Beam-foil Spectroscopy Research Articles

Atomic Transition Probabilities: Past and Future Measurements

The study of atomic and ionic lifetimes and transition probabilities has undergone a period of intense activity, facilitated largely by the application of the technique of Beam-Foil Spectroscopy. This area is now a mature field of study (having been pursued for 40 years), and has provided much new data, elucidated fundamental atomic processes, and furthered applications in many fields. However, technical advances have now broadened the field to permit new types of measurements of unprecedented accuracy and scope. Thus, this occasion offers an appropriate opportunity to review some of the successful applications of the method, to survey the present state of knowledge, and to indicate areas that hold promise for future research.

1s2s2p23s6P–1s2p33s6SoTransitions in O IV

The energies and lifetimes of doubly excited sextet states of boron-like O IV, F V, and Ne VI are calculated with the multiconfiguration Hartree-Fock approach, including QED and higher-order corrections, and also with the multiconfiguration Dirac-Fock GRASP code. The wavelengths and transition rates of electric-dipole transitions from the inner-shell excited terms ${1s2s2p}^{2}3s{}^{6}P--{1s2p}^{3}3s{}^{6}{S}^{o}$ are investigated by beam-foil spectroscopy in the XUV spectral region. The predicted transition wavelengths agree with the experiment. The higher-order corrections, fine structures, and spectrum with high wavelength resolution are found to be critically important in these comparisons. Nine new lines have been identified. The ground sextet states of boronlike atoms are metastable and well above several ionization levels. These are possible candidates for XUV and soft x-ray lasers.

The continuing saga of the Si-like intercombination lines in highly charged ions, Si-like rhodium

The 3s23p23P1,2–3s3p35S2 intercombination lines for Si-like Rh have been investigated using the techniques of beam-foil spectroscopy. Two lines have been identified as belonging to these transitions having, however, different wavelengths from what would be expected based on published iso-electronic trends. The identifications suggested here are strengthened by requirements on (a) the known fine structure splitting in the ground configuration, (b) an intensity ratio based on branching ratios, (c) similar decay characteristics and (d) expected behaviour of the lines as a function of the ion beam energy.

Precise atomic lifetime measurements with stored ion beams and ion traps

For many years, atomic lifetime measurements on multiply-charged ions have been done almost exclusively by beam-foil spectroscopy. For low ion charges, however, spin-changing "intercombination" transitions have a rate that renders them too slow for traditional fast-beam techniques. Here ion traps and fast-ion beams have been combined in the concept of heavy-ion storage rings. These devices have permitted not only an extension of intercombination lifetime measurements down to singly charged ions, but they also facilitated similar measurements on electric-dipole forbidden transitions. The electron-beam ion trap (EBIT) complements the storage-ring work for work on highly charged ions. Achievements, technical issues, and prospects are outlined. PACS Nos.: 32.70Cs, 32.30Jc, 34.50Fa

Radiative lifetime and oscillator strength determination in Mg-like potassium (K VIII)

Theoretical transition probabilities have been obtained for 54 n = 3 transitions depopulating the 3s3p 1Po, 3p2 3P, 1D, 1S, 3s3d 1D, 3D and 3p3d 3Po, 3Do, 3Fo, 1Fo, 1Do, 1Po levels, including 14 transitions not yet observed. Some of these predictions have been compared with experimental lifetimes obtained by beam-foil spectroscopy for four n = 3 levels of K7+. An excellent agreement is observed between theory and experiment for all the levels.

The Spectroscopy of High Spin States for Highly Charged Mg-like Ions

Transitions between high spin core-excited states of highly charged Mg-like Ions (Ti, Fe and Ni) have been investigated using beam-foil spectroscopy. The identification of the 2p53s3p2 5D4–2p53s3p3d 5F5 transition has been aided by calculations and iso-electronic methods.

