Experimental Triple Differential Cross Sections Research Articles

A dynamical (e,2e) investigation into the ionization of pyrazine

We report an experimental and theoretical investigation into the dynamics of electron-impact ionization of the unresolved outermost molecular orbitals (6ag + 1b1g) of pyrazine (C4N2H4). Experimental triple differential cross sections (TDCS) are compared with cross sections calculated within a molecular 3-body distorted wave (M3DW) framework. The M3DW theory was able to reproduce the qualitative scattering behaviour in the binary region, but failed to describe the absolute intensity of the TDCS and the recoil scattering behaviour. Momentum profiles were used to identify possible sources where the M3DW may mask the quantum character of the ionized orbitals in describing the ionization dynamics.

Triple-differential cross sections for (e,2e) electron-impact ionization dynamics of tetrahydrofuran at low projectile energy

We study the triple-differential cross section (TDCS) for the electron-impact ionization of the highest occupied molecular orbital of tetrahydrofuran at a projectile energy ${E}_{0}=91\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$. The experimental data were measured using a reaction microscope, which covers a large part of the full solid angle for the secondary electron emission with energies ranging from 6 to 15 eV, and projectile scattering angles ranging from $\ensuremath{-}{10}^{\ensuremath{\circ}}$ to $\ensuremath{-}{20}^{\ensuremath{\circ}}$. The experimental TDCSs are internormalized across all measured scattering angles and ejected energies. They are compared with predictions from the multicenter distorted-wave (MCDW) approximation and a modified MCDW-Nee method which includes the postcollision interaction (PCI) using the Ward-Macek factor. Additional calculations were obtained using a molecular three-body distorted-wave (M3DW) approach which accounts for PCI in an exact treatment. Generally, the MCDW-Nee and M3DW models show better agreement with experiment than the MCDW calculations. This shows the importance of accounting for PCI for low-energy outgoing electrons in electron-impact ionization processes.

A dynamical (e,2e) investigation into the ionization of the outermost orbitals of R-carvone.

We report an experimental and theoretical investigation into the dynamics of electron-impact ionization of R-carvone. Experimental triple differential cross sections are obtained in asymmetric coplanar kinematic conditions for the ionization of the unresolved combination of the three outermost molecular orbitals (41a-39a) of R-carvone. These cross sections are compared with theoretical cross sections calculated within a molecular 3-body distorted wave (M3DW) framework employing either a proper orientation average or orbital average to account for the random orientation of the molecule probed in the experiment. Here, we observe that the overall scattering behavior observed in the experiment is fairly well reproduced within the M3DW framework when implementing the proper average over orientations. The character of the ionized orbitals also provides some qualitative explanation for the observed scattering behavior. This represents substantial progress when trying to describe the scattering dynamics observed for larger molecules under intermediate-impact energy and asymmetric energy sharing scattering conditions.

Triple-differential cross sections for single ionization of CO2 by 100 eV electron impact

We present a combined experimental and theoretical study for electron-impact ionization of carbon dioxide (CO2) for the projectile energy E0 = 100 eV. Experimental triple-differential cross sections (TDCS) are obtained using a multi-particle momentum spectrometer (reaction microscope). For projectile scattering angles between −5° and −20° a large part of the full solid angle is covered for the slow ejected electron with energies between 5 and 15 eV. The experimental data are measured for the ionization of the three outer valence molecular orbitals 1πg, 1πu, and 3σu which lead to a non-dissociating ion. The measured TDCS summed over all three orbitals are internormalized across the scattering angles and ejected electron energies. They are compared to the theoretical results from the multi-center distorted wave (MCDW) approximation, and from the MCDW-WM approximation which includes post-collision interaction using the Ward–Macek factor (WM). Reasonable good agreement is found between the experiment and the MCDW-WM calculations for the angular dependence and the relative magnitude of the cross sections in the coplanar plane, while for the perpendicular and full perpendicular planes larger discrepancies exist. Since post-collision interaction is not considered the MCDW method shows strong discrepancies with experiment for small mutual angles of the two outgoing electrons in the final state.

Electron- and photon-impact ionization of furfural.

The He(i) photoelectron spectrum of furfural has been investigated, with its vibrational structure assigned for the first time. The ground and excited ionized states are assigned through ab initio calculations performed at the outer-valence Green's function level. Triple differential cross sections (TDCSs) for electron-impact ionization of the unresolved combination of the 4a″ + 21a' highest and next-highest occupied molecular orbitals have also been obtained. Experimental TDCSs are recorded in a combination of asymmetric coplanar and doubly symmetric coplanar kinematics. The experimental TDCSs are compared to theoretical calculations, obtained within a molecular 3-body distorted wave framework that employed either an orientation average or proper TDCS average. The proper average calculations suggest that they may resolve some of the discrepancies regarding the angular distributions of the TDCS, when compared to calculations employing the orbital average.

Positron-impact ionization of atoms and molecules

This paper presents the level of agreement between theory and experiment for positron-impact ionization of atoms and molecules. Distorted-wave theoretical models are able to reproduce the experimental integrated cross sections for atoms and diatomic molecules. The ECC phenomenon seen in the experimental triple differential cross sections can be obtained with a BBK-type model, but for very low impact energies the theory requires the inclusion of the coupling with the positronium formation channel.

