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In atomic systems for which the total oscillator strength of excitations from the ground state is dominated by the transition to the lowest resonance level, the f-sum rule provides a bracketing inequality connecting the lifetime τ of that level to the dipole polarizability αd. This relationship has been used previously to deduce αd from τ. It is shown here that improved spectroscopic accuracies now permit this procedure to be inverted, with τ deduced from a value for αd obtained spectroscopically using the core polarization model. A similar quantitative relationship exists connecting the nonadiabatic correlation factor β to τ, and thus also to αd. The method is applied to a recent measurement of αd for Kr6+ to obtain the values τ(4s4p1P1) = 0.096 ± 0.003 ns and β(Kr6+) = 1.71 ± 0.03a50. It is shown that the use of this method to make precision lifetime determinations for a small number of ions in an isoelectronic sequence permits the exploitation of observed semiempirical regularities to specify the lifetimes of all ions in that sequence.

Much progress has been made in the measurement of oscillator strengths in neutral and singly-ionized atoms, providing a database for use in important applications. However, for multiply-charged ions, measured data for oscillator strengths are almost exclusively limited to low-lying unbranched transitions. Although extensive measurements of ionic lifetimes exist, the lack of branching fraction measurements in multiply-charged ions prevents these data from being converted to oscillator strengths. A significant factor leading to this deficiency involves the lack of adequate line intensity calibration standards in the vacuum ultraviolet spectral region 2000‐400 A. Here we review the interrelationships connecting these rate parameters, indicate some of the important applications for which they are needed, describe the experimental limitations that currently exist, and suggest possible methods for extending these measurements below 1000 A.

The decays of the 2p J = 1/2 and J = 3/2 levels of Li-like ions and of the 2s2p 1,3 P o levels of Be-like ions can be used as simple-atom test beds for lifetime measurements and for the development of accurate calculations of the transition rates. We have summarized and filtered the experimental data in order to obtain consistent data sets and isoelectronic trends that can be compared to theoretical predictions. The graphical presentation of line strength data enables direct comparison and evaluation of the merit of data along extended isoelectronic sequences. From this, the precision that is necessary in future meaningful experiments can be deduced.

We have studied the spectrum of highly ionized krypton in the extreme-ultraviolet wavelength region (50–300 Å), using beam-foil excitation of fast krypton ions at the Argonne ATLAS accelerator facility. We report measurements of transition wavelengths and excited-state lifetimes for n = 2 states in the lithiumlike, berylliumlike, and boronlike ions, Kr31+,32+,33+. Excited state lifetimes ranging from 10 ps to 3 ns were measured by acquiring time-of-flight-delayed spectra with a position-sensitive multichannel detector.PACS Nos.: 32.70.Cs, 32.30.Jc, 34.50.Fa

Semiempirical methods are used to characterize and parametrize the effects of intermediate coupling and configuration interaction on the energy levels of the 3s3p, 3p2 and 3s3d configurations for ions in the Mg isoelectronic sequence. These parametrizations are then used to predict the branching fractions for the 3s3p–3p2 and 3s3p–3s3d transition arrays. The predictions are compared with the MCHF calculations for S V and Fe XV, and good agreement is obtained. The application of this method to deduce transition probabilities and oscillator strengths from lifetime measurements is discussed.

Recent studies of the Si and Ge isoelectronic sequences have shown that branching fractions within the ns2np2–ns2np(n + 1)s supermultiplets are accurately described by a semiempirical model that uses spectroscopic energy level data to characterize intermediate coupling. It was also recently shown that a similar model applied to alkaline-earthlike ns2–nsnp transitions can be advantageously generalized to include the effects of spin-other-orbit interaction and differences between the singlet and triplet radial wave functions. These two formalisms are combined, extended, and applied here to obtain a predictive systematization of branching fractions for the 5s25p2–5s25p6s supermultiplet of the ions Sn I–Cs VI in the Sn sequence.

