Dependence Of Partial Cross Sections Research Articles

Semtex 1A and H negative ion resonances for explosives’ detection

We present the results obtained in dissociative electron attachment to Semtex in the gas phase by making use of a crossed electron-molecular beam experiment. Energy dependence of partial cross sections has been measured in an electron energy range from 0 to 12 eV with an energy resolution of ∼80 meV. Semtex 1A and H are mixtures of two other explosives, i.e., royal demolition explosive – RDX (hexogen) and pentaerythritol tetranitrate – PETN (pentrite), appearing in different fractions. In fresh samples product anions from both explosives can be detected together with anions originating from volatile markers. After some time we only observe product ions formed upon dissociative electron attachment to PETN, which are not observed for a mixture of pure RDX and PETN. For both Semtex samples, an additional anionic feature at 210 m/ z can be assigned to the binder butyl rubber.

A time-dependent wave packet quantum scattering study of the reaction HD+(v=–3;j=1)+He→HeH+(HeD+)+D(H)

Time-dependent wave packet quantum scattering (TWQS) calculations are presented for HD(+) (v = 0 - 3;j(0)=1) + He collisions in the center-of-mass collision energy (E(T)) range of 0.0-2.0 eV. The present TWQS approach accounts for Coriolis coupling and uses the ab initio potential energy surface of Palmieri et al. [Mol. Phys. 98, 1839 (2000)]. For a fixed total angular momentum J, the energy dependence of reaction probabilities exhibits quantum resonance structure. The resonances are more pronounced for low J values and for the HeH(+) + D channel than for the HeD(+) + H channel and are particularly prominent near threshold. The quantum effects are no longer discernable in the integral cross sections, which compare closely to quasiclassical trajectory calculations conducted on the same potential energy surface. The integral cross sections also compare well to recent state-selected experimental values over the same reactant and translational energy range. Classical impulsive dynamics and steric arguments can account for the significant isotope effect in favor of the deuteron transfer channel observed for HD(+)(v<3) and low translational energies. At higher reactant energies, angular momentum constraints favor the proton-transfer channel, and isotopic differences in the integral cross sections are no longer significant. The integral cross sections as well as the J dependence of partial cross sections exhibit a significant alignment effect in favor of collisions with the HD(+) rotational angular momentum vector perpendicular to the Jacobi R coordinate. This effect is most pronounced for the proton-transfer channel at low vibrational and translational energies.

Partial Cross Sections for Electron Impact Dissociation of CF4 into Neutral Radicals

The electron energy dependence of partial cross sections for dissociation of carbon tetrafluoride (CF4) into neutral radicals CF3, CF2 and CF has been measured. This was accomplished with threshold-ionization mass spectrometry in a differentially pumped dual-electron-beam device. The threshold energy for dissociation into each neutral radical was measured for the first time to be 12.5 eV, 15.0 eV and 20.0 eV for CF3, CF2 and CF radicals, respectively. The absolute cross sections for electron impact energy between 10 and 300 eV were determined through measurements of the radical sticking coefficient and electron-impact nitrogen dissociation. The magnitude of the cross section at its maximum is 0.39×10-20 m2 at 120 eV for CF3 radical, 0.22×10-20 m2 at 130 eV for CF2 radical, and 0.21×10-20 m2 at 110 eV for CF radical.

Electron-Impact Dissociation of Methane into CH3 and CH2 Radicals II. Absolute Cross Sections

The electron energy dependence of partial cross sections for dissociation of methane into CH3 and CH2 radicals has been reported in part I of this paper. In order to assign absolute values to these relative cross sections, two additional experiments were carried out. One was cross-section measurement for electron-impact nitrogen dissociation (N2→N+N, N+N+) using threshold-ionization mass spectrometry. The other was the measurement of sticking coefficients of CH3, CH2, and atomic nitrogen. According to these measurements, the absolute partial cross sections for neutral dissociation were determined: maximum cross section is 1.4×10-20 m2 at 24 eV for CH3 radical and 0.72×10-20 m2 at 18 eV for CH2 radical.

