Ionization of Simple Molecules by Ion or Electron Impact in a Reaction Microscope*

We have studied single ionization of simple molecules by fast charged particle impact in a reaction microscope. By measuring the momenta of the emitted electron and the recoil ionic fragment in coincidence, channel-selective low-energy electron spectra have been recorded. For non-dissociative ionization of H$_{2}$ by 6 MeV protons, the electron energy distribution agrees well with a CDW-EIS prediction [1] except for E$_{e}$ $<$ 1 eV where an significant enhancement is observed. It is due to the autoionization of rovibrational levels of Rydberg states of H$_{2}$, which occurs by converting vibrational energy into kinetic energy of the emitted electron. First fully differential cross sections have been obtained bearing the ``signature'' of this molecular mechanism, which lies beyond the Born-Oppenheimer approximation [2]. \newline Recently, considerable interest has been raised by the observation of two-center interference effects in the electron emission from H$_{2}$, in analogy to Young's double-slit experiment [3]. They are predicted to be more pronounced if one could fix the orientation of the molecular axis at the instant of the collision [4]. For dissociative ionization of H$_{2}$ by 6 MeV protons we had access to this information. Molecular-frame angular distributions of the emitted electrons have been compared to the CDW-EIS calculation [5]. \newline Argon dimers as well as atomic Ar, both present in the same gas-jet, are ionized by 1keV electron impact in a kinematically complete experiment carried out in an upgraded reaction microscope. The obtained electron spectra for Ar$_{2}$ and Ar are compared directly in order to identify interference structures, which are expected to be much more visible than for H$_{2}$ since the interatomic distance of Ar$_{2}$ is comparable to the de Broglie wavelength of the emitted electron. \newline \newline [1] M.E. Galassi \textit{et al.,} Phys. Rev. A \textbf{66}, 052705 (2002)\newline [2] C. Dimopoulou \textit{et al.,} Phys. Rev. Lett. \textbf{93}, 123203 (2004)\newline [3] N. Stoltherfoht \textit{et al}., Phys. Rev. Lett. \textbf{87}, 023201 (2001)\newline [4] G. Laurent \textit{et al.}, J. Phys. B \textbf{35}, L495 (2002)\newline [5] C. Dimopoulou \textit{et al.,} J. Phys. B \textbf{38}, 593 (2005)