Peak Cross Section Research Articles

Velocity dependence of rotationally inelastic collisions: 7Li*2(A 1Σ)+Ne, Ar, and Xe

We present velocity-selected measurements of the cross sections for rotationally inelastic collisions of 7Li*2(A 1Σ) with Xe, Ar, and Ne, with various pairs of initial and final rotational levels, ji and jf: Li*2(vi=9, ji) +X→Li*2(vf=9, jf=ji+Δ)+X. For each target gas measurements were made with ji=8, 22, and 42 for approximately eight values of Δ with an overall range of −20 to +24; a total of 69 cross sections were measured. Velocity selection is accomplished by a Doppler-based laser-selection technique combined with further numerical inversion methods. We achieve a measurable relative velocity range of ≂0.5→3×105 cm/s, corresponding to kinetic energies of 120 to 3500 K. In contrast to the few previous similar studies, our data show a dramatic and varied velocity dependence. We have observed a few cross sections which increase substantially with velocity, some which rise faster than v−1r at low velocity, some that fall like v−2r at high velocity, and numerous examples of peaks in endoergic cross sections near threshold. We have seen two distinct types of rotationally adiabatic reduction in certain cross sections: ‖Δ‖=2 in Li*2–Xe and ji=42, Δ≲−16, in all three systems, particularly in Li*2–Ne. The velocity dependent behavior of these cross sections is sensitive to the long-range and short-range interaction potential, respectively. Adiabatic effects in the latter case result in the existence of classical ‘‘dynamical threshold velocities’’ for these large −Δ collisions. Some analytical results of a quantitative investigation into these rotationally adiabatic effects are also given.

Search for a threshold enhancement in theγp→charmed baryon + charmed meson cross section

Results of a search for a predicted enhancement of several microbarns in the charm-photoproduction cross section just above threshold are reported. No charm decays were detected, from which an upper limit to the charm cross section of 94 nb (90% C.L.) at E/sub ..gamma../approx. =10 GeV was obtained. Upper limits in the range 270 to 450 nb were also obtained for the peak cross sections for threshold enhancements in ..gamma..p..-->..D/sup -/..sigma../sub c//sup + +/ and similar channels.

Electron-impact double ionization of rare-gas ions

Cross sections for the removal of two electrons from rare-gas ions by single collisions with incident electrons have been both measured using crossed beams of ions and electrons, and calculated using Hartree-Fock distorted-wave theory. For initial ions ${\mathrm{Ar}}^{4+}$, ${\mathrm{Kr}}^{4+}$, and ${\mathrm{Xe}}^{4+}$ the measured peak cross sections are 1.4 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}18}$, 6.5 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}18}$ ${\mathrm{cm}}^{2}$, respectively. These measurements confirm and extend the measurements of M\"uller and Frodl and of Achenbach et al. Calculations were performed for the charge-state-4 + ions and for ${\mathrm{Xe}}^{+}$, ${\mathrm{Xe}}^{2+}$, and ${\mathrm{Xe}}^{3+}$. Comparison of experiment and theory indicates that the double ionization of rare-gas ions is dominated by the indirect mechanism of inner-shell single ionization followed by autoionization. The distorted-wave calculations for the $4d$ ionization cross section of ${\mathrm{Xe}}^{q+} (q=1 \mathrm{to} 4)$ ions are strongly influenced by term dependence in the ejected-electron continuum, and by ground-state correlations.

K-shell one-electron capture in asymmetric collisions at intermediate and high energies

One-electron capture from the K shell of a target ion (atom) by impact of a bare ion at intermediate and high energies has been studied in the continuum distorted wave approximation. Double-scattering structures on differential cross sections d sigma /d Omega , predicted by Rivarola and Miraglia (1982) for large velocities, are analysed for non-relativistic high energies in near-symmetric and very asymmetric monoelectronic systems. Multielectron systems are also considered in order to determine the influence of passive electrons on these two-step processes. Theoretical results are in good agreement with the first recent experimental data on the Thomas's differential cross section peak from Horsdal-Pedersen et al. (1983) for the H+-He system. The authors compare the continuum distorted wave approximation for asymmetric systems at intermediate velocities with experimental differential cross sections and impact parameter probabilities from Horsdal-Pederson et al. (1983). The systems considered are: p+Ne at 1.5 MeV laboratory energy and p+C at 0.4 and 0.6 MeV laboratory energies. The agreement between theory and experiment is remarkable.

