Angle-Resolved Electron Scattering from H2O near 0°

Abstract

An electron beam, characterized by a high-angular discrimination (≃0.7°), has been used to measure the total (elastic plus inelastic) cross section of H 2 O in the energy range 3-100 eV. Broad coincidence is found with recent experiments, including a pronounced shoulder in the 6-12 eV region. However, at energies ≲6 eV, the present cross sections are ≃30% higher. Furthermore, forward scattering has been probed in the angular range 0°-3.5°and measures of the average (rotationally and vibrationally summed) differential elastic cross sections for incident energies ≤12 eV are obtained at a scattering angle ≃1 ∘ . The measurements, which provide the first test of theoretical predictions in an angular region experimentally unexplored until now, are found to be within 1 standard deviation of corresponding ab initio R-matrix calculations.

“Recommended” cross sections for electron collisions with molecules

“…As shown recently for water by Kadokura et al [53], experiments with well defined, high angular resolution can discern even the electrons rotationally scattered into small angles. In the case of H 2 O, this rises the experimental TCS significantly, towards the theoretical values (see Ref.…”

“…Zhang et al [54] derived, after an extensive discussion of possible methods, an analytical formula for low-angle corrections of elastic and rotationally inelastic DCS (both for positrons and electrons). This triggered further experimental work [53], including detailed studies of the angular resolution effect in TCS, what also allows one to evaluate, a-posteriori, other experimental results [58].…”

Cross Sections for Electron Collisions with H2O

“…25 Already, these early experiments indicated a rapid rise in the TCS in the zero energy limit. In spite of numerous past experiments and comparisons, cross sections for water vapor are still subject to intense research 26,27 and the low-energy cross sections, important for the biochemistry, remain ambiguous.…”

Recent total cross section measurements in electron scattering from molecules

“…Studies of Sullivan et al [58] show that even small changes in the degree of the forward-angle discrimination can permanently distort the shape of the TCS energy dependence, especially at the low impact energies. Recent experiment of Kadokura et al [79] disclosed also considerable influence of an acceptance angle of the electron detector on the magnitude of the measured lowenergy TCS, particularly considerable for polar targets. One can roughly estimate the portion by which the measured TCS might be lowered due to inability to discriminate between unscattered electrons and those scattered at small angles in the forward direction.…”

“…Very recently, Kadokura et al [79] measured TCS in the energy range 3-300 eV, using a high-angular resolution experimental system. Figure 5 shows their TCS results together with some selected previous TCS data [24,67,[80][81][82][83][84] obtained in experiments using electron beams with a poorer angular discrimination.…”