The absolute photoionization cross section of the ground-state C+ ion has been measured from the ionization threshold at 24 eV to 31 eV using a merged ion-photon beam setup with synchrotron radiation from an undulator. The experimental results have been compared with nonrelativistic theoretical data from the Iron Project and the agreement is generally good in the near-threshold region, but differences in the magnitude of the continuum cross section of 15%-35% at higher energies and several deviations in the resonance structure are observed. The present measurements are a part of an experimental program to test the extensive theoretical photoionization cross section calculations performed within the framework of the Opacity Project and the Iron Project.

The absolute photoionization cross section of ground state K+ ions has been measured from the 3p threshold to above the 3s threshold (31-49 eV) by VUV radiation using a merged ion-photon beam set-up. The cross section, which reaches a maximum value of 32 Mb ±10% at 31.8 eV and then decreases with energy, is in overall good agreement with absolute measurements by Peart and Lyon, but the present investigation also reveals the 3s3p6np 2P (n = 4-6) resonances and eliminates a previously unassigned structure at 35.5 eV. The energies, widths and shape parameters of the autoionizing 2P resonances are determined and compared with recent data from dual-laser plasma measurements and with theoretical predictions. The present experimental set-up will allow absolute photoionization cross sections 0.1 Mb to be measured, making it possible to measure the absolute photoionization cross sections of ions of astrophysical importance.

An experimental study of the previously unobserved and controversial ${\mathrm{Cs}}^{\ensuremath{-}}(6s6p^{3}P_{J}^{o})$ states has been conducted using a combination of infrared laser and storage ring experiments. This first application of photodetachment spectroscopy to the study of low-lying resonances in atomic negative ions has resulted in the observation of a narrow resonance structure, lying only 8.0(3) meV above the first detachment threshold of ${\mathrm{Cs}}^{\ensuremath{-}}$. The storage ring experiments suggest that all fine structure levels of the $6s6p$ configuration are unbound, thereby eliminating possibly the only serious candidate for a stable atomic negative ion with opposite parity bound states.

An experimental study of the previously unobserved and controversial Cs-(6s6p P-3(J)0) states has been conducted using a combination of infrared laser and storage ring experiments. This first application of photodetachment spectroscopy to the study of low-lying resonances in atomic negative ions has resulted in the observation of a narrow resonance structure, lying only 8.0(3) meV above the first detachment threshold of Cs-. The storage ring experiments suggest that all fine structure levels of the 6s6p configuration are unbound, thereby eliminating possibly the only serious candidate for a stable atomic negative ion with opposite parity bound states. [S0031-9007(97)05108-9].

A new nonlinear laser technique, yielding accurate and detailed information about the photodetachment process, is applied to the study of excited states in atomic negative ions. The method has facilitated the first experimental observation of the controversial $^{4}P$ state in the ${\mathrm{Ca}}^{\ensuremath{-}}$ ion and the homologous state in the ${\mathrm{Sr}}^{\ensuremath{-}}$ ion and also provided comprehensive information about their binding energies, fine-structure intervals, and autodetachment lifetimes, together with the absolute strengths of the intercombination transitions to the $^{2}P$ ground states.

Recent progress in experimental and computational methods for the study of negative atomic ions has greatly improved the knowledge about the structural and dynamic properties of the negative alkaline-earth ions. This review deals in particular with the stable ground states in , , and and with the metastable quartet states (, , and ) observed in , , , and . Future perspectives are discussed.

A new nonlinear laser technique, yielding accurate and detailed information about the photodetachment process, is applied to the study of excited states in atomic negative ions. The method has facilitated the first experimental observation of the controversial P-4 State in the Ca- ion and the homologous state in the Sr- ion and also provided comprehensive information about their binding energies, fine-structure intervals, and autodetachment lifetimes, together with the absolute strengths of the intercombination transitions to the P-2 ground states.

This paper presents theoretical ab initio calculations and experimental measurements of the binding energy of the metastable He(-)1s2s2p P-4 ion. The calculated 77.518+/-0.011 meV and the measured 77.516+/-0.006 meV values for the binding energy are in excellent agreement and they represent a significant improvement in the accuracy compared to previous studies. The experimental technique is based on the determination of the epsilon s-wave Wigner threshold associated with detachment to the 1s3s S-3 state of the neutral He atom. The yield of the 1s3s S-3 He level was monitored by applying resonant ionization spectroscopy.

The two lowest-lying members of the {sup 1}P{sup 0} dipole series of autodetaching resonances in H{sup {minus}} located just below the H(n=2) threshold have been observed and characterized, utilizing Doppler-tuned collinear laser spectroscopy. The resonance positions have been determined for both H{sup {minus}} and D{sup {minus}}, allowing the first critical test of the predicted isotope effects. The experimental isotope shifts are consistent with a small specific mass shift. We interpret these results as evidence for a rather small electron momentum correlation. {copyright} {ital 1997} {ital The American Physical Society}

Recent progress in experimental and computational methods for the study of negative atomic ions has greatly improved the knowledge about the structural and dynamic properties of the negative alkaline-earth ions. This review deals in particular with the stable ground states in , , and and with the metastable quartet states (, , and ) observed in , , , and . Future perspectives are discussed.

