M.-W. Ruf

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

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 earths 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.

Attachment of electrons to molecules at submillielectronvolt resolution

Abstract Using a novel laser photoelectron method, we have measured electron attachment to neutral molecules with sufficiently high energy resolution to investigate the limiting E − 1 2 behaviour of the cross section, predicted for s -wave attachment at electron energies E approaching zero. Our results for SF 6 , obtained with an effective energy width of about 0.2 meV in the energy range 0–200 meV, show that the E − 1 2 dependence is only reached at very low energies (≲1 meV) and that the cross section decreases more strongly towards higher energies, exhibiting a pronounced downward cusp at the threshold for ν 1 vibrational excitation of SF 6 . At the lowest energies, the experimental rate constant is compatible with the theoretical value for capture due to the e − -SF 6 polarization potential.

Dissociative electron attachment to CCl4 molecules at low electron energies with meV resolution

Abstract Using the laser photoelectron attachment (LPA) method we investigated dissociative electron attachment to carbon tetrachloride [e−(E) + CCl4 → Cl− + CCl3] in the electron energy range 0

Photoionization dynamics of excited Ne, Ar, Kr and Xe atoms near threshold

A review of experimental and theoretical studies of the threshold photoionization of the heavier rare-gas atoms is presented, with particular emphasis on the autoionization resonances in the spectral region between the lowest two ionization thresholds 2P3/2 and 2P1/2, accessed from the ground or excited states. Observed trends in the positions, widths and shapes of the autoionization resonances depending on the atomic number, the principal quantum number n, the orbital angular momentum quantum number l and further quantum numbers specifying the fine- and hyperfine-structure levels are summarized and discussed in the light of ab initio and multichannel quantum defect theory calculations. The dependence of the photoionization spectra on the initially prepared neutral state are also discussed, including results on the photoionization cross sections and photoelectron angular distributions of polarized excited states. The effects of various approximations in the theoretical treatment of photoionization in these systems are analysed. The very large diversity of observed phenomena and the numerous anomalies in spectral structures associated with the threshold ionization of the rare-gas atoms can be described in terms of a limited set of interactions and dynamical processes. Examples are provided illustrating characteristic aspects of the photoionization, and sets of recommended parameters describing the energy-level structure and photoionization dynamics of the rare-gas atoms are presented which were extracted in a critical analysis of the very large body of experimental and theoretical data available on these systems in the literature.

Electron transfer from laser excited rydberg atoms to molecules. Absolute rate constants at low and intermediate principal quantum numbers

Using mass spectrometric detection of positive and negative ions, we have investigated ionizing reactions of Ne(ns,nd) Rydberg atoms, efficiently excited by resonant two-photon excitation of metastable Ne(3s3P2) atoms, with electron attaching moleculesBC (BC=SF6, CCl4, CS2, O2) at thermal collision energies. Absolute rate constants have been determined in the range of low and intermediate principal quantum numbersn(5≦n≲30) by utilizing the photoionization signal caused by room temperature black-body radiation and the loss of Ne(3s3P2) atoms, associated with the laser excitation. Substantially differentn-dependences of the electron transfer cross section have been found for the larger molecules (BC = SF6, CCl4) and the smaller molecules (BC = CS2, O2). Simple model calculations have been performed to gain new insight into the dynamics of the electron transfer process; forBC = SF6, our results at lown(5 ≦n ≦ 10) suggest that internal energy conversion in the Coulombic complex Ne+ — SF6− is important for the formation of the detected ions.

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

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.

Formation of negatively-charged cluster ions in thermal energy collisions with state-selected rydberg atoms

Using crossed atomic, molecular cluster, and cw laser beams in conjunction with mass spectrometric ion detection, we have obtained for the first time results for electron transfer fromstate-selected Rydberg atoms to molecular clusters. We report negative ion mass spectra for (CO2)k− (4≦k≦25) and (O2)k− (1≦k≦13) cluster ions, resulting from collisions of Ar** (nd) Rydberg atoms (12≦n≦40) with (CO2)m and (O2)m clusters at relative velocities around 830 m/s, and, for comparison, positive ion mass spectra due to Ne(3s3P2, 0) Penning ionization. For both CO2− and O2-clusters, the negative and the positive ion mass spectra are very different. For (CO2)kEmphasis>/− cluster ions, the mass spectra show distinct variations with principal quantum number of the Rydberg atom, corresponding to differentn-dependences of the effective rate constant for selected cluster ions, as measured relative to the knownn-dependence for SF6− formation in collisions with SF6. For (O2)k− cluster ions, on the other hand, the mass spectra are almost independent ofn with ion intensities, which clearly reflect their thermochemical stabilities (O4− as dominant species).

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

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|.

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.