Domination of dissociative double-electron excitation over dissociative single-electron excitation in electron collisions with ${\mathrm{NH}}_3$

Double-electron excitations in electron collisions with ${\mathrm{NH}}_3$ are investigated in the range of the outer valence and inner valence excitations by means of angle-resolved electron-photon coincidence measurements. Double-electron and single-electron excitations are termed double and single excitations, respectively, in this article. It is found that the dissociative double excitation resulting in $\mathrm{H}\left(2p\right)$ formation becomes more dominant over the dissociative single excitation at 100-eV incident energy as the electron scattering angle increases just from $8^{\circ }$ to ${15}^{\circ }$. This remarkable domination is not expected from the independent particle model and strongly supports the double-excitation mechanism through the electron correlation induced by the penetration of the incident electron depth into the valence orbitals of a ${\mathrm{NH}}_3$ molecule.

Figure 1

Electron energy-loss spectra of ${\mathrm{NH}}_3$ in coincidence with detection of Lyman-$\alpha$ photons measured at 100-eV incident energy and electron scattering angles of (a) ${15}^{\circ }$ and (b) $8^{\circ }$. Differential cross sections are shown on the same relative scale of the vertical axes in (a) and (b). The resolution of the energy loss is approximately 650 meV and the short vertical bars attached to the data points show the statistical uncertainties. Upper axes show the values of $K^2$, where $K$ is the magnitude of the momentum transfer vector. Long vertical bars show the dissociation limits of the $\mathrm{H}\left(2p\right)$ formation indicated. Curves show the results of the fits of Eq. (1) (see the text).

Figure 2

Ratios of the differential cross sections for dissociative double excitations resulting in $\mathrm{H}\left(2p\right)$ formation to that for dissociative single excitation against the electron scattering angle, $〈d^2{\sigma }_{\alpha }/d{\mathrm{\Omega }}_{\mathrm{e}}d{\mathrm{\Omega }}_{\mathrm{ph}}〉/〈d^2{\sigma }_{{\left(1e\right)}^{-1}\left(\mathrm{mo}\right)}/d{\mathrm{\Omega }}_{\mathrm{e}}d{\mathrm{\Omega }}_{\mathrm{ph}}〉$ vs the electron scattering angle. Squares, $\alpha =\mathrm{the}$ doubly excited $D2$ state; triangles, $\alpha =\mathrm{the}$ doubly excited ‘$\left(2a_1\right){}^{-1}\left({\mathrm{mo}}^{\prime }\right)$’ state. Open symbols show ratios at the optical limit (see the text). Vertical bars represent errors derived from the uncertainties in fitting Eq. (1), and horizontal bars represent the resolution of the electron scattering angle.