Absolute Cross Section Measurements Research Articles

Accurate measurement of absolute experimental inelastic mean free paths and EELS differential cross-sections

Methods are described for measuring accurate absolute experimental inelastic mean free paths and differential cross-sections using DualEELS. The methods remove the effects of surface layers and give the results for the bulk materials. The materials used are VC0.83, TiC0.98, VN0.97 and TiN0.88 but the method should be applicable to a wide range of materials. The data was taken at 200keV using a probe half angle of 29mrad and a collection angle of 36mrad. The background can be subtracted from under the ionisation edges, which can then be separated from each other. This is achieved by scaling Hartree-Slater calculated cross-sections to the edges in the atomic regions well above the threshold. The average scaling factors required are 1.00 for the non-metal K-edges and 1.01 for the metal L-edges (with uncertainties of a few percent). If preliminary measurements of the chromatic effects in the post-specimen lenses are correct, both drop to 0.99. The inelastic mean free path for TiC0.98 was measured as 103.6±0.5nm compared to the prediction of 126.9nm based on the widely used Iakoubovskii parameterisation.

Determination of absolute cross sections for cluster-specific decays

Fluorescence spectrometry is used to determine absolute cross sections of cluster-specific decay processes following photon excitation. Absolute values for selected processes in partially condensed supersonic jets are determined using atomic benchmarks. This method is applicable to a wide variety of processes in clusters in which a photon is emitted. In a proof-of-principle experiment, absolute cross sections of processes in two prototype systems were measured: (I) inner-valence excitation and ionization of Ar clusters and (II) resonant interatomic Coulombic decay (rICD) in Ne clusters. Example II is the first measurement of absolute cross sections of ICD whatsoever.

Investigations of L-shell x-ray production cross sections of In and Sn by low-energy electron impact

Results from the measurements of absolute L-shell x-ray production cross sections of In (Z = 49) and Sn (Z = 50) by the impact of electrons with energies in the range of 6–28 keV are presented. The experimental setup has been improved and thin films with thick aluminum substrates were used as targets in the experiments. The effect of directional and energy spreads of the incident electron beam within the films and the characteristic x-ray enhancement due to backscattered electrons and bremsstrahlung photons generated from the substrates were corrected by means of Monte Carlo simulations. The experimental results of In and Sn elements by electron and positron impacts were compared with the DWBA and PWBA-C-Ex theories. It was shown that the L-shell x-ray production cross sections of In and Sn elements by electron impact measured in this paper were in good agreement with the theoretical predictions within the uncertainties and that the discrepancies existed between the experimental results by positron impact, available in the literature, and the theoretical values. The influence of the atomic relaxation parameters (fluorescence yields and Coster–Kronig coefficients) has also been discussed.

Absolute single-photoionization cross sections ofSe2+: Experiment and theory

Absolute single-photoionization cross-section measurements for ${\mathrm{Se}}^{2+}$ ions were performed at the Advanced Light Source at Lawrence Berkeley National Laboratory using the merged-beams photo-ion technique. Measurements were made at a photon energy resolution of $24\ifmmode\pm\else\textpm\fi{}3$ meV in the photon energy range 23.5--42.5 eV, spanning the ground state and low-lying metastable state ionization thresholds. To clearly resolve the resonant structure near the ground-state threshold, high-resolution measurements were made from 30.0 to 31.9 eV at a photon energy resolution of $6.7\ifmmode\pm\else\textpm\fi{}0.7$ meV. Numerous resonance features observed in the experimental spectra are assigned and their energies and quantum defects tabulated. The high-resolution cross-section measurements are compared with large-scale, state-of-the-art theoretical cross-section calculations obtained from the Dirac Coulomb $R$-matrix method. Suitable agreement is obtained over the entire photon energy range investigated. These results are an experimental determination of the absolute photoionization cross section of doubly ionized selenium and include a detailed analysis of the photoionization resonance spectrum of this ion.

Electronic excitation of furfural as probed by high-resolution vacuum ultraviolet spectroscopy, electron energy loss spectroscopy, and ab initio calculations.

The electronic spectroscopy of isolated furfural (2-furaldehyde) in the gas phase has been investigated using high-resolution photoabsorption spectroscopy in the 3.5-10.8 eV energy-range, with absolute cross section measurements derived. Electron energy loss spectra are also measured over a range of kinematical conditions. Those energy loss spectra are used to derive differential cross sections and in turn generalised oscillator strengths. These experiments are supported by ab initio calculations in order to assign the excited states of the neutral molecule. The good agreement between the theoretical results and the measurements allows us to provide the first quantitative assignment of the electronic state spectroscopy of furfural over an extended energy range.