Exact second Born calculations for electron capture for systems with various projectile and target charges

Results of exact numerical calculations of differential and total $1s\ensuremath{-}1s$ electron-capture cross sections evaluated in the second Born approximation are presented for targets and projectiles of various charges ${Z}_{T}$ and ${Z}_{P}$ at velocities between 10 and 200 MeV/amu. For symmetric systems with ${Z}_{P}={Z}_{T}=Z$ the Thomas peak in the differential cross section, characteristic of a free-wave second Born-approximation process, appears at velocities above ${Z}^{2}$\ifmmode\times\else\texttimes\fi{}(5 MeV/amu), where $Z$ is the nuclear charge of the target (or projectile). The shape of this Thomas peak contains information about real and virtual intermediate states of the system. For total cross sections at velocities below ${Z}^{2}$\ifmmode\times\else\texttimes\fi{}(2 MeV) the second Born-approximation cross section is larger than the first Born-approximation cross section indicating a breakdown of the second Born approximation using the free-wave Green's function. Results using the peaking approximation of Drisko converge to our exact second Born-approximation results only at velocities well above ${Z}^{2}$\ifmmode\times\else\texttimes\fi{}(10 MeV/amu). For systems asymmetric in ${Z}_{P}$ and ${Z}^{T}$ no exact scaling is found, although the systematics are qualitatively similar to the symmetric case using $Z=\frac{1}{2}({Z}_{P}+{Z}_{T})$. For $p$ + Ne at 100 MeV, the exact Born-approximation results lie somewhat above exact impulse-approximation calculations. It is found that the peaking approximation of Briggs and Simony converges to exact second Born-approximation results as the asymmetry of the projectile and target charges increases. At very high velocities the peaking approximation of Drisko also converges slowly to the exact second Born-approximation result.

Dependence of the stimulated emission cross section of Yb<sup>3+</sup>on host glass composition

The stimulated emission cross section for the <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> F <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5/2</inf> → <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> F <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7/2</inf> transition of Yb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> has been determined from absorption and emission measurements of 41 different oxide, fluoride, and oxyfluoride glasses at 293 K. The effective peak cross sections for transitions to Stark levels above the ground state range from approximately 0.3 to 0.8 pm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The largest values occur in borate and phosphate glasses; the smallest values occur in silicate and low-refractive-index fluoride glasses. Radiative lifetimes calculated from integrated absorption spectra are also reported and range from 0.6 to 2.7 ms. Systematic variations in cross sections with changes in modifier ions can be used to tailor stimulated emission cross sections and fluorescence lifetimes.

Measurement of concentration and photoionization cross section of indium in silicon

Variable-temperature Hall-effect measurements, capacitance–voltage (C–V) measurements and flameless atomic absorption have been used to assess the concentration of indium in indium-doped silicon (Si:In) crystals. It was found that the flameless atomic absorption and C–V results were in very good agreement, but the extraction of an indium concentration from Hall-effect data was not a straightforward exercise due to the fact that the indium did not enter the ‘‘exhaustion’’ region. However, by analyzing the Hall data using a recent determination of the hole effective mass, and by using a physically reasonable value for the Hall scattering factor, the Hall data gave indium concentrations in very good agreement with the other two techniques. The Si:In substrates were also characterized optically using infrared absorption. These results, together with the indium concentrations determined above, yielded a peak indium photoionization cross section of (1.55±0.25)×10−16 cm2 at a 4-μm wavelength.

Schematizing spectrograms for speech recognition

An initial step in wide vocabulary, continuous speech recognition is proposed that roughly consists of a schematization of the features seen in conventional spectrograms—e.g., peaks and edges of spectral energy concentrations, temporal discontinuities, and spectral balance information. In a second step, these features can be mapped onto their acoustic‐phonetic correlates—e.g., formant distribution, voice onsets, articulatory closures. A method for locating spectral energy concentrations is given that takes advantage of their usual continuity in time, and thus performs superior to locating peaks in spectral cross sections. It begins with smoothing and flattening convolutions in both the time and frequency dimensions of narrow‐band spectrograms to select the appropriate temporal and spectral scales. Ridges in the resulting two‐dimensional (time‐frequency) surfaces correspond to local spectral energy concentrations. The tops of these ridges are found by the application of a two‐dimensional differential operator at each point in the time‐frequency plane. The operator's definition in terms of the relationship between the gradient and principal directions will be given, along with justification and examples.

Near-threshold excitation of He 23S by electron impact

A trapped-electron experiment is used to study the total inelastic cross section in helium from threshold to 3 eV above threshold. By normalising to the positive ionisation cross section of Rapp and Englander-Golden (1965) the authors find the peak in the 23S excitation cross section at 20.35 eV to be 6.2*10-18 cm2+or-10%. The authors result is substantially larger than previous measurements but is in agreement with recent theoretical work. They discuss several systematic errors to which the trapped-electron method is subject and derive reasonable bounds for the magnitude of the errors.

