Peak Cross Section Research Articles

Absorption cross section measurements of carbon dioxide in the wavelength region 118.7–175.5 nm and the temperature dependence

Laboratory measurements of the relative absorption cross sections of CO 2 at the temperatures 195 and 295 K have been made throughout the wavelength region 118.7–175.5 nm. Laboratory measurements of the absolute absorption cross sections of CO 2 at the temperature 195 and 295 K have been made at 12 different wavelengths in the region 118.7–175.5 nm. These absolute values have been used to put the relative cross section measurements of CO 2 at 195 and 295 K on an absolute basis throughout the region 118.7–175.5 nm. Almost no temperature dependence was observed in the peak cross sections of the strong bands at 130 nm region, but large temperature effects are seen in the other regions.

Velocity Dependence of Quasi-Resonant Vibrotational Transfer in Li2*−Rare Gas Collisions

We report measurements of velocity-dependent cross sections for vibrotationally inelastic scattering in the system Li2 + X → Li2 + X, with vi = 9, ji = 42, and X = Xe, Ar, and Ne. The measurements range over a factor of 30 in energy. Quantum levels were chosen to elucidate the quasi-resonant vibrotational transfer process studied previously. A reduction in collision velocity results in both an increase in total vibrationally inelastic cross section and an enhancement of the quasi-resonant effect, with final rotational state distributions as narrow as 2.5ℏ (fwhm) observed. The largest cross section for Δv < 0 is given by the formula Δjpeak = −4Δv and possesses a roughly 1/vrel velocity dependence. For Δv = +1, energy thresholds shift the peak cross section at low velocity to the nearly energy resonant value of Δjpeak = −6. The similarity of the final state distributions for different target gases observed in previously measured rate constants does not hold in the velocity-dependent data; this results in part from the appearance of low-velocity dynamical thresholds for exoergic cross sections in the Ar and Ne systems. We compare the experimental results for X = Ne with cross sections calculated from quasi-classical trajectories on an ab initio potential energy surface; for jf ≥ 46, agreement is quantitative, while for jf < 46, the calculation overestimates the cross sections.

Formation of antihydrogen by the charge-transfer reaction.

The cross sections for antihydrogen formation in the n=1, 2, 3, 4, 5, 6, and 7 levels from antiproton-positronium collisions are computed in the unitarized Born approximation (UBA). Twenty-seven physical states of antihydrogen (1s\ensuremath{\rightarrow}7h) and ten physical states (1s\ensuremath{\rightarrow}4f) of positronium are included in the UBA basis. The peak cross section for antihydrogen formation from excited positronium targets is much larger than that from a ground-state positronium target at low incident energies. The high-n antihydrogen levels make a significant contribution to the total antihydrogen formation cross section, especially for incident positronium atoms in the Ps(n=3) and Ps(n=4) levels.

Near-threshold photoionization from the Sr I (5s5p) 1Po1 state

Absolute photoionization cross sections from the laser-excited Sr I (5s5p) 1Po1 state are reported showing a giant resonance with a peak cross section of 5600 Mb. The absolute calibration was obtained with a saturation method as published by Burkhardt et al. In the present study, however, the spatial distribution of the photons in the laser pulse was measured with a CCD camera and applied in the evaluation procedure as a Gauss profile. The uncertainties in the absolute values are estimated to be less than 20%. Based on a 12-state non-relativistic close-coupling R-matrix calculation, the giant resonance is assigned as predominantly (5p2) 1D2 with a strong admixture of (4d2) 1D2. While an ab initio calculation including only discrete physical target states was unable to reproduce the resonance structure correctly, reasonable agreement between experiment and theory was obtained by a semi-empirical simulation of additional short-range correlation and relaxation effects.

Dissociative attachment from monochlorinated hydrocarbons in the gas phase

Gas phase measurements of the dissociative attachment (DA) process provide fundamental data useful for the interpretation of studies incorporating molecules on surfaces and in films. In this report, we review recent DA studies in a series of monochlorohydrocarbons to examine the role of the hydrocarbon framework, or “substrate”, to which the Cl is attached. We find that the peak cross section decreases roughly exponentially as the vertical attachment energy to form the temporary negative ion increases. We consider also the problem of measuring the small cross section of methyl chloride and show that observation of the temperature dependence of the process provides information about the cross section. We discuss briefly the recent theoretical studies of this process in methyl chloride. Finally, we examine the effect of placing a region of unsaturation, i.e., a double bond, in a location remote from the CCl bond and show that this enhances the DA cross section, providing evidence for intramolecular transfer of the electron.

