The giant (or shape) resonance in the photodetachment cross section of D- above the n=2 threshold has been subjected to high-resolution vacuum ultraviolet spectroscopy performed by Doppler tuning of ions stored in the ASTRID storage ring. In order to prevent changes in the overlap between laser and ion beams over the large kinetic-energy range employed in the experiment, a new ion-beam-positioning method based on a quadrupole-shunt technique was applied. The study presents an accurate measurement of parameters for the resonance, which is also denoted (2){0}(2)(+) P-1(0). The resonance has a width of 26(2) meV while the asymmetry parameter q of the Fano profile is 3.2(0.4), which is at variance with the most recent theoretical calculations.

This paper reviews the knowledge of the structure, dynamics and collisions of atomic negative ions, as accumulated at the end of 2003, and describes how the research exploring these ions developed during the last decade. New experimental information has mainly been obtained from photon–negative-ion interactions using lasers and more recently also synchrotron radiation as the photon source. Additional insights have been gained from the use of new experimental techniques like heavy-ion storage rings, which made long-time observations of negative ions possible and promoted the study of electron–negative ion-interactions. Substantial progress has also appeared on the theoretical side with computational methods leading to reliable predictions for many of the lighter negative ions.

Recording the yield of He(1snl(3)L) Rydberg states for n=11-14, we measure the photodetachment cross sections of metastable He-(1s2s2p(4)P(o)) ions in the vicinity of the two-electron escape threshold. We observe a large number of double Rydberg He- quartet state resonances and report energies and widths of intrashell states in the n=13-15 manifolds. Sharp thresholds are measured at He((3)P(o)) and He((3)D(e)) Rydberg states with preference for population of the former, whereas the He((3)S(e)) states are not populated, in agreement with qualitative theoretical arguments.

The photon-ion merged beam technique has been used to measure the ionization cross sections of I-, Cs+, Ba+ and Ba2+ ions in the energy region from 40 to 185 eV, which is dominated by photoexcitation from the 4d shell (the experimental data are available at http://www.ifa.au.dk/amo/atomphys/atomphys.htm). Within the experimental accuracy, the total oscillator strengths for these ions and for the I+, I2+, Xe+ and Xe2+ ions, that were recently studied by some of the present authors, are identical, with the contribution from the 4d → np or nf resonances becoming more important with increasing nuclear charge and ionization stage, and with the maximum value of the continuum cross section increasing as the nuclear number is enlarged. The present data support the assumption that the 4f wavefunctions contract gradually with increasing ionization, both along isonuclear and isoelectronic sequences, for ionic charges ranging from -1 to +2.

The lifetime of the metastable Be-(2s2p2 4P3/2) ion has been measured to be 43.40±0.10 µs by means of an electrostatic heavy-ion storage ring. This value is in good agreement with the previously reported lifetime obtained using a magnetic storage ring, but more than 3% longer than the lifetime recently measured utilizing a newly constructed electrostatic ion trap.

The absolute single- and double-photoionization cross sections of singly charged sulfur ions have been measured from 20 to 200 eV by merging a 2 keV beam of S+ ions with a synchrotron-radiation beam from an undulator. The continuum cross section for single photoionization is strongly influenced by a broad Fano profile near the S+ (S-4) ionization threshold at 23.34 eV and a Cooper minimum located near 35 eV. Structures resulting from the excitation of 3s or 2p electrons, either from the S+ (S-4) ground-state ion or from the S+ (D-2) metastable ion, are resolved. identified, and characterized. Significant deviations exist between the experimental data and the photoionization cross sections calculated as part of the Opacity Project.

The absolute single-photoionization cross sections of singly charged nitrogen and oxygen ions have been measured from 29-80 eV and 30-150 eV, respectively, by merging a synchrotron-radiation beam from an undulator with a 2 keV ion beam. In the case of O+, separate data sets were obtained for the S-4 ground state and the D-2/P-2 metastable states by attenuating the target-ion beam with gas, N-2 exploiting the fact that cross sections for charge exchange between O+ ions, in the S-4 or in the D-2/P-2 states, and are different. The spectral structures were very different for the ground and the metastable N-2 states, whereas the cross sections for photoionization into the continua were nearly identical. The agreement between the experimental data and data calculated using the model function by Verner et al. or obtained from R-matrix calculations performed within the Opacity and the Iron projects is rather good. Finally, double photoionization (direct) was observed for O+ ions, with an approximately linear onset at 90 eV and a maximum cross section of 0.04 Mb.

The absolute single-and double-photoionization cross sections of singly charged Fe ions have been measured from 15.8 to 180 eV using the merged-beam technique. The data yield information about the photoionization continua and the resonance structures resulting from excitation of the outer 3d and 4s electrons as well as the inner 3p and 3s electrons. The vast majority of the Fe+ target ions were present in the ground-state configuration, 3d64s, and term,6D. The experimental data have been compared with several calculations, for example R-matrix calculations from the Opacity Project and data obtained using the central-field approximations. The experimental data are available at http://www.ifa.au.dk/amo/atomphys/atomphys.htm.

We report the observation of the predicted metastable 4p(3) S-4 state in the Ca- ion utilizing a spectroscopic technique based on nonlinear resonant multicolor absorption via an autodetaching state. The binding energy of the S-4 state is determined to be 586.86(10) meV with respect to the parent Ca(4p(2) P-3) state, a value less than 3 meV from the predicted one. The lifetime of the SS state is determined to be larger than 7 ps.

