Experimental Cross-section Ratios Research Articles

Molecular-frame (e,\xa02e) ionization dynamics of H2 at high impact-energy

We report a combined experimental and theoretical study on the electron-impact ionization dynamics of H2 at an impact-energy of 520 eV. The molecular-frame fivefold-differential cross sections were measured for electron emission in the plane perpendicular to the incoming projectile beam. An (e, 2e + ion) triple coincidence method was used covering projectile scattering angles of 6.5°, 10.0° and 20.0° and ejected energies of 20 eV and 30 eV. The experimental cross sections are compared with results from the multi-center distorted-wave (MCDW) as well as the molecular three-body distorted wave (M3DW) approaches. M3DW is in overall better agreement with the measured data in the binary lobes than MCDW, while the intensity of recoil lobes are underestimated by both theories. Furthermore, we examine the presence of two-center interference patterns by comparing the experimental cross section ratios between mutually perpendicular alignment angles of the molecular axis with that predicated by the interference model. Agreement with the interference model is found only for Bethe ridge kinematics, i.e. in the binary peak region and with the ejected electron momentum being roughly equal to the momentum transfer. Finally, we suggest a modified interference formula for the recoil peak which takes into account the backscattering of the ejected electron in the ionic potential.Graphical abstract

Independent Calibration of Water Vapor Raman Lidar by using Additional Elastic Measurements at Water Vapor Raman Wavelength

A new method for independent calibration of water vapor Raman lidar by using additional measurements of elastic signals at water vapor Raman wavelength was introduced. The water vapor mixing ratio calibration constant was derived from nitrogen mixing ratio calibration constant and Rayleigh to water vapor Raman backscattering cross section ratio. The nitrogen mixing ratio calibration constant with uncertainties of 1~2% can be determined from the nitrogen mixing ratio measurement in an aerosolfree zone in the upper troposphere with the additional elastic measurement and nitrogen Raman measurement, the Rayleigh to water vapor Raman backscattering cross section ratio can be calculated with uncertainties of about 8%, or can be determined experimentally with uncertainties of less than 5%. Using the experimental cross section ratio, a total uncertainty of the water vapor calibration less than 6% is possible to achieve. Because this calibration method utilizes actual lidar signals in nitrogen and water vapor detection channels, the calibration can fully capture lidar state changes.

Unimolecular Dissociation of H<sup>+</sup><sub style="margin-left:-6px;">2n+1</sub> Hydrogen Clusters: Measured Cross Sections and Theoretically Calculated Rate Constants

In this paper, we studied the process of dissociation unimolecular of the evaporation of H+2n+1 hydrogen clusters according to size, using the Rice-Ramsperger-Kassel-Marcus (RRKM) theory. The rate constants k(E) were determined with the use of statistical theory of unimolecular reactions using various approximations. In our work, we used the products frequencies instead of transitions frequencies in the calculation of unimolecular dissociation rates obtained by three models RRKM. The agreement between the experimental cross section ratio and calculated rate ratio with direct count approximation seems to be reasonable.

High-resolution electron-momentum spectroscopy of the valence orbitals of the iodine molecule

The valence orbitals of the iodine molecule (I${}_{2}$) have been studied using the high-resolution electron-momentum spectroscopy at impact energies of 1200 and 600 eV. Experimental momentum distributions of outer valence orbitals were compared with the nonrelativistic, scalar relativistic, and spin-orbital relativistic calculations, as well as the relativistic pseudopotential calculation. The experimental cross-section ratios of $8{J}_{g}^{3/2}$ to $17{J}_{g}^{1/2}$, $8{J}_{u}^{3/2}$ to $16{J}_{u}^{1/2}$, and the electron-momentum profiles of orbital $16{J}_{g}^{1/2}$ clearly manifested the relativistic effects. In the inner valence region, the symmetry-adapted-cluster configuration-interaction theory was used to interpret the ionization spectrum and the electron-momentum distributions.