The use of lifetime measurements for the specification of transition probabilities along isoelectronic sequences is hindered by the fact that virtually no branching fraction measurements exist for multiply charged ions. Part of the reason for this lies in the lack of calibration standards for intensity as a function of wavelength in the ultraviolet region. It has been shown that transitions of the form ns2np2 - ns2npn´s in Si I and Ge I are virtually free of configuration interaction, hence their branching fractions can be accurately predicted from intermediate coupling amplitudes deduced from measured spectroscopic energy level data. Reported here is the extension of these semiempirical methods to the ions P II, S III, Cl IV and Ar V in the Si sequence and As II, Se III and Br IV in the Ge sequence. This provides a set of branching fractions that can both specify transition probability rates from lifetime measurements for these ions, and serve as a set of lines of known relative intensities for use in the calibration of detection apparatus in the UV region.

Using the beam-foil spectroscopy technique, we have measured the decay time of the 3pthinsp{sup 2}P{sub 3/2} level in sodiumlike bromine. The results, using an arbitrarily normalized decay curves analysis, are in good agreement with the most recent theoretical calculations. They also confirm an earlier experimental result in sodiumlike krypton, suggesting that some previous measurements of the lifetimes of 3pthinsp{sup 2}P{sub 3/2} states of nearby isoelectronic ions are too high. {copyright} {ital 1998} {ital The American Physical Society}

k is the direction of observation. The anisotropy arises from interference between the M1 and induced E1 amplitudes for the decay. Theory predicts that the pseudo-T-odd invariant should contribute to the angular distribution through a term associated with the damping (finite lifetime) of the 2 p levels. The decay rate for the interference term is found to be 21.64s0.12d s 21 sr 21 sVycmd 21 . This corresponds to a 2s Lamb shift for Ar 171 of 38.6(1.4) THz. [S0031-9007(97)04308-1]

We report a measurement of the spectral distribution of photons from the two-photon decay of the ${1\mathrm{s}2\mathrm{s}}^{1}$${\mathrm{S}}_{0}$ level in heliumlike krypton. In the experiment, a beam of ${\mathrm{Kr}}^{34+}$ ions was excited using a thin carbon foil and the two-photon decays were observed as coincidences in an array of x-ray detectors. An important part of this work was the measurement of the efficiency of the detection system as a function of photon energy. A Monte Carlo model of the experiment was used to compare the data with theoretical calculations and demonstrated good agreement with the nonrelativistic calculations for Z=36.

New observations of the 3s23p 2P–3s3p2 4P intercombination transitions in Al-like ions have been made for Cl V from spark spectra recorded at Lund and for Kr XXIV and Mo XXX from spectra obtained at the JET tokamak. The new results have been combined with other identifications of these transitions along the sequence and empirically systematized and compared with theoretical calculations. A set of smoothed and interpolated values for the excitation energies of the 3s3p2 4P levels in P III-Mo XXX is presented.

We have measured the 1s2s 3S1-1s2p 3P0 and 1s2s 3S1-1s2p 3P2 fine structure transition wavelengths in the helium-like Ar16+ ion, using the beam-foil spectroscopy technique with position-sensitive photon detection. The wavelength results have precisions of < 30 ppm and are sensitive to the total QED corrections to < 0.5%. The measurements confirm the significance of (Zα)4-dependent relativistic corrections to the n = 2 state fine structures suggested by recent calculations. The small remaining discrepancies between these measurements and the accurate theoretical values suggest that the magnitude of uncalculated higher order QED terms is about 0.15(Zα)4 a.u. The status of our experimental search for the 3S13P0 transition in Ni26+ is discussed.

We report Progress on a measurement of the spectral distribution of photons from the two-photon decay of the 2{sup 1}S{sub 0} level in heliumlike krypton. A measurement of the exact shape of the continuum singles spectrum of the two-photon decay in heliumlike ions provides a sensitive probe of the calculation of the transition probability. In our experiment a beam of Kr{sup 34+} ions is excited using a thin carbon foil and the subsequent decays are observed in an array of four x-ray detectors.

