Foil Excitation Research Articles

Polarization ofn=3-terms after electron capture by beam-foil interaction

The electron capture by protons into excitedn=3 states as function of the distance from the surface is calculated in OBK-approximation. The target electrons are treated as free electron gas bounded by a step potential. Electron loss processes are considered by weighting the density matrix with a survival probability for different capture depths. The result is used to determine the Stokes parameters of the emitted light. Comparison is made with polarization measurements of HeI (n=3) states after beam foil excitation.

Beam-Foil Study of Titanium in the EUV Using Foils of Different Materials

The ultraviolet spectrum (λ8-60 nm) of highly ionized titanium after foil excitation of 15 and 20 MeV fast ions has been recorded. A number of new lines, mainly in sodiumlike Ti XII, are identified. Line intensity ratios and decay curves are measured in selected cases. Using foils of different materials, relative intensities of yrast transitions n = 3-6 are evaluated and intercompared with the aid of cascade models. Decay curves of yrast levels are studied for resonance effects in the population mechanism. Little evidence is found for such effects.

The optical decay of high l rydberg states in beam foil excited o 7+

Delayed optical radiation following beam foil excitation of Rydberg states in hydrogenic oxygen ions has been observed in the time range from 1.5 ns to 63 ns. The intensity of the n = 9 → 8 and n = 8 → 7 optical transitions ( n is the principle quantum number) is found to be ten times smaller than for the delayed X-ray radiation ( n = 2 → 1) measured in previous work. The results rule out initial populations of levels with only angular momentum l = n − 1 as well as populations of levels with only l = 0.

Absolute Auger yield measurements of Al +, Si +, P +, S +, and Cl + projectiles following foil excitation

Absolute yields of L-Auger electrons of 50–500 keV projectiles with 13⩽ Z⩽18 emerging from thin carbon foils have been measured. Exceptionally high yields (in some cases > 50%) have been found.

Beam-foil level lifetimes in Krypton III

The radiative lifetimes of levels in doubly-ionized Krypton have been measured after foil excitation of a beam of Krypton particles in the wavelength range 120nm–500nm. The results are compared with the experimental values obtained by Fink et al. [1], as well as theoretical values resulting from Coulomb approximation (C.A.) and single configuration Hartree-Fock (H.F.) calculations.

Cascade effects on quantum beats: a case of foil excitation

Calculations have been made of cascade effects on Zeeman quantum beats for the case of beam-foil excitation of certain levels in Ar II. It is noted that cascade contributions significantly affect the quantum beat or oscillatory part of the total light intensity. However, whilst determination of the lifetime of the intermediate level via quantum beat measurements is significantly affected by cascade contributions, the determination of the Lande g factor for the intermediate level is essentially unaffected.

Doubly excited quartet states in Be II

The doubly excited quartet spectrum of Be II, produced by foil excitation of fast Be+ ion beams, has been studied in the wavelength region 600-5000 AA. On the basis of new wavelength determinations for the quartet transitions it has been possible to identify the lower lying, doubly excited levels 1s2snd 4D(n=3-5), 1s2pnd 4D degrees (n=3-5), 1s2p3p 4S, 4P and 4D, and 1s2s4p 4P degrees besides obtaining further evidence for the identification of the 1s2s4s 4S and 1s2s3p 4P degrees levels recently proposed by Mannervik et al. (1981). Recent theoretical calculations support the present identifications.

Information-theoretical interpretation of level populations and charge distribution in beam-foil spectroscopy

The consequences of the assumption that, under prescribed experimental conditions, the ions emerge randomly excited from the foil are examined using the maximum entropy principle. For that purpose constraints are identified by examining the time-resolved intensity of the electromagnetic radiation emitted by the ions after the foil. This leads to the most likely level or multiplet population density matrix compatible with the experimental information. The main consequences of the foil excitation mechanisms are accounted for by incorporating the framework of atomic structure into our information-theoretical approach. In the case of transitions in individual Rydberg series it is shown that for large $n$ and a given $l$ the level population functions decrease as ${n}^{*\ensuremath{-}3}$ and depend universally on the kinetic energy of the incoming beam. It is also shown that the identification of the radiation from the emerging ions in terms of one-electron states leads, according to the maximum entropy principle, to a unique factorized representation of the probability that an ion has a given configuration immediately after the foil. This representation is used for a derivation of the charge distribution which for heavy ions is shown to be approximately Gaussian or chi-squared in accordance with experiments. The passage of ${\mathrm{He}}^{+}$ ions through the foil is treated as an example of a consistency test based on the information-theoretical derivation of the charge distribution. Following Levine and co-workers a surprisal analysis of level population functions is suggested as a well-established alternative to detailed models of foil excitation which usually invoke perturbation theory and consequently often have an ill-defined range of validity.

Contributions to Fusion Research from Studies of Foil-Excited Heavy Ion Beams

XUV spectra in the wavelength region from 5-55 nm have been measured at Brookhaven National Laboratory for foil-excited ion beams of Au, W, Zr, Mo, Fe, and Ti. These atoms are of particular interest in fusion research. In sharp contrast to the wide-ranging time-dependent charge state evolution of heavy ions in a plasma discharge, foil excitation of a heavy ion beam produces relatively well-defined narrow charge-state distributions. Existing accelerators, such as the Brookhaven MP tandems, can produce sufficiently energetic heavy ion beams to span the range of charge states found in present-day tokamaks. The comparison of spectra from beam-foil excitation and tokamak plasmas can therefore be used to infer the average ion and charge state makeup of the plasma discharge. Such comparisons with Au, W, and Mo features in tokamak spectra are discussed and spectra for Zr and Fe (which are expected contaminants in new tokamaks) are presented. Additional contributions to fusion research from beam-foil experiments are demonstrated. These include: wavelengths and identifications for lines which can be resolved in plasma spectra and direct determinations of radiative lifetimes and absorption oscillator strengths for these transitions which are used to determine impurity concentrations in plasmas.