Vibrational effects in the photoionization shape resonance leading to theCΣg+2state of CO2+

The effect of vibrational averaging on the photoionization cross section and photoelectron asymmetry parameter for photoionization of C${\mathrm{O}}_{2}$ leading to the $C^{2}\ensuremath{\Sigma}_{g}^{+}$ state of C${\mathrm{O}}_{2}^{+}$ has been studied. We have computed the photoionization continuum wave function in the frozen-core Hartree-Fock approximation using the iterative Schwinger variational method. Averaging over the symmetric stretching mode of C${\mathrm{O}}_{2}$ reduced the peak resonant cross section by about 15%. This is in contrast to the earlier published results using the continuum multiple-scattering method where a much larger reduction in the peak cross section was obtained. A similar difference between these two theoretical methods was seen in their predicted asymmetry parameters. The present results have also been compared to available experimental data.

Spin Dependence in Neutron-Proton Charge Exchange at 790 MeV

The analyzing power A, and spin transfer parameters K/sub N/N, K/sub S/S, K/sub S/L, and K/sub L/L have been measured in the np charge-exchange (np..-->..pn) region at 790 MeV. These data provide new and unique information on the spin dependence of the np interaction in the charge-exchange region. Models which explain the charge-exchange peak in the np elastic differential cross section as being due to interference between one-pion exchange and a slowly varying background are in basic agreement with the data.

Absorption cross sections and line shape for the NO3(0–0) band

The absorption cross section of the nitrate free radical has been scanned by a tunable dye laser with 0.05 nm resolution (FWHM) between 613 and 672 nm, and portions of the region were scanned at high resolution (200 MHz) with the laser in single-frequency mode. These data, combined with other recent measurements, give a peak cross section at 661.9 nm of 1.90×10−17 cm2; and the integrated cross section over the 0–0 band from 14 910 to 15 290 cm−1 is 2.02×10−15 cm. Two recent studies that were said to disagree because of different peak cross sections are shown to agree in terms of integrated absorption cross section over the 0–0 band. It is confirmed that the spectrum is diffuse even under very high resolution. The 0–0 band is well approximated by two closely spaced Lorentzian functions.

Elastic photon scattering between 9.5 and 12 MeV inPb208andPb206

The elastic photon scattering cross sections of $^{208}\mathrm{Pb}$ and $^{206}\mathrm{Pb}$ were measured at 90\ifmmode^\circ\else\textdegree\fi{} and 135\ifmmode^\circ\else\textdegree\fi{} in the energy range from 9.5 to 12 MeV with a tagged photon beam whose energy spread was about 125 keV. The $^{206}\mathrm{Pb}$ cross section rises monotonically with energy, and is consistent with a total photon interaction cross section which has a Lorentzian energy dependence with a peak cross section of 650 mb at 13.6 MeV and a width $\ensuremath{\Gamma}\ensuremath{\simeq}3.8$ MeV. The $^{208}\mathrm{Pb}$ scattering cross section is larger and has some rapid variations with energy; there is a narrow extra peak near 10.04 MeV and there are abrupt increases in the cross section just below 10.6 and 11.3 MeV. The relative scattering observed at the two angles indicates that all of the scattering, including the rapid variations with energy, is dominated by dipole interactions. This dipole assignment for the fine structure is important for the proper interpretation of inelastic electron scattering by $^{208}\mathrm{Pb}$. Some of the observed fine structure in inelastic electron scattering must be dipole; the fine structure previously reported as being due to electric quadrupole excitation should be considered as tentative until the correct dipole contributions are included.NUCLEAR REACTIONS $^{206,208}\mathrm{Pb}$($\ensuremath{\gamma}\ensuremath{\gamma}$), $E=9.5\ensuremath{-}12$ MeV; measured $\ensuremath{\sigma}(E;\ensuremath{\theta})$; resolution 125 keV; observed fine structure; inferred dipole excitation.