Nonperturbative Treatment of Ionization with Excitation of Helium by Electron Impact

We present cross sections for electron-impact ionization and simultaneous ionization plus excitation of helium by electron impact. The results are obtained from a fully nonperturbative close-coupling formalism using our B-spline R-matrix approach. A large number of pseudostates in the expansion of the wave function represent the coupling to the ionization continuum. We obtain excellent agreement with the directly measured experimental cross section ratios (Bellm et al., Phys. Rev. A 75, 042704 (2007)) for ionization leaving the residual He⁺ ion in either the 1s ground state or the n = 2 (2s + 2p) excited states.

Slow isocharged sequence ions with helium collisions: Projectile core dependence

The collisions of the isocharged sequence ions of $q=6$ (${\text{C}}^{6+}$, ${\text{N}}^{6+}$, ${\text{O}}^{6+}$, ${\text{F}}^{6+}$, ${\text{Ne}}^{6+}$, ${\text{Ar}}^{6+}$, and ${\text{Ca}}^{6+}$), $q=7$ (${\text{F}}^{7+}$, ${\text{Ne}}^{7+}$, ${\text{S}}^{7+}$, ${\text{Ar}}^{7+}$, and ${\text{Ca}}^{7+}$), $q=8$ (${\text{F}}^{8+}$, ${\text{Ne}}^{8+}$, ${\text{Ar}}^{8+}$, and ${\text{Ca}}^{8+}$), $q=9$ (${\text{F}}^{9+}$, ${\text{Ne}}^{9+}$, ${\text{Si}}^{9+}$, ${\text{S}}^{9+}$, ${\text{Ar}}^{9+}$, and ${\text{Ca}}^{9+}$) and $q=11$ (${\text{Si}}^{11+}$, ${\text{Ar}}^{11+}$, and ${\text{Ca}}^{11+}$) with helium at the same velocities were investigated. The cross-section ratios of the double-electron transfer (DET) to the single-electron capture (SEC) ${\ensuremath{\sigma}}^{\text{DET}}∕{\ensuremath{\sigma}}^{\text{SEC}}$ and the true double-electron capture (TDC) to the double-electron transfer ${\ensuremath{\sigma}}^{\text{TDC}}∕{\ensuremath{\sigma}}^{\text{DET}}$ were measured. It shows that for different ions in an isocharged sequence, the experimental cross-section ratio ${\ensuremath{\sigma}}^{\text{DET}}∕{\ensuremath{\sigma}}^{\text{SEC}}$ varies by a factor of 3. The results confirm that the projectile core is another dominant factor besides the charge state and the collision velocity in slow ($0.35\ensuremath{-}0.49{v}_{0}$; ${v}_{0}$ denotes the Bohr velocity) highly charged ions (HCIs) with helium collisions. The experimental cross-section ratio ${\ensuremath{\sigma}}^{\text{DET}}∕{\ensuremath{\sigma}}^{\text{SEC}}$ is compared with the extended classical over-barrier model (ECBM) [A. B\'ar\'any et al., Nucl. Instrum. Methods Phys. Res. B 9, 397 (1985)], the molecular Coulombic barrier model (MCBM) [A. Niehaus, J. Phys. B 19, 2925 (1986)], and the semiempirical scaling laws (SSL) [N. Selberg et al., Phys. Rev. A 54, 4127 (1996)]. It also shows that the projectile core properties affect the initial capture probabilities as well as the subsequent relaxation of the projectiles. The experimental cross-section ratio ${\ensuremath{\sigma}}^{\text{TDC}}∕{\ensuremath{\sigma}}^{\text{DET}}$ for those lower isocharged sequences is dramatically affected by the projectile core structure, while for those sufficiently highly isocharged sequences, the autoionization always dominates, hence the cross-section ratio ${\ensuremath{\sigma}}^{\text{TDC}}∕{\ensuremath{\sigma}}^{\text{DET}}$ is always small.

