The authors attempt to give a comprehensive discussion of observations of atomic negative-ion resonances throughout the periodic table. A review of experimental and theoretical approaches to the study of negative-ion resonances is given together with a consideration of the various schemes that are used for their classification. In addition to providing, where possible, tabulated data for the energies, widths, and symmetries of these states, the authors also attempt to highlight regularities in their behavior both within groups of the periodic table and along isoionic sequences.

The production of free-free radiation in collisions of low-energy electrons with Ar atoms has been measured using the drift-tube technique. The excitation coefficients were obtained from measurements of the absolute intensity of continuum radiation at wavelengths of 500, 650, and 1300 nm. At 650 nm the electric-field—to—gas-density ratio $\frac{E}{N}$ was varied from 0.25×${10}^{-21\mathrm{}}$ to 10×${10}^{-21\mathrm{}}$ V ${\mathrm{m}}^2$ corresponding to mean electron energies from 1.2 to 5.4 eV. As expected, the emission was proportional to the argon density from 3×${10}^{24}$ to 15×${10}^{24}$ atoms/${\mathrm{m}}^3$. The experimental excitation coefficients are in good agreement with calculations using theoretical free-free emission cross sections and electron energy distributions and serve to demonstrate the usefulness of simple formulas relating the free-free emission cross section to measured momentum-transfer cross sections.

Exited atoms may relax to the ground state by radiative decay, a process which is usually very fast (of order nanoseconds). However, quantum-mechanical selection rules can prevent such rapid decay, in which case these “metastable” states can have lifetimes of order seconds or longer. In this Letter, we determine experimentally the lifetime of the longest-lived neutral atomic state—the first excited state of helium (the $2{}^{3}S_1$ metastable state)—to the highest accuracy yet measured. We use laser cooling and magnetic trapping to isolate a cloud of metastable helium (${\mathrm{He}}^{*}$) atoms from their surrounding environment, and measure the decay rate to the ground $1{}^{1}S_0$ state via extreme ultraviolet (XUV) photon emission. This is the first measurement using a virtually unperturbed ensemble of isolated helium atoms, and yields a value of 7870(510) seconds, in excellent agreement with the predictions of quantum electrodynamic theory.

We have carried out a comprehensive theoretical and experimental study of electron scattering from molecular nitrogen at energies below 10.0 eV. In the theoretical component of this project we have generated differential and integral cross sections for elastic scattering and vibrational excitation in converged vibrational close-coupling calculations. In the experiments, we have measured differential cross sections for these processes at scattering angles from 20° to 130° in a crossed-beam experiment at a large number of energies between 0.55 and 10 eV and, in a complementary time-of-flight experiment, total cross sections at energies between 0.08 and 10.0 eV. The measured angular distributions have been extrapolated to 0° and 180° using a procedure based on a nonlinear least-squares fit constrained by known physical properties of the e-${\mathrm{N}}_2$ scattering matrix; numerical integration of the resulting extrapolated distributions yields integrated cross sections with almost no error beyond that inherent in the measured angular data. We find generally good agreement between the present experimental and theoretical cross section, particularly at energies near the ${\mathrm{\Pi }}_{\mathit{g}}$ resonance near 2.39 eV. In previous studies of scattering in this region, such comparisons have been made problematical by the difficulty of ascertaining the appropriate theoretical scattering energy. We recommend here a protocol for resolving this problem for both elastic scattering and vibrational excitation.