Intramolecular Electron Scattering and Electron Transfer Following Autoionization in Dissociating Molecules

Resonant Auger decay of core-excited molecules during ultrafast dissociation leads to a Doppler shift of the emitted electrons depending on the direction of the electron emission relative to the dissociation axis. We have investigated this process by angle-resolved electron-fragment ion coincidence spectroscopy. Electron energy spectra for selected emission angles for the electron relative to the molecular axis reveal the occurrence of intermolecular electron scattering and electron transfer following the primary emission. These processes amount to approximately 25% of the resonant atomic Auger intensity emitted in the studied transition.

Figure 1

Schematic of the ${\mathrm{O}}_{\mathrm{2}}$ $1s\to 3{\sigma }_u^{*}$ resonant excitation and its decay. Because of the highly repulsive intermediate potential surface, excited ${\mathrm{O}}^{\mathrm{*}}$ fragments may reach the atomic region before they deexcite.

Figure 2

Experimental layout and kinematical correlation between electron and ion momenta. All ions, but only electrons emitted in the direction of the hemispherical analyzer, are detected.

Figure 3

Main panel: Histogram of autoionization electron, fragment ${\mathrm{O}}^{\mathrm{+}}$ ion pairs detected in coincidence. Ion emission angles with respect to the spectrometer axis $\theta$ are given for fragments with a kinetic energy of 3.5 eV. Top panel: Electron kinetic energy distribution summed over ion time of flight.

Figure 4

Kinetic energy spectra of electrons emitted parallel and antiparallel to the ions at relative angles of 135°–180° and 0–45°, respectively. The solid curves represent a fit result based on all available relative angles. Components $A$ and $B$ correspond to resonant Auger emission in the atomic region, components $A^{*}$ and $B^{*}$ to processes, in which after the electron emission a charge transfer to the ionic fragment occurs, components $C_{\mathrm{B}}$ and $C_{\mathrm{F}}$ to backward and forward scattering of resonant Auger electrons on the neutral fragment, and $C_{\mathrm{B}}^{*}$ and $C_{\mathrm{F}}^{*}$ to processes involving energy exchange by scattering and charge exchange between the fragments. The peak labeled “$m$” we assign to decay into a molecular ionic state occurring before dissociation.