$\left(e,e+\mathrm{ion}\right)$ study on electron-induced dissociative ionization of ${\mathrm{O}}_2$

Electron-induced dissociative ionization of ${\mathrm{O}}_2$ has been investigated by means of $\left(e,e+\mathrm{ion}\right)$ spectroscopy. The linear momenta of the fragment ions have been measured in coincidence with electron energy-loss spectra for ${\mathrm{O}}_2$ at scattering angles of $2.2^{\circ }, 4.2^{\circ }$, and $8.2^{\circ }$ using an incident energy of 1.4 keV. From the analysis of the data, partial ion yield spectra have been obtained for the $B{}^2\mathrm{\Sigma }, \{c{}^4\mathrm{\Sigma }+2{}^2\mathrm{\Sigma }+2{}^4\mathrm{\Sigma }\}, \{2{}^2\mathrm{\Sigma }+2{}^4\mathrm{\Sigma }\}$, and $3{}^2\mathrm{\Sigma }$ ionization, allowing us to assess state-specific momentum-transfer dependence of the relative ionization cross sections. It has been shown that the shape resonance in the $B{}^2\mathrm{\Sigma }$ channel is substantially suppressed by the contributions of electric nondipole transitions. To get insight into the stereodynamics in the electron-${\mathrm{O}}_2$ collision processes, the angular distributions of the fragment ions have been examined. It has been revealed that the $\left(e,e+\mathrm{ion}\right)$ cross section for the $2{\sigma }_{\mathrm{g}}\to 1{\pi }_{\mathrm{g}}$ autoionization band exhibits a characteristic angular distribution reflecting the anisotropic shape of the molecular orbital to which the target electron is excited.

Figure 1

(a) Electron-energy-loss and kinetic-energy correlation diagram of electron-${\mathrm{O}}^{+}$ coincidence events for $\theta =2.2^{\circ }$. (b) Fragment-ion-yield spectra of ${\mathrm{O}}_2$ constructed from the electron-${\mathrm{O}}^{+}$ coincident data for $\theta =2.2^{\circ }, 4.2^{\circ }$, and $8.2^{\circ }$. The solid line represents the result of photoionization experiment [43].

Figure 2

(a) Potential-energy curves of the electronic states of ${\mathrm{O}}_2{}^{+}$ [19, 45, 46]. The Franck-Condon region (2.2–2.4 bohr) is denoted by the vertical lines. (b) The kinetic-energy distributions of ${\mathrm{O}}^{+}$ constructed from the coincidence data at $E=19–30\mathrm{eV}$. For ease of comparison, the distributions are scaled so that their intensities coincide with each other at $\mathrm{KE}=1.9\mathrm{eV}$.

Figure 3

Fragment-ion-yield spectra constructed from the coincidence data with detection of ${\mathrm{O}}^{+}$ having $\mathrm{KE}=$ (a) 0.46–1.15, (b) 1.8–2.5, (c) 3.2–4.0, and (d) 4.6–5.8 eV, which are mainly associated with ionization to the$B{}^2\mathrm{\Sigma }, \{c{}^4\mathrm{\Sigma }+2{}^2\mathrm{\Sigma }+2{}^4\mathrm{\Sigma }\}$,$\{2{}^2\mathrm{\Sigma }+2{}^4\mathrm{\Sigma }\}$, and $3{}^2\mathrm{\Sigma }$ states, respectively. See text for details.

Figure 4

Angular distributions of ${\mathrm{O}}^{+}$ having $\mathrm{KE}=0.46–1.15\mathrm{eV}$, which are dominantly attributed to dissociation from the $B{}^2\mathrm{\Sigma }$ state to the dissociation limit $L_1$: ${\mathrm{O}}^{+}\left({}^{4}S\right)+\mathrm{O}\left({}^{3}P\right)$. Solid lines represent the analytical function $I\left({\varphi }_{\mathit{K}}\right)=C\{1+{\beta }_2{P}_2(cos{\varphi }_{\mathit{K}})+{\beta }_4{P}_4(cos{\varphi }_{\mathit{K}})\}$ fitted to experiment. For comparison in shape, all results are scaled so that the proportional coefficient $C$ is equal to 1. See text for details.

Figure 5

Generalized asymmetry parameters, (a) ${\beta }_2$ and (b) ${\beta }_4$, for the $B{}^2\mathrm{\Sigma }$ ionization at $K^2=0.16$, 0.56, and 2.10 a.u. Dashed lines are drawn as a guide for the eye.

Figure 6

Left-hand panel: Angular distributions of ${\mathrm{O}}^{+}$ having $\mathrm{KE}=1.8–2.5\mathrm{eV}$ at $E=25–27\mathrm{eV}$, which are mainly associated with the $c{}^4\mathrm{\Sigma }$ ionization, for $K^2=$ (a) 0.16, (b) 0.56, and (c) 2.10 a.u. Solid lines represent the analytical function $I\left({\varphi }_{\mathit{K}}\right)=C\{1+{\beta }_2{P}_2(cos{\varphi }_{\mathit{K}})+{\beta }_4{P}_4(cos{\varphi }_{\mathit{K}})\}$ fitted to the experiment. For comparison in shape, all results are scaled so that the proportional coefficient $C$ is equal to 1. The right-hand panel depicts ${\beta }_2\left(K\right)$ and ${\beta }_4\left(K\right)$ against $K^2$, which were obtained by fitting $I\left({\varphi }_{\mathit{K}}\right)$ to the experimental angular distributions. Dashed lines are drawn as a guide for the eye.

Figure 7

Angular distributions of ${\mathrm{O}}^{+}$ having $\mathrm{KE}=1.8–2.5\mathrm{eV}$, which are mainly associated with the $\{c{}^4\mathrm{\Sigma }+2{}^2\mathrm{\Sigma }+2{}^4\mathrm{\Sigma }\}$ ionization, at $E=27–29$, 29–31, and 31–33 eV. Solid lines represent the analytical function $I\left({\varphi }_{\mathit{K}}\right)=C\{1+{\beta }_2{P}_2(cos{\varphi }_{\mathit{K}})+{\beta }_4{P}_4(cos{\varphi }_{\mathit{K}})\}$ fitted to the experiment. For comparison in shape, all results are scaled so that the proportional coefficient $C$ is equal to 1.

Figure 8

Generalized asymmetry parameters (a) ${\beta }_2$ and (b) ${\beta }_4$, for the $\{c{}^4\mathrm{\Sigma }+2{}^2\mathrm{\Sigma }+2{}^4\mathrm{\Sigma }\}$ ionization at $K^2=0.16$, 0.56, and 2.10 a.u. Dashed lines are drawn as a guide for the eye.

Figure 9

Schematic representations of the $2{\sigma }_{\mathrm{g}}\to 1{\pi }_{\mathrm{g}}$ excitation when the momentum-transfer vector is (a) perpendicular and (b) parallel to the molecular axis. For both cases, reflection in a mirror plane depicted by a dashed line changes the sign of the $1{\pi }_{\mathrm{g}}$ orbital, while the symmetry operation leaves the $2{\sigma }_{\mathrm{g}}$ orbital and the exponential term $exp(i\mathit{K}·\mathit{r})$ unchanged. The integrand in the transition matrix $\langle 2{\sigma }_{\mathrm{g}}|exp(i\mathit{K}·\mathit{r})|1{\pi }_{\mathrm{g}}\rangle$ is therefore odd under the reflection operations and the scattering cross section vanishes at ${\varphi }_{\mathit{K}}=0^{\circ }, {90}^{\circ }$, and ${180}^{\circ }$.