We have measured the 1s2s $^{3}$${\mathit{S}}_{1}$--1s2p $^{3}$${\mathit{P}}_{0}$ and 1s2s $^{3}$${\mathit{S}}_{1}$--1s2p $^{3}$${\mathit{P}}_{2}$ fine-structure transition wavelengths in the heliumlike ${\mathrm{Ar}}^{16+}$ ion using the beam-foil spectroscopy technique with position-sensitive photon detection. The wavelength results are 559.944(16) \AA{} for the J=1--2 transition and 661.533(18) \AA{} for the J=1--0 transition, which correspond to measurement precisions of about 2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$ a.u. in the transition energies. These values are sensitive to the relativistic contributions in the transition energies to 500 ppm and to the total QED corrections to 0.5%. The measurements confirm the significance of (Z\ensuremath{\alpha}${)}^{4}$-dependent relativistic corrections to the n=2 state fine structures suggested by recent calculations. The small remaining discrepancies between these measurements and the accurate theoretical values suggest that the magnitude of uncalculated higher-order QED terms is about 0.15(Z\ensuremath{\alpha}${)}^{4}$ a.u. Our experimental results disagree with an earlier recoil-ion measurement for the $^{3}$${\mathit{S}}_{1}$-- $^{3}$${\mathit{P}}_{0}$ transition wavelength.

The empirically observed isoelectronic linearization of alkali-metal-like line strengths [ital Z][sup 2][ital S]=[ital A]+[ital B]/([ital Z][minus][ital C]) is examined for the Li, Na, Cu, and Ag sequences. It is found that small deviations from this linearity observed at high [ital Z] can be removed by use of a correction factor obtained from the relativistic formulation of the hydrogenlike line strength. With this correction, low-[ital Z] data can be linearly interpolated to the nonrelativistic hydrogenic value in the high-[ital Z] limit. The linearization is tested in the data-rich Li, Na, and Cu sequences and is utilized to make precise predictions for the Ag sequence on the basis of a small number of accurate measurements.

We have observed radiation from highly charged molybdenum ions in the JET tokamak plasmas. Spectra were recorded in the wavelength range 70-110 Angstrom, resulting in the classification of 15 new n = 3. Deltan = 0 transitions in Mo XXIV-Mo XXVIII.

Beam-foil lifetime data on the intercombination line 4s2 1S0-4s4p 3P10 in the Zn-like ions are reviewed and compared to various theoretical predictions. These predictions spread by about a factor of three. After massive corrections for experimental term energies and singlet-triplet splittings, this spread is reduced to a much narrower band, and most of the predictions then agree with the experimental lifetime findings and with the trend predicted by calculations which explicitly take core polarization into account. A screening parameterization of the experimental lifetime and term structure data can then be used to represent singlet and triplet resonance level lifetimes by a single, common curve.

We report a measurement of the lifetime of the 1s2s $^{1}$${\mathit{S}}_{0}$ state in two-electron ${\mathrm{Br}}^{33+}$. This state decays to the 1${\mathit{s}}^{2}$ $^{1}$${\mathit{S}}_{0}$ ground state only by the simultaneous emission of two photons. Ions in the 1s2s $^{1}$${\mathit{S}}_{0}$ level are produced by excitation of energetic bromine ions in a thin carbon foil. The lifetime is determined by measuring the change in the rate of two-photon coincidences as a function of the foil-detector separation. The lifetime measurement yields 39.32(32) ps in agreement with a theoretical value of 39.63(16) ps.

Experimental and theoretical studies of lifetimes and transition probabilities for doubly ionized mercury, Hg III, are reported. Such data are currently of great astrophysical interest because observations with the Goddard high resolution spectrograph on board the Hubble space telescope have shown some 5d(9)6s-5d(9)6p transitions of Hg III in the spectrum of the chemically peculiar star chi Lupi. We now report experimental lifetimes for some selected 5d(9)6p and 5d(8)6s6p levels, determined by beam-foil spectroscopy, and compare the results with theoretical transition probabilities based on relativistic multiconfiguration Dirac-Fock calculations.