Measurements of photoionization cross section of the 4p levels and oscillator strength of the 4p→nd 2D3/2,5/2 transitions of potassium

We have carried out measurements of absolute photoionization cross sections of the 4p excited levels and oscillator strengths of the 4p→nd Rydberg transitions in potassium using a two-step photo-excitation and ionization technique in conjunction with a thermionic diode ion detector. The measurements were conducted using the linearly polarized laser light and the absolute values of the cross sections from the 4p 2P3/2 and 2P1/2 excited levels have been determined at the ionization threshold as (6.3±0.9)Mb and (5.4±0.8)Mb respectively. In addition, photoionization cross sections have been determined at various ionizing wavelengths above the first ionization threshold to explore different energy regions of the continuum. The oscillator strengths for the 4p 2P1/2→nd 2D3/2 and 4p 2P3/2→nd 2D3/2,5/2 Rydberg transitions have been deduced by using the measured cross sections of the 4p 2P1/2 and 2P3/2 levels at the ionization threshold. The new results are in good agreement with the available theoretical and experimental data.

Toluene Valence and Rydberg Excitations as Studied by ab initio Calculations and Vacuum Ultraviolet (VUV) Synchrotron Radiation.

The electronic spectroscopy of isolated toluene in the gas phase has been investigated using high-resolution photoabsorption spectroscopy in the 4.0-10.8 eV energy range, with absolute cross-section measurements derived. We present the first set of ab initio calculations (vertical energies and oscillator strengths), which we use in the assignment of valence and Rydberg transitions of the toluene molecule. The spectrum reveals several new features not previously reported in the literature, with particular relevance to 7.989 and 8.958 eV, which are here tentatively assigned to the π*(17a') ← σ(15a') and 1π*(10a″) ← 1π(14a') transitions, respectively. The measured absolute photoabsorption cross sections have been used to calculate the photolysis lifetime of toluene in the upper stratosphere (20-50 km).

Precision studies of deep-inner-shell photoabsorption by atomic ions

With emphasis on K-shell ionization and K-shell excitation mediated by the absorption of a single photon the present paper provides an overview of the status of experiments in which deep-inner-shell photoabsorption by atomic ions has been investigated. For inner-shell processes not involving the K-shell only photon energies beyond about 350 eV are considered. Mechanisms of photoabsorption and subsequent relaxation as well as relations between photoionization and photorecombination are discussed. New possibilities for precise measurements of transition energies have been provided by the high photon-energy resolution available at 3rd generation synchrotron light sources. This is particularly interesting for studies of K-shell excitation of few-electron systems. By using atomic ions as targets, either in form of a collimated beam or trapped in electromagnetic fields, it has been possible to reach a high level of precision in the spectroscopy of transitions involving the decay of K vacancies by photoemission or Auger processes. Examples for absolute cross section measurements and precision spectroscopy are presented.

A novel technique for the measurement of the electron neutrino cross section

Absolute neutrino cross section measurements are presently limited by uncertainties on $\nu$ fluxes. In this paper, we propose a technique that is based on the reconstruction of large angle positrons in the decay tunnel to identify three-body semileptonic $K^+ \rightarrow e^+ \pi^0 \nu_e$ decays. This tagging facility operated in positron counting mode ("event count mode") can be employed to determine the absolute $\nu_e$ flux at the neutrino detector with ${\cal O}(1\%)$ precision. Facilities operated in "event by event tag mode" i.e. tagged neutrino beams that exploit the time coincidence of the positron at source and the $\nu_e$ interaction at the detector, are also discussed.

Total and partial cross sections of theSn112(α,γ)Te116reaction measured via in-beamγ-ray spectroscopy

An extended database of experimental data is needed to address uncertainties of the nuclear-physics input parameters for Hauser-Feshbach calculations. Especially $\alpha$+nucleus optical model potentials at low energies are not well known. The in-beam technique with an array of high-purity germanium (HPGe) detectors was successfully applied to the measurement of absolute cross sections of an ($\alpha$,$\gamma$) reaction on a heavy nucleus at sub-Coulomb energies. The total and partial cross-section values were measured by means of in-beam $\gamma$-ray spectroscopy. Total and partial cross sections were measured at four different $\alpha$-particle energies from $E_\alpha = 10.5$ MeV to $E_\alpha = 12$ MeV. The measured total cross-section values are in excellent agreement with previous results obtained with the activation technique, which proves the validity of the applied method. The experimental data was compared to Hauser-Feshbach calculations using the nuclear reaction code TALYS. A modified version of the semi-microscopic $\alpha$+nucleus optical model potential OMP 3, as well as modified proton and $\gamma$ widths, are needed in order to obtain a good agreement between experimental data and theory. It is found, that a model using a local modification of the nuclear-physics input parameters simultaneously reproduces total cross sections of the $^{112}$Sn($\alpha$,$\gamma$) and $^{112}$Sn($\alpha$,p) reactions. The measurement of partial cross sections turns out to be very important in this case in order to apply the correct $\gamma$-ray strength function in the Hauser-Feshbach calculations. The model also reproduces cross-section values of $\alpha$-induced reactions on $^{106}$Cd, as well as of ($\alpha$,n) reactions on $^{115,116}$Sn, hinting at a more global character of the obtained nuclear-physics input.

