The scattering coefficients for elastic collisions between electrons and molecules of methane, acetylene and ethylene have been measured over the following ranges: for C${\mathrm{H}}_4$, 10 to 625 volts, 10 to 150 deg., and for 800 volts, 10 to 50 deg.: for ${\mathrm{C}}_2$${\mathrm{H}}_2$, 10 to 100 volts, 10 to 150 deg.; for ${\mathrm{C}}_2$${\mathrm{H}}_4$, 10 to 225 volts, 10 to 150 deg. Total absorption coefficients due to elastic scattering were computed by integration. For 100 volts and above, the scattering coefficients of electrons by the molecules with two C atoms were far smaller than the coefficient for C${\mathrm{H}}_4$ with one C atom, but for 10 volt electrons the scattering was much less in C${\mathrm{H}}_4$ than in the other two gases.

Evidence for interference effects between the electron waves scattered by individual atoms was indicated by the presence of maxima in the curves for the ratios of the scattering coefficients for ${\mathrm{C}}_2$${\mathrm{H}}_4$ and C${\mathrm{H}}_4$ (also for the pair ${\mathrm{C}}_2$${\mathrm{H}}_2$ and C${\mathrm{H}}_4$) expressed as functions of scattering angles. The ratios were also computed by the well-known formula for the intensity of the resultant waves emitted by a molecule, viz., ${\Psi }^2=\Sigma \stackrel{n}{1}i\Sigma \stackrel{n}{1}j{\psi }_i{\psi }_{j}sin\frac{x_{\mathrm{ij}}}{x_{\mathrm{ij}}},$ where ${\psi }_i$ and ${\psi }_j$ are the electron wave amplitudes due to the atoms $i$ and $j$, and $x_{\mathrm{ij}}$ is a parameter involving the distance apart of these atoms. Fairly satisfactory agreement between experiment and calculations was obtained for the ratio for ${\mathrm{C}}_2$${\mathrm{H}}_4$ and C${\mathrm{H}}_4$, provided that we assume that the scattering is done by the H atoms alone. Presumably the slow electrons do not penetrate enough to come under the influence of the C atom. To get satisfactory agreement in the case of ${\mathrm{C}}_2$${\mathrm{H}}_2$, however, it was necessary to assume that the C atoms exerted their full effect.

• Received 25 September 1933

DOI:https://doi.org/10.1103/PhysRev.44.876