Experimental study on ratios of coherent scattering to Compton scattering for elements with atomic numbers 26⩽ Z⩽82 in 59.5 keV for 55° and 115°

Differential cross-sections for coherent and Compton scattering of 59.5 keV γ-rays at the scattering 55° and 115° by Fe, Ni, Cu, Zn, Zr, Nb, Mo, Ag, Sn, Ta, Au and Pb were measured using a Ge(Li) detector. To measure theoretical coherent and Compton scattering differential cross-sections are extracted by interpolating atomic form factors and incoherent scattering functions corresponding to 2.216 and 4.048 Å −1 photon momentum transfer. The experimental cross-section ratios are compared with predictions of nonrelativistic and relativistic form factors theories.

Ionization widths for Ne(3l)-Ar systems(l=s,p): Application to ionization and intramultiplet mixing cross sections

{ital Ab initio} autoionization widths for the Ne(3s)-Ar and Ne(3p)-Ar systems are calculated in a two-electron approximation. Using correctly normalized wave functions, the calculations result in absolute values for the autoionization widths. We use a modified coupled-channel code with {ital ab initio} potentials and ionization widths as input to calculate cross sections for ionization and intramultiplet mixing. The influence of ionization on the intramultiplet mixing process can thus be studied. Experimental results for intramultiplet mixing cross sections for the Ne(3p)-Ar system at a collision energy {l_angle}E{r_angle}=1.7 eV are presented. These data and ionization cross sections obtained by Bussert {ital et al.} [Z. Phys. A {bold 320}, 105 (1985)] are in good agreement with coupled-channel calculations. For the Ne(3s)-Ar system the calculated ionization widths are in good agreement with empirical ionization widths derived in a multiproperty analysis of a wide range of scattering data. For this system we have to conclude that the {ital ab initio} results cannot explain the experimental cross section ratio Q({sup 3}P{sub 0})/Q({sup 3}P{sub 2})=1.3. Therefore, {ital ab initio} calculations of the real part of the potential are essential for a correct quantum-mechanical coupled-channel analysis. {copyright} {ital 1997} {ital The American Physical Society}

Theoretical Study on the Stability of Low-Spin Hydridomethyl Complexes of the First-Row Transition Metal Cations

Theoretical calculations have been performed in order to study the stability of the low-spin hydridomethyl complexes HMCH3+ for the first-row transition metals (M+ = Sc+−Cu+). Originally experimental results have been rationalized by assuming a low-spin hydridomethyl complex as a stable intermediate in the reactions of methane with singly charged metal cations. Recently, theoretical studies showed that for some late transition metals of the first row (Fe+ and Co+) no stable low-spin insertion product could be located on the potential energy surface. For the early elements of this row (Sc+−V+) the experimental cross section ratios σ(MH+)/σ(MCH3+) indicate that the elimination reactions for these cations proceed via a statistically behaved intermediate. Our CASPT2 calculations indeed confirm a stable hydridomethyl complex for these cations. The reason for the stability of the insertion complexes could be traced back to the relative position of the lowest lying low-spin s0dn state and the lowest lying low-spin s1dn-1 state in the electronic spectrum of the corresponding free transition metal cations. Further, an analysis of the wave function clearly reveals a correlation between the extent of the participation of the 4s orbital in the metal−ligand bonds and the experimentally observed dominance of the H2 elimination over the other elimination reactions for the cations Sc+ to Cr+. An explanation in terms of the frontier orbital approach is given.

Radiative capture of deuterons in Ba and Au

138Ba(d,γ)140La and197Au(d,γ)199mHg cross sections were measured for deuteron energies ranging from 5 to 26 MeV. A survey of existing experimental (p,γ), (d,γ) and (α,γ) cross sections is given. A comparison of all the experimental data with calculated compound nucleus cross sections is performed. An increase in the ratio of experimental cross sections and calculated compound nuclear cross sections with increasing mass numberA is observed for all capture processes.

The dσ(p, p'd)/dσ(p, nd) ratio for7Li at Tp=670 MeV calculated in a simple model

The experimental cross section ratio of 7Li (p, p'd) and 7Li (p, nd) large-angle quasifree scattering at Tp=670 MeV is compared with theoretical estimates based on the cluster model of 7Li.