X-ray spectra and satellite classification of foil-excited Mg and Al

Foil excitation of fast ion beams has been used to produce few-electron ions of magnesium and aluminium. The emission spectra have been analysed with a 10 cm curved crystal X-ray spectrometer. The spectra show series of transitions in H- and He-like ions and numerous satellite lines. By comparison with Hartree-XR calculations, these lines are seen to arise from core-excited two- to five-electron ions.

EUV spectra of Mo XIV to Mo XXIX

Observations of the EUV spectra of Mo XIV to Mo XXIX have been made in the wavelength region from 5 to 50 nm by foil excitation of 22 to 200 MeV Mo ion beams. The results are compared with spectra observed in tokamak plasmas and light sources and with theoretical predictions.

Wavelength measurements of 1s2s3S-1s2p3P transitions in helium-like28Si12+,32S14+and35Cl15+

The authors have measured the wavelengths of the 1s2s 3S-1s2p 3P transitions in two-electron Si XIII, S XV, and Cl XVI, produced by foil excitation of 50-90 MeV silicon, sulphur, and chlorine ions. Our wavelength precision provides sensitivity to Lamb shift contributions to the transition energies to +or-3%. The results for S XV represent the first measurements of the 3S-3P transitions for this ion.

Energies and lifetimes of excited states in copperlike Kr viii

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.

Beam-Foil Lifetime Studies of Highly Ionized Silicon

Previous beam-foil lifetime studies in the EUV range are extended to cover all n = 2 states of Si VII and Si IX. The lifetime values obtained agree with theoretical data of Nicolaïdes and Beck. For some states of Si X and Si XI information about initial population ratios of singly and doubly excited states is obtained. After foil excitation, the contribution of cascades from doubly excited states in the n = 2 shell to the intensities of decays from singly excited to ground states amounts to 20-40%.

EUV Spectrum and Lifetimes of Foil-Excited Phosphorus

Foil excitation of 31P ions at 19 MeV has been used to produce singly and doubly excited states of highly ionized phosphorus atoms. The EUV spectrum in the wavelength range 10 nm < λ < 55 nm has been observed and analyzed by means of a 2.2 m grazing incidence monochromator. General agreement with tabulations based on observations with other light sources has been found. Decay curves of 22 n = 2, Δn = 0 transitions in P IX-P XII have been taken. The evaluation procedure accounts for the spatial resolution function. As in the case of silicon, the decays of singly excited states appear to be strongly fed by cascades from doubly excited states in the n = 2 shell. The lifetime results are in general agreement with theoretical predictions.

The X-ray spectra of H-like, He-like and Li-like silicon ions after foil excitation

Radiation emitted from silicon beams at 35, 38 and 45 MeV excited by passage through thin carbon foils has been studied in the range 3-9 AA using a curved-crystal X-ray spectrometer. More than 100 lines have been observed, of which only one quarter had been observed or predicted from solar or laboratory plasma spectra. Most of the previously unobserved lines may be attributed to satellites of H- and He-like transitions arising in three- and four-electron Si ions. The lifetime of the Si XIII 23P1 state has been measured, and a preliminary experimental value of 6.5+or-1.3 ps obtained.

Absolute Auger Yield Measurements of P+, O+, N+, C+, and B+ Projectiles Following Foil Excitation

Absolute K-and L-shell vacancy fractions of 50 to 500 keV P+-, O+-, N+-, C+-, and B+-ions excited by thin carbon foils have been measured using the technique of Auger-electron spectroscopy. The fraction of inner-shell vacancies in the excited projectile ions is proportional to the absolute Auger yield for low Z atoms. The projectile energy dependence of the vacancy fractions is duscussed.

Projectile Auger spectra of gas- and foil-excited oxygen ions at MeV energies

High resolution projectile Auger spectra of singly and doubly core-excited oxygen ions are studied as a function of incident beam energy (2--18 MeV), exciter gas (helium, neon, argon, and krypton) or foil (carbon), incident charge state (1+--4+), and delay time. Auger electron production from excited oxygen states varies considerably for collisions with different beam energies and exciter gases. A striking similarity in both the linewidths and relative Auger line intensities is observed for single collision gas and multiple collision foil excitation. The incident ion charge state does not appreciably affect the excitation cross sections and the charge state of the outgoing projectile. In order to assign the measured Auger peaks numerous excitation and Auger transition energies for singly and doubly core-excited states are calculated using semiempirical and ab initio methods. Calculated transition energies are compared with experimental line structures. Possible excitation and deexcitation mechanisms are suggested and discussed within a quasimolecular single-particle model.

Observation of zero field quantum beats in gas-excited helium projectiles

Zero-field quantum beats in the decay of the 3p3P level in HeI have been observed under approximately single-collision conditions using fast4He+ projectiles excited in a short gas-cell. The frequency, amplitude and phase of the beat signal are compared with theory and results from multiple collision experiments using foil excitation.

Alignment effects in heavy ionized beam foil excited atoms

We have measured the polarization of light emitted in optical transitions in P V, O VII and O VIII, produced by beam foil excitation, using a 400 keV phosphorus beam from a heavy ion accelerator and a 36 MeV oxygen beam from a Tandem Van de Graaff generator.