The absolute cross section for photoionization of CO+ ions leading to formation of longlived metastable CO2+ ions has been measured from the threshold at 27.3 eV to 50 eV utilizing a merged ion-photon beam set-up with synchrotron radiation from an undulator. The continuum cross section reaches a value of 2.5(4) Mb near threshold and decreases slowly with increasing photon energy. Superimposed on this continuum discrete structures are observed, which can be assigned to CO+(4σ5σ 3Σ+(v = 0))nlλ Rydberg series with n⩾5.

We present measurements on the absolute photoionization cross section of doubly-charged Ca ions in the photon energy range 50-70 eV, which encompasses resonances of the type 3s→np. The energy dependence of the continuum cross section differs from the earlier measurement by van Kampen et al (1997), but is reproduced quite well by a recent R-matrix calculation (Wilson et al 1999), although the estimates of the oscillator strengths of the resonance lines are too low.

Absolute photoionization cross sections are presented for Mg+ ions in the photon energy range 25-160 eV. The assignments of the structures seen are discussed and compared with earlier assignments derived from photographic measurements. Good agreement is found with theoretical calculations of the continuum cross section, whereas the agreement with calculations of the oscillator strengths and of the line positions is less satisfactory.

The partial and total absolute cross sections for the photoionization of Xe+ and Xe2+ ions, respectively, have been measured in the 4d excitation region (50-130 eV). The experimental cross sections are compared with calculated values obtained by the random phase approximation method taking exchange and rearrangement into account. The maximum total cross section values are 27(3) Mb for both ions, a value also reported for the photoionization of the Xe atom. The reliability of the experimental cross sections has been tested by measuring the absolute photoionization cross section for the He+ ion.

The absolute cross section for photoionization of C+ ions has been measured from the ionization threshold at 24 eV to 105 eV by overlapping an ion beam with a monochromatized synchrotron-radiation beam from the ASTRID undulator. The measurements, which are important for astrophysical modeling of, for example, stellar atmospheres, have been compared with R-matrix calculations from the Opacity Project and the Iron Project. The general agreement between theory and experiment is good, yet differences in the magnitude of the cross section of up to 50% are observed as well as some deviations concerning the resonance structure.

The absolute photoionization cross section for the astrophysically important C+ ion has been measured for the first time thus allowing a test of the predicted cross section from the Opacity Project. The measurements are performed with a new ion-photon merged beam setup at the ASTRID storage ring utilizing an undulator beam line. In addition to the predicted 2s2p(3P)np 2D and 2S autoionizing resonances, the 2P states which are not included in the theoretical predictions also contribute significantly to the ionization yield below the 2s2p(3P) limit. The cross section is determined with a precision of 10% and lies 5–25% below the theoretical prediction at 24–37 eV.

Experimental investigations of the photodetachment cross section for the negative hydrogen ion in the region near the $\mathrm{H}(n=2)$ threshold are discussed. Doppler-tuned spectroscopy in a collinear geometry is used to obtain a comparatively high resolution in this photon-energy range. The ions are accelerated in the ASTRID storage ring, which allows an accurate velocity measurement and further enables the application of momentum-spread reduction techniques. The fixed-frequency vacuum-ultraviolet laser beam (118 nm) is generated by sum-frequency mixing in a xenon gas cell. The photodetachment cross section exhibits pronounced resonances that correspond to the rich spectrum of doubly excited ${}^{1}P$ states near the $\mathrm{H}(n=2)$ threshold. The position of the resonances is determined with an accuracy that challenges the current theoretical developments. The experimental observations are used to predict the behavior of a dipole series below $\mathrm{H}(n=2).$ The measurement on both hydrogen and deuterium facilitates a study of isotope effects. By means of momentum-spread-reduction techniques it should be possible to resolve the natural linewidth of the narrow dipole resonances. Preliminary studies show that improvements in electron cooling of the ${\mathrm{H}}^{\ensuremath{-}}$ beam promise to reach this limit. Finally, the feasibility of extending the high-resolution work to higher-lying $(n=3)$ resonances is briefly discussed.

The absolute photoionization cross sections of I+ and I2+ ions have been measured from 45 to 140 eV, covering the region of 4d ionization where a large maximum, also known as a "giant resonance," occurs in the cross section. For both ions, the maximum cross section appears near 90 eV and is measured to be 23(3) Mb and 24(4) Mb, respectively. This is significantly larger than that previously reported for atomic iodine, and only slightly smaller than the values calculated using the random-phase approximation with exchange.

The shape parameters for the lowest-lying P-1(o) resonance, (2){0}(3)(-), of D- have been measured using high-resolution vacuum-ultraviolet spectroscopy. The experiment was performed at the storage ring ASTRID, and the resonance was resolved by applying electron cooling to reduce the velocity spread of the ion beam. The resonance has a width of 37(3) mu eV while the asymmetry parameter cl of the Fano profile is - 16(3). These values present the first critical test of a large number of theoretical calculations.

Long-lived metastable states of N-2(-) are generated by sputtering surfaces of TiN or BN with keV Cs+ ions. The lifetimes of the metastable ions have been measured at the storage ring ASTRID. Experimental investigations of the charge exchange of N-2(+) ions in alkali-metal vapor and theoretical calculations support the conclusion that the long-lived N-2(-) species are sextet states, originating from the attachment of an electron to a quintet state of the N-2 molecule. [S1050-2947(99)07911-1].

This paper reviews the results obtained in recent years from spectroscopic studies of the negative hydrogen ion at the ASTRID storage ring. 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 characterised, using Doppler-tuned collinear laser spectroscopy. The resonance positions have been determined for both H- and D-, allowing also a critical test of the predicted isotope effects. Further studies are in progress based on electron cooling of the negative hydrogen ion beam.