The oscillator strength in atomic absorption spectroscopy

The role of the oscillator strength, f, in the theory of atomic absorption is investigated. For a pure natural broadened absorption line, the peak absorption coefficient α o is independent of the oscillator strength. The peak absorption coefficient becomes dependent on f only through additional broadening processes such as Doppler or collisional broadening. The peak cross section for resonance absorption, α 0/ N 1, for a closed transition with equal statistical weights is given by σ 0 = 2πXXX 2 = ( 2 π )/[c n(ω 0)] (where XXX = λ 2π and n(ω 0) is the spectral mode density of the radiation field at the resonance frequency ω 0) and physically represents the cross-sectional area per allowed mode of the radiation field per unit time per unit frequency interval, multiplied by a lineshape factor 2/π. A summary is presented of some recent determinations of oscillator strengths of atomic absorption lines, based on lifetime measurements made in this laboratory. The results are used to revise values of the theoretical characteristic mass for Ag, Al, Au, Ca, Cu, Mo, Na, Ti and V used in absolute analysis by graphite furnace atomic absorption spectroscopy.

Measurements of the lineshape of the Z 0 and determination of electroweak parameters from its hadronic and leptonic decays

During the LEP running periods in 1990 and 1991 DELPHI has accumulated approximately 450 000 Z 0 decays into hadrons and charged leptons. The increased event statistics coupled with improved analysis techniques and improved knowledge of the LEP beam energies permit significantly better measurements of the mass and width of the Z 0 resonance. Model independent fits to the cross sections and leptonic forward- backward asymmetries yield the following Z 0 parameters: the mass and total width M Z = 91.187 ± 0.009 GeV, Γ Z = 2.486 ± 0.012 GeV, the hadronicf and leptonic partials widths Γ had = 1.725 ± 0.012 GeV, Γ ℓ = 83.01 ± 0.52 MeV, the invisible width Γ inv = 512 ± 10 MeV, the ratio of hadronic to leptonic partial widths R ℓ = 20.78 ± 0.15, and the Born level hadronic peak cross section σ 0 = 40.90 ± 0.28 nb. Using these results and the value of α s determined from DELPHI data, the number of light neutrino species is determined to be 3.08 ± 0.05. The individual leptonic widths are found to be: Γ e = 82.93 ± 0.70 MeV, Γ μ = 83.20 ± 1.11 MeV and Γ τ = 82.89 ± 1.31 MeV. Using the measured leptonic forward-backward asymmetries and assuming lepton universality, the squared vector and axial-vector couplings of the Z 0 to charged leptons are found to be g V ℓ 2 = (1.47 ± 0.51) × 10 −3 and g A ℓ 2 = 0.2483 ± 0.0016. A full Standard Model fit to the data yields a value of the top mass m t = 115 −82 +52 (expt.) −24 +52(Higgs) GeV, corresponding to a value of the weak mixing angle sin 2 θ eff lept = 0.2339±0.0015 (expt.) −0.0004 +0.0001 (Higgs). Values are obtained for the variables S and T, or ϵ 1 and ϵ 3 which parameterize electroweak loop effects.

Deeply bound pionic atoms with resonant Compton scattering

We study the contribution to ordinary Compton nuclear scattering of the resonant channel γ + A → (A′ π −) → γ + A with the π − bound in the nucleus. We show that the interference of this resonant channel with background amplitudes produces significant peaks in the elastic backward differential cross section as a function of the incoming photon energy for light and medium-size nuclei. We also find that for light nuclei the signals in forward Compton scattering or in the total photonuclear cross section are big enough to be observed, provided there is a good resolution in the photon beam. The reaction is particularly suited to find deeply bound pionic states in medium-size nuclei, or ordinary levels in unstable light nuclei.

Absolute cross sections for the dissociative electron impact ionization of the CF x ( x = 1−3) free radicals

We studied the dissociative ionization of the CF x ( x = 1−3) free radicals by electron impact and measured absolute partial cross sections and appearance energies for the formation of the various fragment ions. Fast beams (3–3.5 kV) of CF 3, CF 2 and CF were prepared by near-resonant charge transfer of CF + 3, CF + 2 and CF + with Xe under conditions where the neutral beams contained primarily ground state ions with little vibrational excitation (0.5 eV or less). Absolute cross sections for the various molecular fragment ions were measured from threshold to 200 eV with peak cross sections of up to 1.3 Å 2. The cross sections for the formation of the atomic C + and F + ions were found to be small (0.3 Å 2 or less) except for the F + cross section from CF 2. The measured appearance energies indicate that the fragment ions are formed with excess kinetic energies ranging from essentially zero to about 3 eV per fragment. The CF + cross section from CF 2 shows a second onset which corresponds to the double positive ion formation CF 2 → CF + + F +. Extensive ion trajectory calculations were carried out in order to quantify the collection efficiency of our apparatus for fragment ions formed with excess kinetic energy.