Effects of Foil Aging Properties on the Intensities of Atomic Transition Lines Following Ion-Foil Interaction

In this work we measured the time dependence of the light intensity of the 1s22s 1S1/2–1s12p 1P3/2 resonance transition in Li-like Al ions after passing through a 10 µg/cm2 carbon foil. The measurements were done at beam energies of 0.8 MeV/u and 2.5 MeV/u. Also, the 1s5g–1s6h transition in He-like Al ions, after passing through the same thickness carbon foil, at 2.5 MeV/u was investigated. The results show very obvious decreases of the light intensity for the Li-like transition with beam exposure time of the foil at both beam energies, while a slow increase was observed for the He-like line intensity. In this work several different foil aging effects on line intensities are discussed and a new normalization scheme is proposed to improve the reliability of beam foil spectroscopy method in lifetime measurements.

High-spin inner-shell excited states in few-electronneon: quintets and sextets

Quintet states in Be-like Ne VII and sextet states in B-like Ne VIhave been investigated by beam-foil spectroscopy in the XUVspectral region. The wavelengths of the n = 2-n' = 3 transitionsfrom the inner-shell excited terms 1s2l2pn'l' 5L;l = s,p; l' = s,p,d and 1s2l2p2n'l' 6L; l = s,p; l' = s,p,d are obtainedusing Hartree-Fock calculations and linear Z-expansions in therespective isoelectronic sequences. Lifetime measurements arepresented for some n = 3 quintet levels. Spectral lines areassigned to n = 2-n' = 4 transitions supported by 32 newline identifications in Ne VI, VII and IX. A class of metastablelow-spin inner-shell excited states in Be-like ions, possiblecandidates for XUV-laser schemes, are also discussed. Finally,metastable 3l3l' sextet states in the B sequence areintroduced, one of which might be bound in Be-.

High precision polarimeter system for atomic polarization and tilted-foil experiments with 1.7 keV/amu N+14 beam

A high precision atomic polarimeter system for the use in beam-foil spectroscopy experiments with a few keV/amu heavy-ion beams has been developed. The polarimeter measures the circular polarization of fluorescences from the beam ion in-flight after the beam–foil interaction. The present system has two identical such polarimeters in both sides of the beam axis to reduce the systematic errors such due to the fluctuations of beam current, background and so on. A successful use of an ultrathin carbon foil (1.5 μg/cm2), which was durable for several hours against a few hundred nA beam irradiation, enabled the beam–foil experiments with such low energy heavy-ion beams. A performance test of the polarimeter system was carried out in the tilted foil experiments with a 1.7 keV/amu N+14 beam. The atomic polarization was observed for the transition 1s22s22p3p 1D→1s22s22p3p 1P, whose fluorescence wavelength is 399.5 nm, in the N+ ion (N II). The polarization was approximately −2% for the tilt angle of −40° and showed monotone increasing with increasing tilt angle up to +2% for +40°. The polarization at 0° was (0.002±0.25)%, which is highly consistent with the expected polarization of 0%. This result indicates the high reliability of the present polarimeter system. This is the first tilted-foil experiment at such low beam energy. The present experimental technique will be very useful for studies of the polarization mechanism of the beam–foil interaction.

Analysis of the 2p53s, 3p and 3d Configurations of Ne-like Br XXVI

We report a study of the 2p53s, 3p and 3d configurations of Ne-like Br XXVI, by means of beam foil spectroscopy. In the region 100 – 300 Å more than 20 Br XXVI lines were identified, 5 of which were known earlier, from x-ray laser work. Experimental energies for the 3s, 3p and 3d levels are compared with results of relativistic Hartree-Fock (HFR) calculations. We also present predicted 3s-3p and 3p-3d wavelengths, obtained by combining the new Br results with data for other Ne-like ions.

Low-lying quartet states in Li-like neon

Inner-shell excited quartet states in Li-like neon have been investigated by beam-foil spectroscopy in the XUV domain. The wavelengths of the transitions n = 2 - n´ = 3 and 2 = 2 - n´ = 4 from the states 1s2ln´l´ 4L; l = s,p; l´ = s,p,d in Ne VIII were obtained using Hartree-Fock calculations and linear Z-expansions in the Li sequence. The results of these expansions are in good agreement with the measured wavelengths. Our high resolving power allows us to enhance the wavelength accuracy of quartet transitions previously reported. In addition, lifetimes of n´ = 3 quartet states are measured. Finally, an assignment for a transition from the non-autoionizing inner-shell excited doublet state Ne VIII 1s2p(3Po)3d 2Do is made and a Li-like quartet E2 transition in our spectra is discussed.