Oscillations in the collision-energy dependence of partial cross sections and of product energy distributions for the backward scattering in the reaction D+H 2(ν=1, j=0)→HD(ν′, all j′)+H

Results of three-dimensional quasiclassical trajectory calculations for the backward scattering in the reaction D+H 2(ν=1, j=0)→HD(ν′, all j′)+H are reported. Significant oscillations in the collision-energy dependence of partial cross sections for the range of scattering angles 170°–180° are observed. These oscillations are stronger than those found earlier for the ground vibrational-state reaction D+H 2(ν=0, j=0)→HD(ν′, all j′)+H, and are accompanied by oscillations in product energy distributions.

Two-meson doorway model of low energy antiproton-proton annihilation

We investigate a semiclassical model for pp̄ annihilation into any number of pions and kaons at antiproton lab momenta p lab < 3 GeV/ c, based on the hypothesis of two-meson doorway states as intermediate channels in the annihilation process. All relevant selection rules are incorporated. We find that with baryon radii of about 0.6 fm, the energy dependence of partial cross sections in all important annihilation channels can be reproduced successfully. Angular distributions for p p → π +π − and K + K − can also be described, provided one admits a substantial s-wave suppression in this model.

Measurement of the reaction K+n

In an experiment carried out at the CERN Proton Synchrotron and using the CERN polarized deuteron target, the reaction ${K}^{+}$${n}_{\ensuremath{\uparrow}}$\ensuremath{\rightarrow}${K}^{+}$${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$p was measured in the region -t=0.1--1.0 (GeV/c${)}^{2}$ and m(${K}^{+}$${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$)=0.812--0.972 GeV at incident momenta of 5.98 and 11.8 GeV/c. The experiment yields the m and t dependence of 14 linearly independent spin-density-matrix elements describing the coherent production of ${K}^{+}$${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$ states with dimeson spins J=0 and J=1. This first measurement of the KN\ensuremath{\rightarrow}K\ensuremath{\pi}N reaction on a polarized target enables us to study experimentally pion production on the level of production amplitudes. Although the mass dependence of partial cross sections averaged over nucleon spins is smooth, we observe large and systematic structures in the moduli squared of individual nucleon transversity amplitudes which reveal an unexpected but essential role of nucleon spin in the pion production process. Our polarization data suggest the possibility of a new state I=(1/2 ${0}^{++}$(860) with a width of 20--40 MeV. At present we consider this conclusion as only tentative. The predictions of the additive quark model relating ${K}^{+}$${n}_{\ensuremath{\uparrow}}$\ensuremath{\rightarrow}${K}^{\mathrm{*}0}$p and ${p}_{\ensuremath{\uparrow}}$p\ensuremath{\rightarrow}${\ensuremath{\Delta}}^{++}$n are well satisfied at 6 GeV/c. Our results emphasize the need for a systematic study of single-pion production in a new generation of dedicated experiments with spin at the recently proposed high-intensity hadron facilities.

An LPS analysis of the reactions K +p→K +π+π−p and K +p→K oπ+πop

Results are presented on the reactions K +p→K + π − π +p and K +p→K 0 π 0 π + p studied with the method of the longitudinal phase space. The analysis is based on data at 7 incoming K + momenta, ranging from 5 to 12.7 GeV/ c. The energy dependence of partial cross sections in various regions of the LPS is discussed. Evidence is presented for the dominance of diffractive processes in particular regions of the phase space and for the hypothesis of factorization.

Energy dependence of partial cross sections for multipion photonproduction

The energy dependence of the cross section for multiple pion production by photons is found in agreement with the simple power law suggested by Hofmokl and Wroblewski for inelastic hadron-hadron collisions. Fits of available data to this law are presented.