Extreme ultraviolet emission spectrum of CO 2 induced by electron impact at 200 eV

We present the extreme ultraviolet (EUV) emission spectrum of CO 2 induced by electron impact at 200 eV. There are 36 spectral features which are identified with a resolution of 0.5 nm over the wavelength range of 40 to 125 nm. Absolute emission cross sections were obtained for each of these features. The EUV emission spectrum induced by electron impact consists of atomic multiplets of C I, II and O I, II, III as well as CO and CO + molecular band systems produced by dissociative excitation. The C I (119.4 nm) multiplet is the strongest feature of C I with a peak cross section of 3.61 × 10 −19 cm 2 at 200 eV. The strongest feature of O I in the EUV spectrum is the O I (99.0 nm) multiplet with a peak cross section of 3.59 × 10 −19 cm 2 at 200 eV.

Observation of quasielastic light scattering in the one-dimensional heisenberg antiferromagnet KCuF 3

Using a conventional Raman spectrometer, we have observed inelastic light scattering having a peak in the differential cross section at zero frequency in the one-dimensional Heisenberg antiferromagnet KCuF 3 over an extremely wide range of temperatures above T N = 39 K. The scattered light is completely polarized and appears only for the ZZ configuration of the polarization of the incident and scattered light where Z is parallel to the c axis. The line profiles Lorentzian. Since the magnetic short-range order of this compound remains even at room temperature, the observed quasielastic scattering line is very likely due to fluctuations in the magnetic-energy density.

Temperature dependent kinetics of the formation and self-reactions of FC(O)O2 and FC(O)O radicals

This paper presents the ultraviolet (UV) absorption spectrum of FC(O)O2 and the temperature dependent rate constants for its formation via the addition of O2 to FCO, its self-reaction to form FC(O)O, and for the subsequent dimerization of FC(O)O. The UV spectrum of FC(O)O2 shows two absorption bands in the 190–300 nm range. The lower energy band has a peak cross section of (3.0±0.3)×10−18 cm2 at 232 nm. A higher energy band begins at ∼210 nm and reaches an absorption cross section of 4.3×10−18 cm2 at 190 nm. The FC(O)O2 self-reaction exhibits a negative temperature dependence with k2(T)=(2.5±0.4)×10−12 e(286±40)/T cm3 s−1 over the 213–358 K temperature range. The oxygen addition to FCO has a rate constant of k1=(8.0±0.8)×10−13 cm3 s−1 and the FC(O)O dimerization rate constant is in the range k3a=(2.3–6.5)×10−12 cm3 s−1, at 293 K and 300 Torr total pressure. Both of these rate constants show little variation over the 213–358 K temperature range.

Photoinduced charge-transfer dissociation in van der Waals complexes. II. Na2...ClCH3, Na2...(ClCH3)2, and Na...FPh

van der Waals complexes Nam...(XR)n (R=CH3 and Ph; X=Cl and F) have been generated by crossing a beam of Na with a supersonic jet of halide molecules XR seeded with Ar. The complexes were identified and measured by photoionization time-of-flight mass spectrometry. Tunable visible radiation (530–680 nm) was used to measure the photodissociation spectrum (an ‘‘action spectrum’’) of these clusters. The peak cross section for photodissociation of Na2...(ClCH3)n (n=1 and 2) was found to be large (∼6 Å2) with its maximum at 635 nm for n=1 and at 615 nm for n=2. These peak wavelengths suggest that the primary route for photodissociation is excitation of the Nam followed by charge-transfer dissociation: Na2...(ClCH3)1,2 + hυ → [Na*2...(ClCH3)1,2]‡ → [Na+2...(ClCH3)1,2−]‡→products (the double dagger indicates a transition state). By contrast, the photodissociation of Na2+...(ClCH3) appeared to follow a direct mechanism involving the dissociation of the [Na–Na]+. For Na...FPh, two excited states of the complex (Π and Σ) were thought to be responsible for the two broad peaks in the action spectrum, each with vibrational structure.

Electron excitation of the H2(a 3Sigma(g)(+) - B 3Sigma(u)(+)) continuum in the vacuum ultraviolet

The electron excitation function for the H2(a 3Sigma(g)(+) - b 3Sigma(u)(+)) continuum emission has been measured from 11 to 30 eV at 195.0 nm. The emission cross, section data have been combined with electron energy loss cross section measurements to extend the experimental data to 60 eV. The excitation cross section has been estimated by normalizing the data to the electron energy loss cross section at 20 eV and using a model to account for emission from the entire band system. The spin forbidden excitation of the a 3Sigma(g)(+) state is a major dissociative channel of H2 at low energy with a peak cross section of 1.7 +/- 0.85 x 10 exp -17 sq cm at 15.5 eV. The FWHM of the cross section is 7 eV. The high energy dependence of the cross section has a rapid 1/E-cubed fall off with electron energy, E, above 50 eV. A modified Born approximation analysis was applied to the data to provide an analytic model.