Ultraviolet and Visible Spectroscopy of Rydberg States inHighly Ionized Neon

Ultraviolet and visible beam-foil spectroscopy(1800–5300Å) of Ne at 10MeV reveals many transitionsbetween Rydberg levels with principal quantum numbers5 ≤ n ≤ 15 in Ne VI–X. Most of the low-ltransitions for the strong ground-state core transition lines in NeVIII have been observed. Low-l transitions of Ne VI 6–7and Ne VII 6–7 as well as metastable core-excited quartetRydberg transitions for Ne VIII 6–7, Ne VIII 7–8 and NeVIII 8–9 were identified based on multi-configurationHartree-Fock calculations. New transitions between n = 3 levelshave been assigned in Ne VI, VII and IX.

Limitations of the Beam-Foil Method in Lifetime Measurements

We have measured the decay time of the 1s22p 2P3/2 level of Li-like Ar15+ using beam-foil spectroscopy. The experiment was done at a beam energy of 2.6 MeV/u and with an Al foil instead of a C foil which is usually used in beam-foil experiments. The signal was recorded by means of a CCD detector system installed on a 2.2 meter grazing incidence spectrometer. This system allowed recording of the spectral line-profile decay. The result shows different decay times for different parts of the measured line profile, thereby corroborating our previous studies of the same transition in Ar15+ by using a C foil and the same energy. In the latter work some difficulties were revealed in determing reliable lifetimes for excited levels in highly charged ions. We discuss the causes of the uncertainties in decay time (hence lifetime) measurements and suggest a scheme for increasing the reliability of beam-foil lifetime experiments.

A CCD Detector System for Beam-Foil Spectroscopy

A thin back-illuminated CCD based photon detector system has been installed on the Rowland circle of a 2.2 meter grazing incidence spectrometer, replacing the previously used Ceratron single channel detector. For reasons to be described the CCD chip surface was not angled tangential to the spectrometer Rowland circle. The spectrometer is used for beam-foil spectroscopy experiments at the RIKEN laboratory. The performance of the CCD was tested using a beam of 183 MeV Nb ions.

Measurements of2s2S1/2–2p2P3/2,1/2transition energies in lithiumlike heavy ions: Experiments and results forNi25+andZn27+

Wavelengths of the fine-structure transitions $2s{}^{2}{S}_{1/2}--2p{}^{2}{P}_{3/2}$ and $2s{}^{2}{S}_{1/2}--2p{}^{2}{P}_{1/2}$ in lithiumlike ${\mathrm{Ni}}^{25+}$ and ${\mathrm{Zn}}^{27+}$ have been measured using beam-foil excitation and grazing-incidence spectroscopy. The respective transition wavelengths of $142.461\ifmmode\pm\else\textpm\fi{}0.006\AA{}$ and of $216.061\ifmmode\pm\else\textpm\fi{}0.011\AA{}$ for ${\mathrm{Zn}}^{27+}$ have not been measured so far and present the most precise values in lithiumlike heavy ions measured by beam-foil spectroscopy. The ${\mathrm{Ni}}^{25+}$ results are in very good agreement with data from grazing-incidence spectroscopy at tokamaks. The measurement precision for both ions corresponds to less than or equal to 1% of the quantum electrodynamic contributions to the transition energies. This provides sensitivity to two-photon exchange processes for the screening of the electron self-energy.