The two lowest-lying members of the P-1(0) dipole series of autodetaching resonances in H-located just below the H(n = 2) threshold have been observed and characterized, utilizing Doppler-tuned collinear laser spectroscopy. The resonance positions have been determined for both H- and D-. allowing the first critical test of the predicted isotope effects. The experimental isotope shifts are consistent with a small specific mass shift. We interpret these results as evidence for a rather small electron momentum correlation.

This paper presents theoretical {ital ab initio} calculations and experimental measurements of the binding energy of the metastable He{sup {minus}}1s2s2p{sup 4}P ion. The calculated 77.518{plus_minus}0.011 meV and the measured 77.516{plus_minus}0.006 meV values for the binding energy are in excellent agreement and they represent a significant improvement in the accuracy compared to previous studies. The experimental technique is based on the determination of the {epsilon}s-wave Wigner threshold associated with detachment to the 1s3s{sup 3}S state of the neutral He atom. The yield of the 1s3s{sup 3}S He level was monitored by applying resonant ionization spectroscopy. {copyright} {ital 1997} {ital The American Physical Society}

The binding energy and fine-structure splitting of the ground state of the Sr- (5s(2)5p P-2) ion have been measured by means of the laser photodetachment threshold technique combined with state-selective detection of neutral atoms using resonant ionization spectroscopy. We find binding energies of 32.17+/-0.03 meV (J=3/2) and 52.06+/-0.06 meV (J=1/2), yielding a fine-structure splitting of 19.89+/-0.07 meV. This represents an improvement of two orders of magnitude compared to the recent data based on a combination of laser photodetachment and accelerator mass spectroscopy [D. Berkovits et al., Phys. Rev. Lett. 75, 414 (1995)].

A new technique based on state-selective, resonant-ionization detection of atoms, produced by laser detachment or resonant excess-photon- detachment processes, has made it possible to obtain significant progress in the studies of structural and dynamic properties of negative ions and of nonlinear interaction of an autoionizing state with a near-resonant laser field. New methods are briefly described and illustrated by examples from stable and autoionizing negative ions formed by elements with a closed selectron subshell: He-, Be-, Cd-, Si-, Ba-.

Electron spectra from 20 and 50 keV - Ne and 25, 40, 50, and 60 keV - Ne collisions have been measured at ejection angles of 20 - , and at electron energies down to 1.6 eV. In both collision systems, a low-energy structure is observed at energies below 5 eV superimposed on a broad continuum. It is proposed that the low-energy structure may be due to electrons promoted to the continuum via the so-called T promotion associated with `saddle-point' electrons.

The negative hydrogen ion has been investigated by a new spectroscopic approach in the region near the n = 2 threshold of the neutral hydrogen atom. The technique is based on Doppler-tuned spectroscopy using an H- beam stored in a heavy ion storage ring collinearly overlapped with a fixed-frequency narrow-bandwidth vacuum ultraviolet laser beam (118 nm). The position of the Feshbach resonance is determined as 10.9243(3) eV which is in good agreement with recent theoretical calculations but deviates significantly from the previously reported experimental result.

Measurements of the photodetachment cross section of have been performed in the vicinity of the threshold, with photon energies ranging from 5.23 - 6.04 eV. The experimental cross section is nearly constant throughout this energy region, in marked contrast to recent theoretical predictions of strong resonance structures. The present data indicate that the quasibound state may be located at higher energies than so far predicted.

The presence of two metastable states in the negative beryllium ion, and , which are connected by an optical transition, offers a unique possibility for gaining new information about this ion. Utilizing resonant, two-photon detachment, combined with resonant-ionization detection of the resulting neutral atoms, we have obtained results yielding the lifetimes of the () and () levels, the fine-structure splittings of the levels, and an improved transition wavelength.

Electron spectra from the Li+-Ne and Na+-Ne collisions have been measured for collision energies from 3.5 to 60 keV, ejection angles of 20-160 degrees , and electron energies in the 5-150 eV range. In both systems the electron spectrum consists of a relatively large continuum superimposed by discrete lines arising from doubly-excited atomic states. For the Li+-Ne system the continuum part decreases exponentially with electron energy, with a slope inversely proportional to the collision velocity. The angular distribution is approximately symmetric in the centre-of-mass system. For the Na+-Ne system the behaviour is more complicated, in particular, the continuum part cannot be fitted with a single exponential. The results are discussed in terms of recent models developed for one-electron systems by Solov'ev, Ovchinnikov and collaborators, describing the ionization process in terms of superpromotions, with the electron continuum being parametrized in terms of characteristic properties related to avoided and so-called hidden crossings of the quasimolecule.