Energy levels, with classifications and uncertainties, have been compiled for the spectra of the neutral atom and all positive ions of sodium (Z=11). Wavelengths with classifications, intensities, and transition probabilities are also tabulated. In addition, ground states and ionization energies are listed. Where available, the hyperfine structure constants and the percentages of the leading components of the energy levels are included. For all ionization stages of sodium, at least some experimental data are available; however, for those for which only a few transitions have been measured, theoretical calculations or values obtained by isoelectronic fitting are reported. Similarly, theoretical or isoelectronically determined ionization energies are given when they are thought to be more accurate than the available experimental data would produce.

Wavelengths, Transition Probabilities, and Energy Levels for the Spectra of Sodium (NaI–NaXI)

Optical studies of multiply-excited states are reviewed with emphasis on emission spectroscopy. From optical measurements, properties such as excitation energies, lifetimes and autoionization widths can be determined with high accuracy, which constitutes a challenge for modern computational methods. This article mainly covers work on two-, three- and four-electron systems, but also sodium-like quartet systems. Furthermore, some comments are given on bound multiply-excited states in negative ions. Fine structure effects on transition wavelengths and lifetimes (autoionization) are discussed. In particular, the most recent experimental and theoretical studies of multiply-excited states are covered. Some remaining problems, which require further attention, are discussed in more detail.

Optical studies of multiply excited states

If a single plane cathode in a glow discharge is replaced by a cathode with some hollow structure such as a cylindrical or slit shaped hole, then, in a specific range of operating conditions the negative glow is found to be inside the hollow structure of the cathode. Under such conditions at a constant current the voltage is found to be lower and, at a constant voltage, the current is found to be orders of magnitude larger than for the plane cathode. This effect is called the hollow cathode effect (Pashen, 1916).

Basic Mechanisms Contributing to the Hollow Cathode Effect

Pioneering observations on the luminescent phenomena generated in evacuated tubes belong to the history of spectroscopy and date as far back as the mid-1800s.(1,2) Within this field of research, the first description of a hollow cathode discharge (HCD) published in the scientific literature can be found in the early years of this century, when a German physicist, Friedrich Paschen, reported on the quite unique features of this radiation source.(3) At the time he was mainly engaged in the investigation of the spectral series of H2 in the IR region and in the distribution of energy in the spectra emitted by glowing gases. Together with Back in 1913 he discovered the effect (named for both scientists) that gives rise to the splitting of emission lines when the source is subjected to a very strong magnetic field. This phenomenon is actually a modification of the Zeeman effect that requires less intense fields.

Hollow Cathode Discharges

Systematic calculations of energies and decay rates for the 2p53snl core-excited states (with n<or=7) in Na have been performed using a configuration-interaction method, including spin-orbit interaction and core-polarization correlation through a semi-empirical model core potential. Data obtained extend and clarify considerably the classification of lines in the existing photoabsorption and ejected-electron spectra of Na. Lifetime determination included radiative decay to the bound 2p6nl levels and to the lower core-excited states as well as autoionization. Both correlational and relativistic effects play an important role, as expected, but in addition it was found that the non-orthogonality of bound and continuum orbitals, being a result of core excitation, is also very important in evaluating of autoionization rates. The predicted lifetimes are analysed with respect to their 'quasimetastability', which has recently attracted great attention in atomic spectroscopy and in the construction of XUV lasers.

Energy levels and lifetimes for the 2p53snl core-excited states in Na

By using a pulsed hollow-cathode discharge, metastable atoms are produced in the (2${p}^{5}$3s3p)${}^{4}$${D}_{7/2}$ and (2${p}^{5}$3s3p)${}^{4}$${S}_{3/2}$ levels of Na i. A tunable laser is used to excite these atoms to levels in the 2${p}^{5}$3s3d and 2${p}^{5}$3s4s configurations, and thereby to establish a partial Grotrian diagram for the quartet system. Several possible Na i xuv laser systems are described.

Laser spectroscopy of Na i quartets

We report the measurement, in a pulsed hollow-cathode discharge, of metastable quartet atoms of Li and Na. By using a tunable probe laser, population densities of 3 × 1010 atoms cm−3 and 1011 atoms cm−3 were measured in the Li(1s2s2p)4P° and Na(2p53s3p)4D7/2 levels, respectively. These levels are candidates for energy storage for extreme-ultraviolet lasers.

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Measurement of lithium and sodium metastable quartet atoms in a hollow-cathode discharge.

The paper summarizes the properties of certain radiating levels of the column I metals. XUV emission studies and an absorption experiment on the 1091 A line of neutral Cs are described. A technique for correlating picosecond, broadband XUV pulses is discussed.

Quasi-Metastable Energy Levels and Applications

Certain quartet levels in alkali-like systems retain metastability against autoionization while acquiring large radiative yields. This quasi-metastability occurs through selective coupling to non-autoionizing doublet levels by the spin-orbit interaction. An example of such a level is the 5p/sup 5/5d6s /sup 4/P/sub 5/2/ level of neutral Cs, which has a calculated branching ratio for radiation at 1091 A of 43%. Experimentally, we find that this line has an emission intensity equal to 1/6 of that of the strongest ion line of Cs/sup +/, and is a promising candidate for an extreme ultraviolet laser.

D. E. Holmgren

Papers

Laser spectroscopy of Na i quartets

Measurement of lithium and sodium metastable quartet atoms in a hollow-cathode discharge.

Quasi-Metastable Energy Levels and Applications

Emission at 1091 A in neutral core-excited Cs