Lifetimes of the 2{ital p}{sup 5}3{ital s} {sup 1}{ital P}{sub 1} and {sup 3}{ital P}{sub 1} levels in SVII and ClVIII have been measured using the beam-foil technique. The results are 17.4{plus minus}0.5 (13{plus minus}1) and 52{plus minus}2 (27{plus minus}1) ps for SVII (ClVIII). Many theoretical results, obtained by a variety of methods, are available for these lifetimes. A summary of the theoretical lifetimes reveals a significant scatter of the results, and none of the calculations agrees with all the present experimental data.

We describe recent precision experiments in fewelectron ions including measurements of the lifetimes of two-photon-emitting levels in Ni26+ and Ni27+, a measurement of the lifetime of the 23S1 level in Br33+ and measurements of the 23S1 + 23P0,1,2 transition energies in B3+.

Studies of the useful lifetime of 2-5 µg cm−2 carbon stripper foils have been made using 225 keV heavy ion beam bombardment. The foils were self supporting and were mounted both in a standard and a tension-slackened manner. In determining the quality of the foil during ion bombardment several techniques were used, in particular beam-foil spectroscopy. Here the intensity of the light emitted by the foil-emergent beam was measured as a function of bombardment time. The measured lifetime before foil failure for standard foils was found to agree well with a semiempirical model proposed by Auble and Galbraith, and the slackened foils exhibited usable lifetimes 3-4 times longer than the normal foils. The properties of ordinary and slackened foils were compared under realistic conditions. It is concluded that the slackened foils are very well suited for studies of atomic spectra and meanlives of excited states using beam foil excitation methods.

We report lifetime measurements for three of the 5d 9 6p levels in Au II and relativistic multiconfiguration Dirac–Fock (MCDF) calculations of lifetimes and transitions probabilities in this ion. The work was motivated by significant disagreements between previous experimental 5d 9 6p lifetimes, obtained by means of the beam–foil (BF) method and theoretical data based on Hartree–Fock calculations in which relativistic effects were included as perturbations. The new measurements, based on laser-induced fluorescence, support the BF data and are also in excellent agreement with the MCDF results. We confirm that, while computational methods which include perturbative relativistic corrections appear to be adequate for moderately heavy elements such as Ag, fully relativistic calculations are essential for describing systems as heavy as Au.

The recently installed 330 kV electrostatic positive ion accelerator at the University of Toledo is described. Experiments have been performed using ions ranging from H + to Hg 2+ and exotic molecules such as HeH + . Most of these experiments involve the beam-foil studies of the lifetimes of excited atomic states and the apparatus used for these experiments is also described. Another beamline is available for ion-implantation. The Toledo heavy ion accelerator facility welcomes outside users.

The experimental measurement and theoretical formulation of atomic lifetimes are being extended to systems that have both many electrons stripped away and multiple electrons remaining. The dynamical structure of these systems can be significantly affected by interactions that are negligible in neutral and fewtimes ionized systems, and the lifetimes can sensitively probe interelectronic correlations, relativistic and quantum electrodynamic interactions, and effects of the finite size of the nucleus. These interactions often have characteristic dependences on the central charge, and studies of systematic trends can reveal both predictive regularities and theoretical insights. Besides providing new tests of atomic theory, these systems have important applications in astrophysics, in fusion plasma technology, and in XUV-laser development.

Time-resolved extreme ultraviolet spectra of foil-excited 120-MeV Br ions have been recorded, using a spectrometer equipped with a position-sensitive detector. The spectra near \ensuremath{\lambda}=20\char21{}30 nm were measured at times after excitation of up to 6 ns. Prominent in the delayed spectra were the long-lived decays of the intercombination transitions in the Mg-like ${\mathrm{Br}}^{23+}$ and Al-like ${\mathrm{Br}}^{22+}$ ions. The present paper reports on lifetime measurements for intercombination transitions in highly charged bromine ions using such a position-sensitive detector.

