Low-energy elastic electron scattering by acetylene

We report measurements and first-principles calculations of the differential cross sections for elastic scattering of low-energy electrons by acetylene, C${}_2$H${}_2$, at collision energies from 1 to 100 eV, with an emphasis on energies near and below that of the ${\pi }^{*}$ shape resonance. The measurements cover angles from 5${}^{\circ }$ to 130${}^{\circ }$. We compare our results to previous experimental and theoretical values.

Figure 1

Differential cross section for elastic electron scattering by C${}_2$H${}_2$ at the electron energies indicated in each panel; in the lower right panel, all results are at 2.5 eV except the present calculation. Red circles are the present measurements, and the blue solid line is the present calculation. Also shown are previous calculations by Jain [32] (magenta dashed line) and by Gianturco and Stoecklin [37] (orange dot-dashed line). Green diamonds are the measurements of Kochem and coworkers [20] scaled upward by a factor of 2; except at 2 eV, these results are as quoted in Ref. [32].

Figure 2

As in Fig. 1, at higher energies. At 5 eV and above, the open green squares show the previous measurements of Khakoo and coworkers [33]. At 10 eV, the cyan double-dot-dashed line is the calculation of Thirumalai and coworkers [34]. The lower panels use a logarithmic vertical scale to make details at larger scattering angles more visible.

Figure 3

As in Figs. 1 and 2, at higher energies. At 50 and 100 eV, the maroon triangles show the measurements of Iga and coworkers [14]. A logarithmic vertical scale is again used for clarity.

Figure 4

Differential cross section for elastic scattering of electrons by C${}_2$H${}_2$ at a scattering angle of 90${}^{\circ }$, plotted as a function of energy. Symbols are as in Fig. 1, with the addition of two calculations by Franz and coworkers [42]: the brown dot-dashed line shows their DFT-model results, and the cyan double-dot-dashed line shows their $R$-matrix results.

Figure 5

Symmetry components of the present calculated integral elastic cross section for electron scattering by C${}_2$H${}_2$, within the $D_{2h}$ subgroup of $D_{\infty h}$ used in the calculation. The leading $D_{\infty h}$ contributions are ${}^2{\Sigma }_g^{+}$ to ${}^2{A}_g$, ${}^2{\Sigma }_u^{+}$ to ${}^2{B}_{1u}$, ${}^2{\Pi }_g$ to ${}^2{B}_{2,3g}$, and ${}^2{\Pi }_u$ to ${}^2{B}_{2,3u}$. The small unlabeled components are ${}^2{B}_{1g}$ (upper curve) and ${}^2{A}_u$ (lower curve), for which the leading contributions are ${}^2{\Delta }_g$ and ${}^2{\Delta }_u$, respectively.

Figure 6

${}^2{A}_g$ and ${}^2{B}_{1u}$ contributions to the integral elastic cross section at low energy. The thin lines with points are the results shown in Fig. 5, and the thicker lines are from an enlarged calculation; see text for discussion.

Figure 7

Integral cross section for elastic scattering of electrons by C${}_2$H${}_2$. Red circles are the present measurements, and the blue line is the present calculation. Previous measurements of Khakoo and coworkers [33] are shown by green open squares, and those of Iga and cowokers [14] are shown by maroon triangles. The other calculations shown are those of Jain [32] (dashed magenta line) and of Gianturco and Stoecklin [37] (dot-dashed orange line).

Figure 8

Momentum-transfer cross section for elastic scattering of electrons by C${}_2$H${}_2$. Symbols have the same meanings as in Fig. 7.