EWBA calculations for the relativistic (e,2e) experiments

Eikonal wave Born calculations for the relativistic (e,2e) experiments carried out recently in Tubingen are presented. It is shown that even for large projectile energies the triply differential cross section (TDCS) is very sensitive to the value of atomic field. The idea is put forward that the cross section peak angle shift in the case of symmetric (binary) (e,2e) experiments can be used for direct determination of a mean atomic field value.

Photoenhanced dissociative electron attachment to SO 2

Optically enhanced dissociative electron attachment to electronically excited SO 2 molecules was observed using crossed laser, electron and molecular beams. Irradiation of SO 2 molecules with the 308 nm XeCl excimer laser line showed a sharp O − resonance at near zero electron energy in addition to the O − resonances from ground state SO 2 at 4.55 and 7.3 eV. The additional O − resonance is attributed to the reaction hv (308 nm) + SO * 2 → SO * 2( 1B 1 or 1A 2) → +e−(< 0.5 eV)SO −* 2 →O − + SO. The peak cross section value of this process is at least two orders of magnitude larger than dissociative attachment from ground state SO 2.

Absolute differential and integral electron excitation cross sections for atomic oxygen: 9. Improved cross section for the ³P → ¹D transition from 4.0 to 30 eV

The O I (³P → ¹D) transition cross section has been remeasured for a new set of incident energies. In addition, a new O2 (a ¹Δg ← X ³Σg−) reference cross section set has been used to normalize the relative measurements. The previous results of Doering and Gulcicek (1989) have been revised to reflect the improved reference cross section set. The new results combined with the revised Doering and Gulcicek (1989) values show that theory and experiment are in excellent agreement at incident energies greater than 9 eV. However, at lower incident energies, the experimental cross section rises to a sharp peak near 6 eV. The peak in the experimental cross section is 5.4 × 10−17 cm² (±35%) at 6 eV, approximately a factor of 2 larger than the theoretical cross section. The shape of this cross section vs. incident energy has many implications for aeronomy.

Electron-positron pair production in Coulomb collisions of ultrarelativistic sulfur ions with fixed targets.

Angular and momentum distributions have been measured for positrons from electron-positron pairs created in peripheral collisions of 6.4-TeV sulfur ions with fixed targets of Al, Pd, and Au. The data are compared with results of several theoretical treatments. Measured differential cross sections peak at low momentum (\ensuremath{\le}1 MeV/c), extend significantly to much higher momenta (&gt;17 MeV/c), and concentrate sharply in the forward direction, along the ion-beam axis. Positron yields scale as the square of the target nuclear charge as predicted by theory.

Elastic photon scattering from 4He: Charge symmetry problem.

Elastic photon scattering cross sections have been measured on {sup 4}He in the region of the giant dipole resonance (GDR) using the tagged-photon technique. These data place stringent constraints on the total photoabsorption {sigma}{sub {gamma}} at the peak of the GDR, which to very good approximation is the sum of the ({gamma},{ital p}) and ({gamma},{ital n}) cross sections. We find the peak cross section to be {sigma}{sub 0}=2.86{plus minus}0.12 mb. This result is inconsistent with the sum of the most recent measurements of the ({gamma},{ital p}) and ({gamma},{ital n}) cross sections. Thus any conclusion about charge symmetry based on the ratios of these cross sections is suspect.

The complete UV spectrum of SO2 by electron impact, 2, The middle ultraviolet spectrum

We have measured the middle ultraviolet (MUV) electron impact induced fluorescence spectrum of SO2 from 200 to 430 nm in a crossed beam experiment. The spectrum is dominated by two features at the experimental resolution of 0.5 nm. These two features are referred to as MUV1 and MUV2. MUV1 is the SO(A3∏ → X3 ∑‐) band system extending from 240 to 265 nm produced by dissociative excitation. MUV2 is a blend of the molecular band systems in the range 264‐430 nm. In addition, the excitation function measurements of MUV2 indicate that low‐energy electrons effectively prepare SO2 in one or more electronically excited triplet states that involve the SO2 (ā(3B1)) state by direct excitation and/or cascading. A candidate triplet band system for this emission process is the electronic transition. This emission process is the largest contributor to the MUV2 emission cross section at low electron impact energy. The peak cross section for MUV1 occurs at 20 eV with a value of 25.0 ± 5.5 × 10‐19 cm2. The peak cross section for MUV2 arises at 9 eV with a value of 368 ± 81 × 10‐19 cm2. The laboratory measurements of the excitation functions of both MUV features were made from 0 to 1 keV. The emission cross sections are an important part of the total inelastic cross section of SO2 needed in modeling the Io atmosphere. The laboratory results give a plausible explanation of the Io auroral hot spots observed by Voyager 2.