We report a measurement of the lifetime of 22S1/2 level in hydrogenlike 84Kr and the first determination of the branching ratio for the M1 decay for this Z=36 one‐electron ion. The lifetime of this state was measured by the beam‐foil time‐of‐flight method, in which the foil was moved relative to a fixed, collimated Si(Li) detector. An analysis based on the single photon spectra from a tightly collimated detector led to the measurements of the lifetime of 22S1/2, the two‐photon emission rate, the M1 emission rate and the branching ratio of M1 decay. All these exprimental results are in good agreement with the theoretical calculations of Parpia and Johnson, and Goldman and Drake.

A simple general method is presented whereby measured spectroscopic energy-level data can be used to empirically construct the eigenvectors of intermediate coupling for an arbitrary configuration. These empirical eigenvectors can be used to elucidate the transition rates into, out of, and among the levels of the configuration by separating the effects of intermediate coupling from the radial transition moment. Predictive applications to the ${\mathit{np}}^{3}$ configurations of the N and P isoelectronic sequences are presented.

Analysis of ultraviolet absorption from the dominant ions in interstellar clouds provides information on the mineralogy of the solid material in space and on the synthesis of elements in our Galaxy and beyond. For the most part the spectra are acquired with the spectrometers on the Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer. In order to convert the amount of absorption into an accurate abundance, determinations of oscillator strengths of sufficiently high quality are needed. We present our most recent beam-foil measurements in this area. Lifetimes, branching fractions, and oscillator strengths for all transitions within the P II multiplet at 1154 A are compared with available results. Close agreement between our laboratory measurements and theoretical and semi-empirical calculations suggests a means for calibrating fast-beam instruments in the far ultraviolet. As for Cu II, our results for the line at 1358 A provide further evidence for a short lifetime for the upper state of interest.

The dipole polarizability and adiabatic correlations of atomic ions are related to the energy levels and oscillator strengths of the levels through bracketing inequalities. For systems with a ground state ns 2 1 S0, the total oscillator strength is dominated by the unbranched intrashell ns 2 1 S0‐nsnp 1 P o 1 transition, and the remaining oscillator strength can be narrowly bracketed using the f -sum rule. Thus measurements of the lifetime of the lowest resonance transition can be used to specify the polarizabilities and, alternatively, measurements of the polarizabilities can be used to deduce lifetimes. Moreover, isoelectronic regularities in line strengths can be used to obtain a comprehensive database from a small number of precision lifetime determinations. These methods are applied homologously to produce values for polarizabilities and lifetimes for the Mg, Zn, Cd and Hg isoelectronic sequences. To confirm the reliability of these methods, fast ion beam lifetime measurements in P IV were performed using the arbitrarily normalized decay curve (ANDC) method, and a test of the f -sum was made for Ba II, for which precision measurements of both the polarizability and lifetimes exist.

Higher order photon transitions such as M1, M2 and two-photon decay are conveniently studied using highly-charged electron ions. Here we discuss two examples from recent experiments which were done using the ATLAS facility at Argonne National Laboratory. The first is a test of Relativistic Quantum Mechanics involving a precision measurement of the spectral shape of the two-photon decay of the 1s2s{sup 1}S{sub 0} state in He-like nickel and the second is a test of the theory of damping in quantum mechanics involving observation of E1-M1 interference in the electric field quenching of metastable H-like ions.

In a recent paper Kompitsas et al. [ J. Opt. Soc. Am. B11, 697– 702 ( 1994)] presented new spectroscopic measurements for the 4snf levels in neutral Zn and used these results to deduce a value for the dipole polarizability αd of the Zn+ ion. We point out other data sources that are relevant to this determination and present additional analyses and computations that test the value that they reported. In this manner we have obtained the value αd(Zn+) = 15.4 ± 0.5a03, which confirms with reduced uncertainties the value reported by Kompitsas et al.

