Angle-Resolved Electron Scattering from ${\mathrm{H}}_2\mathrm{O}$ near 0°

An electron beam, characterized by a high-angular discrimination ($\simeq 0.7°$), has been used to measure the total (elastic plus inelastic) cross section of ${\mathrm{H}}_2\mathrm{O}$ 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 $\lesssim 6\mathrm{eV}$, the present cross sections are $\simeq 30\%$ 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 $\le 12\mathrm{eV}$ are obtained at a scattering angle $\simeq 1^{\circ }$. 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.

Figure 1

Example of the radial dependence of the total cross section for $e^{-}+{\mathrm{H}}_2\mathrm{O}$ scattering at 2.8 eV. Top axis shows the angular acceptance corresponding to each value of $r$.

Figure 2

Detection probability versus scattering angle for the current interaction region and for different radii at the PSD.

Figure 3

(a) Total cross sections for electron scattering from ${\mathrm{H}}_2\mathrm{O}$. Solid symbols correspond to direct measurements, hollow symbols to measurements corrected for forward-angle elastic scattering: filled triangle present results, cyan filled circle [30], filled inverted triangle [15], open inverted triangle [13], filled square [16], dark-red filled circle [31], times [32], filled rhombus [17]. (b) Total cross section comparison between present results filled triangle and theoretical predictions: dots SEP(CP)a1 [33], dotted dash SEP(ED)a1 [33]; integral elastic cross section dashed filled circle initial state $J=0$ [34], and dashed filled square thermal average [34]; dash additivity rule [35], dashed open circle semiempirical formula [35], dashed open square Schwinger approach [36], solid line $R$ matrix [37], dashed plus IAM plus BEB [30]. (c) Comparison among present ${\sigma }_T$ for $⟨\theta ⟩\simeq 0.7°$ and $⟨\theta ⟩\simeq 0.9°$, and results from Ref. [30].

Figure 4

Comparison between experimental $⟨d{\sigma }_{el}/d\mathrm{\Omega }⟩$ (symbol) and theoretical ${⟨d{\sigma }_{el}/d\mathrm{\Omega }⟩}_{Th}$ (line) at $⟨\theta ⟩\simeq 1.2°$.