron Attachment: From Diatomics to Biomolecules

The threshold energy and the threshold behaviour for electron impact ionization of CH4 were investigated at two temperatures, 293 and 693 K. The study was performed with a crossed electron–molecule beam apparatus with an electron energy resolution of 120 meV full width at half maximum. The values of the ionization energies (IEs) and the threshold behaviour were determined using a fitting procedure involving a convolution of the cross section and the electron energy distribution function. At 293 K the following IEs were obtained: IE(CH4+/CH 4) = 12.65 ± 0.4 eV, IE1(CH3+/CH 4) = 13.58 ± 0.1 eV (ion pair) and IE2(CH3+/CH 4) = 14.34 ± 0.1 eV. At 693 K a red shift in these IEs of about 0.14 eV was observed for both CH4+/CH 4 and CH3+/CH 4 processes, which reflects the change in the internal energy of CH4 with increasing temperature. In addition, at 293 K IEs were determined also for the small fragment ions, i.e. IE(CH2+/CH 4) = 15.1 ± 0.1 eV, IE(CH+/CH 4) = 19.8 ± 0.1 eV and IE(C+/CH 4) = 20.5 ± 0.2 eV.

https://iopscience.iop.org/article/10.1088/0953-4075/36/2/307/pdf

Electron impact ionization of CH4: ionization energies and temperature effects

Cross sections and rate coefficients for capture of low-energy electrons with polar and polarizable target molecules are calculated in the framework of Fabrikant and Hotop’s extended version of the Vogt–Wannier model and an extension of this approach is given in the present article Analytical approximations are derived in order to facilitate the application to experiments A comparison with a selection of experimental electron attachment rate coefficients provides insight into the competition between anion formation through electron capture and scattering processes which do not follow this pathway

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Modelling low-energy electron-molecule capture processes.

Using a variant of the laser photoelectron attachment (LPA) method with an extended energy range (EXLPA), we have studied low-energy electron attachment to the molecules CCl4 (Cl− and Cl−2 formation) and SF6 (SF−6 and SF−5 formation) in a diffuse gas target (TG = 300 K) from 0 eV up to 2 eV at energy widths down to 14 meV. In the EXLPA method, pulses of near-zero energy photoelectrons are produced in a guiding magnetic field, accelerated by a weak electric field, brought to the energy of interest prior to their traversal through the target region and subsequently accelerated and deflected onto a detecting plate. Anions due to electron attachment are extracted by a pulsed electric field, during which the photoelectron current is interrupted, and detected by a quadrupole mass spectrometer. The EXLPA anion yields are combined with absolute cross sections, obtained at very high resolution (≈1 meV) with the LPA method over the range 0–0.17 eV, to yield new recommended absolute partial and total attachment cross sections over the range 0–2 eV at the well-defined gas temperature TG = 300 K. Our cross sections show characteristic deviations from previously reported results. At least in part, these differences can be attributed to the fact that in the earlier electron beam experiments the gas temperature was higher than 300 K. For SF6, the branching fractions for SF−5 formation at electron energies 0.002–0.43 eV and for different initial rovibrational distributions are compared with those recently predicted from kinetic modelling within the framework of statistical unimolecular rate theory. Satisfactory agreement is observed, but our data provide evidence that an additional path for producing SF−6 and SF−5 ions is available at electron energies above about 0.3 eV.

https://iopscience.iop.org/article/10.1088/0953-4075/42/12/125202/pdf

High-resolution electron attachment to the molecules CCl4 and SF6 over extended energy ranges with the (EX)LPA method

We report the results of the experimental determination of the appearance energy values AE(Xn + /X) for the formation of multiply charged Ne, Ar and Xe ions up to n = 4 (Ne), n = 6 (Ar) and n = 8 (Xe) following electron impact on Ne, Ar and Xe atoms using a dedicated high-resolution electron impact ionization mass spectrometer. The data analysis uses the Marquart-Levenberg algorithm, which is an iterative, nonlinear least-squares-fitting routine, in conjunction with either a two-function or a three-function fit based on a power threshold law. This allows us to extract the relevant AEs and corresponding exponents for a Wannier-type power law from the measured near-threshold data. The values of the AEs determined in this work are compared with other available experimental and spectroscopic values of the AEs and the extracted exponents are compared with other available experimental data and with the predictions of the various Wannier-type power law models.

https://iopscience.iop.org/article/10.1088/0953-4075/35/13/312/pdf

Electron impact multiple ionization of neon, argon and xenon atoms close to threshold: appearance energies and Wannier exponents

Using the laser photoelectron attachment method with an energy width ≤ 1 meV at low energies (≤ 172 meV) and an electron beam method (energy width about 70 meV) at higher energies (up to 2 eV), we have determined absolute cross sections σe (E) for dissociative electron attachment to the dipolar molecules CH2Br2 and CCl3Br by normalization to known electron attachment rate coefficients. At thresholds for vibrational excitation of the CH2Br2 molecule, the DA cross section exhibits pronounced structure due to coupling of the attachment process with scattering channels; below the ν3 = 1 onset for the symmetric CBr2 stretch vibration, a clear vibrational Feshbach resonance is observed. At low energies the cross sections σe (E) show an energy dependence between E−1/2 and E−1, in essential agreement with predictions of an extended Vogt–Wannier (EVW) capture model which includes the long-range electron–dipole interaction in addition to the polarization force. The determined absolute values are, however, substantially smaller than those predicted by the EVW model. Semiempirical R-matrix calculations have been carried out which show that at low electron energies (E < 0.15 eV) Br− formation from CCl3Br proceeds by s-wave attachment to the anion ground state while a broad peak, observed around 0.6 eV and evolving predominantly into the Cl− channel, is due to an excited anion state. Comparisons are made with cross sections and rate coefficients obtained in previous photoelectron attachment work (TPSA) and in electron beam as well as swarm experiments. Based on our joint experimental results for σe (E), we report the electron temperature dependence of the rate coefficients ke (Te) for free electron attachment involving a Maxwellian electron ensemble and a gas at room temperature (TG = 300 K).

http://iopscience.iop.org/article/10.1088/0953-4075/35/20/306/pdf

M. Stano

Papers

Electron impact multiple ionization of neon, argon and xenon atoms close to threshold: appearance energies and Wannier exponents

High-resolution study of dissociative electron attachment to dipolar molecules at low energies: CH2Br2 and CCl3Br