Hyperthermal K–TeF6 molecular beam scattering

Angular distributions of K+ product ions from collisions of a beam of hyperthermal K atoms with a cross beam of thermal K–TeF6 molecules were determined at 13.7 and 23.7 eV (lab). The angular yields of K atom products from the same system were too low to permit measurement of angular distributions. From the integrated yields, the K+ ion/K atom branching ratio was determined to be greater than 103. In addition to the extremely large branching ratio, the differential cross sections exhibited several other unusual characteristics: (a) the lack of small angle scattering, corresponding to virtual absence of covalent scattering, (b) two peaks in the differential cross section with an outer rainbow feature at very large scattering angles (∼275 eV deg). The observations are unexpected from previous experimental and theoretical studies of electron transfer reactions and from the electronic and structural properties of TeF6 and TeF6−. A simplified dynamics model based on formation of electronically excited TeF6− in the initial electron transfer, followed by inner crossings leading to formation of electronically and vibrationally unexcited TeF6− or dissociation to TeF5− and other ionic products, has been developed which accounts for the experimental results. The model suggests that the observed two peaks in the differential cross section are due to the production of TeF6− (inner peak) or TeF5− and other ionic dissociation products (outer peak). The model also suggests that the observed branching ratio requires a vertical electron affinity of ⩽1.9 eV, much lower than its adiabatic electron affinity of 3.3 eV.

Exact second Born calculations of 1s-1s electron capture in p+H

Exact numerical calculations for 1s-1s electron capture by protons on hydrogen show that the second Born approximation is characterised by a peak in the differential cross section at theta cm=0.054 degrees when the projectile energy is above 5 MeV. Below 5 MeV this peak disappears. Between 3 and 1000 MeV the total non-relativistic cross section in the second-order Brinkman-Kramers (BK) approximation lies below the first-order BK total cross section.

Luminescence in stoichiometric and doped chromium borates

The Cr 3+ luminescence in RE(Cr xM 1-x) 3 (BO 3) 4 (RE=Gd, Lu; M=Al, Ga; O <x ≤ 1) is analyzed within a configurational coordinate model. The relative positions of the 4A 2−, 2E− and 4T 2-levels and the correlated observed line shapes yield a 18% anharmonic distortion of the 4T 2 potential. We present the relevant parameters for possible four level laser operation as effective peak cross sections, lifetimes and Stokes shift.

Atomic exchange potentials and angle resolved photoemission

The usefulness of angle-resolved photoemission from core states of adatoms for surface structure studies has been questioned because of the total mismatch between theory and experiment for normal emission from the 4d levels of Te on Ni(100) as a function of photon energy. We point out that the theory was based on the use of an Xα potential for Te with a α value appropriate for ground state properties. We show that such a potential is unsuitable for scattering problems and that much lower a values are required to give a reasonable description of sharp peaks in the atomic photoionization cross section as a function of photon energy. We show that Xα calculations of the phase shifts give much improved correspondence with full Hartree-Fock calculations when α is reduced below its usual range of 1 to 23, and that a corresponding improvement in the matching of experimental and calculated photoelectron azimuthal distributions is found. A realistic exchange potential must be energy dependent, decreasing to zero at high enough energies. We show that the simplest reasonable approximation to the local density exchange potential, i.e. the Hartree-Fock approximation for the uniform electron gas, resolves the problem of the normal emission data from Te 4d on Ni(100).

Deuteron breakup with 3He at 89.4 and 118.9 MeV

Quasifree processes were studied by measuring coincident energy spectra for the 2H( 3He, 3Hep)n, 2H( 3He, tp)p and 2H( 3He, dd)p reactions at 89.4 and 118.9MeV. The spectral shape was reproduced with two types of spectator model, that is, a three-body model (TBM) and a four-body model (FBM). The latter contains the effects of projectile breakup processes. Angular correlations of peak cross sections were explained by calculations in the FBM in contrast with those in the TBM. The absolute cross sections were not explained by either model.

ChemInform Abstract: THE TERBIUM CHLORIDE‐ALUMINUM CHLORIDE VAPOR SYSTEM. II. FLUORESCENCE

AbstractDie Fluoreszenzspektren und die ‐abklingkinetik des bei erhöhten Temp. zwischen TbCl3 und AlCl3 gebildeten flüchtigen Komplexes (TbC13(AlCl3)x(g)) werden zwischen 350 und 700 nm gemessen.

The terbium chloride–aluminum chloride vapor system. II. Fluorescence

Measurements of the fluorescence spectra and fluorescence decay kinetics of the vapor complex formed at elevated temperatures between TbCl3 and AlCl3 have been made in the 350 to 700 nm wavelength range. The principal lines in the spectra were identified as emission from the 5D4, 5D3, and the 5G6 energy levels of the Tb3+ ion. The radiative lifetime of the 5D4 level at low temperatures was found to be 3.88 ms, and was used to determine the oscillator strengths of the 5D4→7FJ (J=0 through 6) transitions after their branching ratios were measured. The peak cross section for stimulated emission on the strong 5D4→7F5 transition was found to be 2.87×10−21 cm2 at λ=543 nm. A population inversion of 1018 cm−3 could produce a small signal gain of 30% per meter on this transition, indicating that laser emission may be possible at temperatures where sufficient vapor density can be obtained.