Single photoionization of aluminum-like P2+ and magnesium-like P3+

Absolute single-photoionization cross sections for aluminum-like P2+ have been measured from 26.5 to 66.5eV using photoion yield spectroscopy. In addition, absolute cross section measurements for the single-photoionization of magnesium-like P3+ have been measured from 43 to 60eV. For both ions, the measured spectra are dominated by Rydberg series resonances superimposed on a non-resonant direct photoionization cross section. Analyses of these data have helped identify the initial and final states of the Rydberg series observed in each photoion. In addition, quantum defects are derived for each series.

Photodetachment cross sections of theC2nH− (n=1–3)hydrocarbon-chain anions

We report theoretical results of the low-energy photodetachment cross sections of the ${\mathrm{C}}_{2}\mathrm{H}{}^{\ensuremath{-}},\phantom{\rule{0.16em}{0ex}}{\mathrm{C}}_{4}\mathrm{H}{}^{\ensuremath{-}}$, and ${\mathrm{C}}_{6}\mathrm{H}{}^{\ensuremath{-}}$ hydrocarbon-chain anions. The complex Kohn variational technique is used to calculate molecular-frame transition dipole moments from the anion ground state to a photoelectron in the continuum of the neutral radical. We employ the Franck-Condon approximation and include interchannel electronic coupling to determine the low-energy photodetachment cross sections and asymmetry parameters. We discuss the behavior of the cross section, especially near thresholds, and describe the role of electronic resonances and excited channels. The theoretical results reproduce the main characteristics of recent measurements of absolute photodetachment cross sections.

Absolute measurements of chlorine Cl+ cation single photoionization cross section

The photoionization of Cl+ leading to Cl2+ was measured in the photon energy range of 19.5–28.0eV. A spectrum with a photon energy resolution of 15meV normalized to absolute cross-section measurements is presented. The measurements were carried out by merging a Cl+ ion beam with a photon beam of highly monochromatic synchrotron radiation at the Advanced Light Source at Lawrence Berkeley National Laboratory. The measured photoionization of Cl+ consists of several autoionization resonances surperimposed on the direct photoionization signal. Most of the prominent resonances are assigned to members of Rydberg series originating from the singlet ground state and from metastable triplet levels within the ground-state configuration of Cl+. The direct ionization cross section is no larger than 12Mb.

Absolute absorption cross-section measurement of a submonolayer film on a silica microresonator

Conventional absorption spectroscopy is not nearly sensitive enough for quantitative overtone measurements on submonolayer coatings. While cavity-enhanced absorption detection methods using microresonators have the potential to provide quantitative absorption cross sections of even weakly absorbing submonolayer films, this potential has not yet been fully realized. To determine the absorption cross section of a submonolayer film of ethylene diamine (EDA) on a silica microsphere resonator, we use phase-shift cavity ringdown spectroscopy simultaneously on near-IR radiation that is Rayleigh backscattered from the microsphere and transmitted through the coupling fiber taper. We then independently determine both the coupling coefficient and the optical loss within the resonator. Together with a coincident measurement of the wavelength frequency shift, an absolute overtone absorption cross section of adsorbed EDA, at submonolayer coverage, was obtained and was compared to the bulk value. The smallest quantifiable absorption cross section is σmin=2.7×10−12 cm2. This absorption cross section is comparable to the extinction coefficients of, e.g., single gold nanoparticles or aerosol particles. We therefore propose that the present method is also a viable route to absolute extinction measurements of single particles.

Absolute measurements of electron impact excitation cross-sections of atoms using cavity ringdown spectroscopy

A new method of using cavity ringdown spectroscopy (CRDS) technique to measure apparent electron impact excitation cross-sections (EIECS) for different energy levels of an atom is described. The method involves measurements of absolute population densities of the energy levels in concern using the high sensitivity CRDS, leading to an analytical expression of EIECS as a function of the population densities. Determination of EIECS in this method does not require an external calibration. The proposed method is validated by determining EIECS of the metastable levels of Hg (6s6p 3P0) and Ne (2p53s 3P2), and the results are in good agreement with the cross-sections reported in the literature. The population densities in both cases were extracted from the published data. This method exhibits promising potential for absolute measurements of EIECS of metastable energy levels.