Empirical predictions are reported for the lifetimes of the 4s2–4s4p resonance and intercombination transitions in the Zn isoelectronic sequence. The approach uses a direct-mapping reduction of available energy-level and lifetime data and imposes no theoretical assumptions or restrictions. Measured spectroscopic energy-level data are used to characterize intermediate coupling effects in terms of an effective singlet–triplet mixing angle, which is then used to convert lifetime data into mixing-reduced effective line strength factors. Through a linearizing screening parameterization, these reduced line strength factors are found to exhibit regularities permit accurate interpolation, extrapolation, and smoothing and also provide a convenient exposition for that sensitive comparison between experimental results and ab initio calculations.

Highly ionised complex atoms, which have many electrons stripped away but many electrons still remaining, are presently a subject of vigorous experimental study. The high precision experimentally attainable for these systems suggests their use in the investigation of higher order theoretical processes, and the specification of these interactions is essential to the development of reliable predictive methods. It is shown that parametric reductions which combine ab initio theoretical calculations with experimental observations can yield predictions of much higher precision than could be obtained from either source separately. These formulations also reveal subtle regularities in the measured data that are not exhibited by the corresponding computations, which suggest possible calculational improvements.

The use of anomalies in emitted intensities due to cancellations of the transition integral is suggested as a probe of small model-dependent effects. Calculations of predicted anomalies in both the total intensity and in the relative intensities of fine-structure components are presented for specific transitions in certain ions of the K I isoelectronic sequence, which indicate the experimental feasibility of this approach.

Semi-empirical values for the lifetimes, transition probabilities and oscillator strengths have been computed for all n = 4, n = 5 and some n ≤ 9 Rydberg transitions for ions in the Cu I isoelectronic sequence through In XXI. Extrapolation and interpolation techniques were utilised to obtain a set of estimated term values and ionisation potentials which, although crude by spectroscopic standards, are of sufficient accuracy to serve as inputs for transition probability calculations by the numerical Coulomb approximation.

A procedure is developed by which M1 decay rates for a system with intermediate coupling can be computed from the corresponding rates in pure LS coupling and simple factors obtained from measured energy separations. Applications are made to the nsnp configurations in the Be, Mg and Zn isoelectronic sequences which are compared with ab initio MCDF calculations.

It is confirmed that the 3489-\AA{} transition observed in beam-foil excited lithium spectra originates from the negative lithium ion. It is most probably the transition $1s2s2{p}^{2}^{5}P\ensuremath{-}1s2{p}^{3}^{5}S^{\ensuremath{\circ}}$, as suggested by Bunge. A new electric field acceleration technique provides identification of the charge state of the emitting ion. The decay curve is a cascade-free single exponential. The polarization and beam energy dependence of the light yield have also been measured. The transition has been identified in heavier isoelectronic ions.

EUV spectra in the range λ = 13−40 nm were measured for foil-excited gold ions at 31–238 MeV. Data were recorded at the higher energies to extend and to complement previous measurements and at the lowest energy to see if the recently predicted 5s–5p resonance lines in the Pm-like ion, Au XIX, could be seen in beam-foil excitation. The experimental results are presented, previous beam-foil measurements are reviewed, and the contributions and relations to fusion research are discussed.

Wavelength and lifetime measurements for the fine-structure components of the $1s2s2p^{4}P^{o}\ensuremath{-}1s2{p}^{2}^{4}P$ multiplet in lithiumlike Ne VIII have been made using foil excitation of a fast-ion beam. The results are compared with recent theoretical calculations and with previous measurements for other ions of the same isoelectronic s\"equence. An apparent discrepancy between theory and experiment for the $1s2{p}^{2}^{4}P_{\frac{3}{2}}$ lifetime is removed.