Isospin and the Bohr Independence Hypothesis

The influence of isospin on statistical nuclear decay at approximately 20 MeV is investigated for two composite systems $^{66}\mathrm{Zn}$ and $^{64}\mathrm{Zn}$ by measurement of the cross sections and the energy spectra of protons and \ensuremath{\alpha} particles emitted from the two composite systems. The composite system $^{66}\mathrm{Zn}$ was formed at an excitation energy of 19.7 MeV in the $^{65}\mathrm{Cu}+p$ and $^{62}\mathrm{Ni}+\ensuremath{\alpha}$ reactions, while the composite system $^{64}\mathrm{Zn}$ was formed at an excitation energy of 19.5 MeV in the $^{63}\mathrm{Cu}+p$ and $^{60}\mathrm{Ni}+\ensuremath{\alpha}$ reactions. In both systems the experimental cross-section ratio $\frac{\ensuremath{\sigma}(\ensuremath{\alpha},{\ensuremath{\alpha}}^{\ensuremath{'}})\ensuremath{\sigma}(p,{p}^{\ensuremath{'}})}{\ensuremath{\sigma}(\ensuremath{\alpha},p)\ensuremath{\sigma}(p,\ensuremath{\alpha})}$ exceeded the value of approximately unity predicted by the Bohr independence hypothesis and the statistical theory of compound-nuclear decay including angular momentum. The cross-section ratios were 2.2 \ifmmode\pm\else\textpm\fi{} 0.2 and 1.3 \ifmmode\pm\else\textpm\fi{} 0.1 for the $^{66}\mathrm{Zn}$ and $^{64}\mathrm{Zn}$ composite systems, respectively. The major part of the deviation of these ratios from unity in both the composite systems is ascribed to an isospin selection rule acting in the ${T}_{g}$ states of both composite systems which causes these states to decay preferentially by proton emission. Other factors which might enhance these ratios, such as direct reactions and reduced moments of inertia, are considered and are found to change the above ratios by a relatively small amount.

Electron-Impact Ionization Cross Sections. IV. Group IVb Atoms

The electron-impact ionization cross sections for Ag, Ge, Sn, and Pb have been measured relative to one another by quantitatively vaporizing known amounts of material in a mass spectrometer. For 60-eV impacting electrons, the relative single ionization cross sections are found to be (1.00), 1.46±0.3, 1.46±0.16, and 1.43±0.1, respectively. The corresponding secondary-electron multiplier gains for 6.4-keV ions are (1.00), 1.04, 1.03, and 0.79, all ±5%. These experimental cross-section ratios are compared to the best available calculated values. From the absolute cross section for Pb of Beilina et al., that for Ag is 5.6×10−16 cm2.

Nuclear Reactions Induced by the Nitrogen Bombardment of Boron-11, Fluorine, Aluminum, Silicon, Phosphorus, and Chlorine

Cross sections as a function of bombarding energy were obtained from thick target yields for twelve reactions induced by 27.5-Mev nitrogen ions accelerated in the ORNL 83-in. Cyclotron. Materials containing the following six elements were bombarded: boron, fluorine, aluminum, silicon, phosphorus, and chlorine. The irradiated materials were gamma counted and the radioactive products were identified by their characteristic gamma rays and half lives. The following nuclides were observed: Ne/sup 23/ from B/sup 11/; P/sup 30/ from F/ sup 19/; K/sup 38/ from Al/sup 27/; Cl/sup 34m/ and K/sup 38/ from silicon; Sc / sup 43/ from P/sup 31/; Sc/sup 43/, Sc/sup 44, Sc/sup 44m/, Ti/sup 45/, and V/sup 47/ from Cl /sup 35/; and Cr/sup 49/ from Cl/sup 37/. The observed cross sections are in general agreement with those obtained from other heavy-ion induced reactions at these energies. For the target nucleus Cl/sup 35/ it was possible to identify five products, and experimental cross-section ratios were compared to the theoretical ratios determined from Monte Carlo evaporation calculations. (auth)