Double−K−vacancystates in electron-impact single ionization of metastable two-electronN5+(1s2s3S1) ions

The role of hollow states intermediately produced in electron-impact ionization of metastable He-like $N{}^{5+}(1s2s{\phantom{\rule{0.16em}{0ex}}}^{3}{S}_{1})$ ions has been investigated in detail. A crossed-beam setup and suitable experimental techniques were employed for the measurement of accurate absolute cross sections and precise energy-scan data. Fine structures arising from $K$-shell excitations and associated resonances have been observed for this two-electron ion with less than $\ifmmode\pm\else\textpm\fi{}$0.5 eV uncertainty on the energy scale. Fine details, such as interference of the reaction pathways of direct ionization and excitation with capture of the incident electron followed by double-Auger decay, could be revealed. Ab initio calculations based on the convergent close coupling (CCC) approach are in good agreement with the experiment.

Measurements of absolute M-subshell X-ray production cross sections of Th by electron impact

Measurements of absolute M-subshell X-ray production cross sections for element Th were made by electron impact for energies ranging from the ionization threshold up to 38keV. Experimental data were obtained by measuring the X-ray intensity emitted from ultrathin Th films deposited onto self-supporting C backing films. The measurements were conducted with an electron microprobe using high-resolution wavelength dispersive spectrometers. Recorded intensities were converted into absolute X-ray production cross sections by means of atomic data and estimation of the number of primary electrons, target thickness, and detector efficiency. Our experimental X-ray production cross sections, the first to be reported for the M subshells of Th, are compared with X-ray production cross sections calculated with the mean of ionization cross sections obtained from the distorted-wave Born approximation. The Mα X-ray production cross section calculated is in excellent agreement with the measurements, allowing future use for standardless quantification in electron probe microanalysis.

Elastic scattering cross section of photons by water: limits for the free atom approximation

Several papers can be found in the literature in which elastic and inelastic experimental scattering cross sections of photons by water with energies in the keV range are measured. Usually, the authors use some kind of normalization with theoretical values, because of the difficulties in obtaining some parameters necessary to obtain absolute cross sections from the measurements. A reasonable range of momentum transfer has been covered, but nevertheless, inconsistencies and gaps still remain.In this work, we address the problem performing absolute cross section measurements in the high momentum transfer region (0.33 Å−1 ≤ x ≤ 4.5 Å−1), where no structure effects can be observed. We compare the results with theory to validate it in an intermediary region (0.33 Å−1 ≤ x ≤ 1.72 Å−1) and then use the experimental values to normalize data in the low‐energy region (0.078 Å−1 ≤ x ≤ 1.72 Å−1) to obtain the experimental cross section in that region.From the comparison, we concluded that for momentum transfers, x between 0.7 and 4.5 Å−1 the scattering can be considered as being due to free atoms in free molecules. Therefore, the scattering cross sections may be considered as a sum of those from free atoms.Theoretically, the scattering cross sections can be obtained from the form factors as well as from the second‐order perturbation theory (S‐matrix) because there is no significant difference between both. On the other hand, for x ≤ 0.7 Å−1, interference between the photon‐scattered amplitudes due to liquid structure generates oscillations in the cross section values. Copyright © 2014 John Wiley & Sons, Ltd.

Electronic states of tetrahydrofurfuryl alcohol (THFA) as studied by VUV spectroscopy and ab initio calculations.

The electronic spectroscopy of isolated tetrahydrofurfuryl alcohol (THFA) in the gas phase has been investigated using high-resolution photoabsorption spectroscopy in the 5.0-10.8 eV energy-range, with absolute cross-section measurements derived. The He(I) photoelectron spectrum was also collected to quantify ionization energies in the 9-16 eV spectral region. These experiments are supported by the first high-level ab initio calculations performed on the excited states of the neutral molecule and on the ground and excited state of the positive ion. The good agreement between the theoretical results and the measurements allows us to quantify for the first time the electronic-state spectroscopy of THFA. The present work also considers the question of the lowest energy conformers of the molecule and its population distribution at room temperature.

The γ-ray spectrometer HORUS and its applications for nuclear astrophysics

A dedicated setup for the in-beam measurement of absolute cross-sections of astrophysically relevant charged-particle induced reactions is presented. These, usually very low, cross-sections at energies of astrophysical interest are important to improve the modeling of the nucleosynthesis processes of heavy nuclei. Particular emphasis is put on the production of the p nuclei during the astrophysical γ process. The recently developed setup utilizes the high-efficiency γ-ray spectrometer HORUS, which is located at the 10MV FN tandem ion accelerator of the Institute for Nuclear Physics in Cologne.The design of this setup will be presented and results of the recently measured 89Y(p,γ)90Zr reaction will be discussed. The excellent agreement with existing data shows that the HORUS spectrometer is a powerful tool to determine total and partial cross-sections using the in-beam method with high-purity germanium detectors.