Electron-capture-to-the-continuum by positron impact

The process of electron-capture-to-the-continuum (ECC) is discussed for positrons and other projectiles. The work leading to the first observation of this phenomenon in the case of positron impact is outlined. The resulting first experimental triple-differential cross-section is compared with recent theories.

Triple-differential cross section of the [formula omitted] Coulomb breakup and astrophysical [formula omitted]-factor of the [formula omitted] reaction at extremely low energies

A method of calculation of the triple-differential cross section of the 208 Pb( 6 Li,αd) 208 Pb Coulomb breakup at astrophysically relevant energies E of the relative motion of the breakup fragments, taking into account the three-body ( α– d– 208 Pb ) Coulomb effects and the contributions from the E1- and E2-multipoles, including their interference, has been proposed. The new results for the astrophysical S-factor of the direct radiative capture d(α,γ) 6 Li reaction at E≤250 keV have been obtained. It is shown that the experimental triple-differential cross section of the 208 Pb( 6 Li,αd) 208 Pb Coulomb breakup can also be used to give information about the value of the modulus squared of the nuclear vertex constant for the virtual decay 6 Li→α+d .

Atomic ionization by electron impact: A hyperspherical approach

An {ital ab initio} calculation, following a hyperspherical partial-wave method [J. N. Das, Aust. J. Phys. {bold 47}, 743 (1994)], satisfactorily represents the recent experimental triple differential cross-section results of R{umlt o}der {ital et al.} [Phys. Rev. A {bold 53}, 225 (1996)] for electron hydrogen atom ionization collisions at low energies. The qualitative agreement with their theoretical results is also good. Moreover, our results are in absolute units and in some instances our results are in better agreement with the experiments compared to their theoretical results. Here it is also exhibited that the hyperspherical partial wave method is a promising formalism for studying atomic ionization problems. {copyright} {ital 1997} {ital The American Physical Society}

Double photoionization: II. Analysis of experimental triple differential cross sections in helium and neon

We use the general formalism established in the companion paper I to analyse recent measurements of the triple differential cross sections for double photoionization of He and Ne, for equal energy sharing and for symmetry of the residual ion. A dynamical factor, which depends on the energy and on the mutual angle between the two electrons, is extracted from the experiments without relying on any dynamical approximation. This factor is expanded with respect to the one-electron angular momentum , up to a maximum value , which measures the degree of angular correlation attained by the electron pair. We discuss the physical meaning of , and the dependence of the dynamical factor on the target, which is observed when comparing helium and neon results.

Triple-differential cross section for electron-impact ionization of argon in a coplanar symmetric geometry at intermediate energies.

We have measured experimental triple-differential cross sections for electron-impact ionization of the outer shell of argon at incident energies of 115.8 and 215.8 eV. The experiments have been performed in coplanar symmetric geometry with [ital E][sub [ital a]]=[ital E][sub [ital b]]=50 eV and [ital E][sub [ital a]]=[ital E][sub [ital b]]=100 eV. The results are compared with recent distorted-wave Born-approximation calculations.

Coplanar asymmetric (e,2e) experiments on xenon 4d and 5p orbitals.

Relative triple-differential cross sections for ionization of Xe 4[ital d] and 5[ital p] orbitals have been measured in coplanar asymmetric kinematics. The scattered-electron energy is held at 1000 eV while the ejected-electron energy is changed so as to select energy-loss values [Delta][ital E] in or out of the region characterized by the giant resonance in the 4[ital d][r arrow][var epsilon][ital f] channel. The ionization mechanism as well as the interference between the direct and resonant processes have been investigated by varying the momentum transfer in a collision at each ejected-electron energy. Strong interchannel coupling between the 5[ital p] and 4[ital d] shells in the region of the giant resonance has been observed in ([ital e],2[ital e]) experiments. Distorted-wave-Born-approximation calculations have been compared with the experimental triple-differential cross section and used to guide the discussion.

Second-order effects in large-angle coplanar symmetric (e, 2e) processes

Theoretical and experimental triple differential cross sections are compared for the ionisation process e-+He(11S) to He+(1s)+2e-, at an incident electron energy of 200 eV, in a coplanar symmetric geometry. At large angles, the theoretical curve obtained by including the second Born contribution due to the initial target state differs markedly from the first Born result and exhibits a characteristic behaviour which is confirmed by the experimental data.

Absolute triple- and double-differential cross sections for ionisation of helium by electron impact

The authors have measured absolute (e,2e) cross sections for electron impact ionisation of helium with a much higher accuracy than obtained before. In a coplanar symmetric geometry ( theta =45 degrees ) both outgoing electrons-with equal velocities-were detected in coincidence. The resulting experimental triple differential cross sections for incoming electrons of 200-2800 eV contain an experimental error smaller than 20%. At the same time the authors have determined the absolute double differential cross sections at 45 degrees to an accuracy of 10%. The data are compared with various theoretical results.