Lifetimes of Doubly-Excited 2p3l Levels in SinglyIonized Boron, B II

Lifetimes of several 2p3s, 2p3p and 2p3d levels in singly ionizedboron, B II, have been determined by means of beam–foilspectroscopy and theoretical calculations, using thesuperposition-of-configurations (SOC) program of Cowan. Thesedoubly-excited terms which lie close to the first ionization limit ofB II make radiative transitions to the singly excited system. For a fewautoionizing 2p3l levels we have calculated excitationenergies and decay rates. The theoretical and experimental data aregenerally in very good agreement.

Particle detected Fourier transform NMR at single crystal surfaces— 6Li on Ru(001)

Fourier transform nuclear magnetic resonance at single crystal surfaces is described. The adsorption of 6Li on Ru(001) is investigated by newly developed techniques utilizing atomic beam sample preparation and optical detection of the NMR signal on desorbing atoms (laser induced fluorescence) or ions (beamfoil spectroscopy). In complete analogy to magnetically detected NMR, the transverse magnetisation is coherently recorded. This allows pulse NMR to be applied to surface problems.

NMR at single crystal surfaces

Nuclear magnetic resonance has developed into a very powerful technique to study the structure and dynamics of atomic and molecular systems, in both the liquid and the solid phase. However, the investigation of single-crystal surfaces with the conventional NMR methods is essentially impossible due to the small sample size of less than 10 15 sites on 1 cm 2 . To overcome this for the important class of alkali adsorbates on metals and semiconductors, two methods are presented. Common to both is the preparation of a highly nuclear spin-polarized atomic beam of 6 Li in the one case and 8 Li in the other. The latter isotope is radioactive and undergoes a β-decay with a half-life of 0.84 s. Li adsorbed on the close-packed Ru(001) surface is investigated. The longitudinal relaxation time, T 1 , is the main observable and is used to deduce the local electronic density of states [LDOS(E F , r = 0)] and Li diffusion barriers. The second experiment uses 6 Li as an adsorbate, also studied on Ru(001). The nuclear polarization is measured by beam foil spectroscopy. A novel particle detected (photon counting) Fourier transform NMR technique is demonstrated. This is done by observing the time-dependent flux of circularly polarized light emitted behind the foil after a 90° pulse has been employed at the surface. Electric field gradients and transverse relaxation times, T 2 , are thus determined. A large difference between T 1 and T 2 is traced to the dimensionality of the system.

Intercombination transitions in beryllium-like and boron-like iron

Beam-foil spectroscopy techniques have been used to study intercombination transitions in Be-like Fe22+ and B-like Fe21+. The method of time delayed spectroscopy was used to identify the Be-like 2s2 1S0-2s2p 3P1 and B-like 2s22p 2P3/2-2s2p2 4P5/2 transitions. Intensity decay curves for these transitions were recorded and level lifetimes were obtained. The latter were found to be 17.5 ± 1.5 ns for the Be-like 2s2p 3P1 level and 14.8 ± 1.0 ns for the B-like 2s2p2 4P5/2 level.

Single crystal surfaces probed by polarized nuclei

Nuclear magnetic resonance has developed into a very powerful technique to study the structure and dynamics of atomic and molecular systems, both in liquid and solid phase. However the investigation of single crystal surfaces with “conventional” NMR methods is essentially impossible due to the small sample size of less than 1015 sites on a cm2. To overcome this for the important class of alkali adsorbates on metals and semiconductors, two methods are presented. Common to both is the preparation of a highly nuclear spin polarized atomic beam of 6Li in the one case and 8Li in the other. The latter isotope is radioactive and undergoes a \beta‐decay with a halflife of 0.84 s. Li adsorbed on the close packed Ru(001) surface is investigated. The T{in1} relaxation rate is the main observable and is used to deduce the local electronic density of states (LDOS(EF,r=0)) and the Li diffusion barriers at low and high adsorbate coverage. The second experiment uses 6Li as an adsorbate, also studied on Ru(001). The nuclear polarization is measured by beam foil spectroscopy. A novel particle detected (photon counting) Fourier‐Transform NMR technique is demonstrated, by observing the time dependent flux of circularly polarized light emitted behind the foil after a 90\circ‐pulse has been employed at the surface. Development and prospects of the latter technique are presented.