H. Hotop

Binding energies in atomic negative ions

This article updates a ten‐year‐old review of this subject [J. Chem. Phys. Ref. Data 4, 539 (1975)]. A survey of the electron affinity determinations for the elements up to Z=85 is presented, and based upon these data, a set of recommended electron affinities is established. Recent calculations of atomic electron affinities and the major semiempirical methods are discussed and compared with experiment. The experimental methods which yield electron binding energy data are described and intercompared. Fine structure splittings of these ions and excited state term energies are given.

Electron affinities of Cu and Ag

In a crossed‐beam experiment, photodetachment of Cu− and Ag− ions by an Ar‐ion laser beam (4880 A) is studied by energy analyzing the detached electrons. The electron affinities are found to be: E.A. (Cu)=(1.226 ± 0.010) eV, E.A. (Ag)=(1.303−0.011+0.007) eV. These electron affinities are approximately 0.5 eV less than those determined by surface ionization studies. The surface ionization technique is discussed and possible reasons for the discrepancies are advanced. The new electron affinities are used to estimate those of the atoms in the long series of the periodic table. Measurements of the angular distribution of the photodetached electrons yield β=2.00 ± 0.03 for the anisotropy parameter β in both cases.

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.

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.

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).

Erratum to: Experimental and theoretical cross sections for photoionization of metastable Xe*(6s 3P2, 3P0) atoms near threshold

Using high resolution laser photoelectron spectrometry we have determined absolute cross sections σJ0J1 and the electron angular distribution parameter for one photon ionization of metastable Xe*(6s3PJ0, J0 = 2, 0) atoms to the resolved Xe+ (2PJ1, J1 = 3/2, 1/2) ion states at several wavelengths near threshold. For comparison with the present and future experimental data we have calculated partial cross sections and ß-parameters for photoionization of Xe*(6s3PJ0, J0 = 2, 0) and of the analogous alkali atom Cs(6s) over the photoelectron energy range (0–5) eV. We have used both a term-dependent Pauli-Fock (PF) approach and a configuration interaction method involving Pauli-Fock atomic orbitals (CIPF). Through the PF method we include relativistic effects on the atomic orbitals; the CIPF method was designed to take into account the important electron correlation effects which are found to be essential for obtaining good agreement between the theoretical and experimental results.