A multiconfiguration Dirac-Fock (MCDF) calculation using the program of Grant and collaborators has been performed for the Ne-like systems S VII, Cl VIII and Ar IX. Excitation energies and transition probabilities have been calculated for all 3s, 3p and 3d levels in these spectra. The theoretical results are in excellent agreement with recent experimental data.

The 1s2s2p2 5P1,2,3-1s2p3 5S20 Ne VII transitions have been observed in the vacuum-ultraviolet spectrum of a foil-excited neon beam. Their wavelengths and the fine structure follow well the trend recently developed for the isoelectronic series of negative Li through F VI. We compare these results to computations using MCDF-OL and MCDF-EAL methods. We also identify transitions from doubly-excited singlet states in Ne VII and note that some of these have been incorrectly classified in the literature.

Lifetime measurements using the beam-foil method are reported for the 2s3p 3P0, 1, 2 levels in N IV, O V and F VI. The 3P1 level lifetime is significantly shorter than those for 3P0 and 3P2. This is due to the 2s2 1S0-2s3p 3P1 intercombination transition. From careful lifetime measurements at several ion energies we obtain transition probabilities of (3.3 ? 2.7) ? 106, (22 ? 6) ? 106 and (88 ? 9) ? 106 s-1 for this spin-forbidden transition in N IV, O V and F VI, respectively. These results are supported by theoretical analyses.

Highly ionized members of the Pm sequence should produce strong resonance lines in the uv spectra of hot plasmas contaminated by heavy elements. These ions for $Zg~74$ have an alkali structure with ground configuration $4{f}^{14}5s$. Hartree-Fock calculations show that in ${\mathrm{W}}_{\mathrm{XIV}}$ through Uxxxii the dominant resonance lines are the $5s\ensuremath{-}5p$ doublets in the range $\ensuremath{\lambda}=100\ensuremath{-}400$ \AA{}. Approximate predictions are given for the doublet wavelengths, line strengths, and mean lives.

The spectrum of Kr VIII has been observed between 180 and 2000 A by using foil excitation of 2.5--3.5-MeV krypton ions. Twenty new transitions have been classified and eleven new excited-state energies have been determined within the n=4 --7 shells. The ionization potential is derived to be 1 015 800 +- 200 cm/sup -1/. The excited-state energies and fine structures are compared with recent relativistic Hartree-Fock calculations. The 4p-state lifetime has been measured by performing a simultaneous analysis of decay data for the 4p level and for its dominant cascade-repopulating levels. The 4p lifetime is found to be 30% shorter than previously measured values and is in excellent agreement with the result of a recent multiconfiguration Hartree-Fock calculation. The source of the discrepancy between this result and earlier measurements is discussed.

Recent developments in the measurement of lifetimes of excited states in highly ionised atoms using fast ion beam excitation methods are reviewed. The applicability and precision of these methods are discussed in the context of specific measurements and compared with existing theoretical capabilities.

An experimental and theoretical study has been made of the 3s-3p and 3p-3d transitions in Ni XIX. The method of beam-foil spectroscopy, using 30-64 MeV Ni ions from a tandem accelerator, has been combined with theoretical calculations using the multiconfiguration Dirac-Fock (MCDF) program.

We report measurements of the lifetimes of the ${2}^{1}$${\mathrm{S}}_{0}$ state of heliumlike ${\mathrm{Ni}}^{26+}$ and the ${2}^{2}$${\mathrm{S}}_{1/2}$ state of hydrogenlike ${\mathrm{Ni}}^{27+}$, both of which decay predominantly by two-photon emission. Our method differs from previous measurements of the lifetimes of similar states in that we require a coincidence between the two photons. Our lifetime of 217.1(1.8) ps for the hydrogenlike decay is in agreement with the theoretical value of 215.45 ps. The heliumlike decay lifetime of 156.1(1.6) ps is in fair agreement with the theoretical value of 154.3(0.5) ps.