Symmetry breaking in electron-impact dissociative ionization of linear symmetric molecules

This work provides experimental evidence of symmetry breaking in electron-impact dissociative ionization of $\mathrm{C}{\mathrm{O}}_2$. Angular correlation between the recoil direction of fragment ions and transferred momentum is measured. It reveals that despite the inversion symmetry of the $\mathrm{C}{\mathrm{O}}_2$ molecule, $\mathrm{C}{\mathrm{O}}^{+}$ is emitted preferentially in the momentum transfer direction near the thresholds of ionization to close-lying ${}^2{\mathrm{\Pi }}_{\mathrm{u}}$ and ${}^2{\mathrm{\Pi }}_{\mathrm{g}}$ states. Similar forward-backward asymmetry is found also for dissociative ionization of ${\mathrm{N}}_2$. The findings cannot be explained if ionization and dissociation are two independent steps, indicating that the Coulomb field of the slow ejected electron induces the mixing of gerade and ungerade states in the molecular ion.

Figure 1

Upper panel: two-dimensional plots of energy correlation between the scattered electrons and $\mathrm{C}{\mathrm{O}}^{+}$ ions for $\theta =2.2^{\circ }$. Lower panel: ion-yield spectra for $\mathrm{C}{\mathrm{O}}^{+}$ with $\mathrm{KE}=1.4–2.3\mathrm{eV}$ at $\theta =2.2^{\circ }$ and 4.2°.

Figure 2

Angular distributions of $\mathrm{C}{\mathrm{O}}^{+}$ ions having $\mathrm{KE}=1.4–2.3\mathrm{eV}$ for $E=28–30$, 34–36, and 40–42 eV. Left- and right-hand panels are the angular distributions for $K^2=0.16$ and 0.56 a.u., respectively. Solid lines represent the analytical function fitted to experiment, $C\{1+{\beta }_2{P}_2\left(\cos {\varphi }_{\mathit{K}}\right)+{\beta }_4{P}_4\left(\cos {\varphi }_{\mathit{K}}\right)\}$, and the dashed line is the associated photoion angular distribution. All results are scaled so that the proportional coefficient $C$ is equal to 1.

Figure 3

Angular distributions of ${\mathrm{N}}^{+}$ ions having $\mathrm{KE}=1.4–2.1\mathrm{eV}$ generated from ($e, e$ + ion) data of ${\mathrm{N}}_2$ [7, 8] at $K^2=0.16\mathrm{a}.\mathrm{u}.$ for $E=38–40$ and 44–46 eV. Solid line is the analytical function fitted to the result for $E=44–46\mathrm{eV}$.