Fragmentation of molecules in fast ion collisions

C. DIMOUPOULOU; R. MOSHAMER; P.D. FAINSTEIN; M.E. GALASSI; R.D. RIVAROLA; D. FISCHER; C. HÖHR; A. DORN; J. ULLRICH

Belfast, Irlanda

Conferencia; 13th International Conference on the Physics of Highly Charged Ions (HCI),; 2006

Single ionization of simple molecules as well as Van der Waals dimers, e.g. H2, CO2, Ar2 by fast charged particle impact has been studied using 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. The experiments have been carried out at the Max Planck Institute for Nuclear Physics in Heidelberg. Experimental cross sections are presented, compared with the predictions of state-of-the-art CDW-EIS (Continuum Distorted Wave-Eikonal Initial State) calculations and discussed in terms of molecular effects such as (i) autoionization and predissociation channels and (ii) interference patterns resulting from the two-center geometry of the diatomic molecule, in analogy to Young’s double-slit experiment. In non-dissociative ionization of H2 by 6 MeV protons, the emission of very low-energy electrons (below 1 eV) is significantly enhanced in comparison to the theoretical results. This is due to the autoionization of rovibrational levels of Rydberg states of H2, 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 [1]. The two-center interference patterns recently observed in the electron emission from ion impact ionization of H2 [2], are predicted to be more pronounced if the orientation of the molecular axis could be fixed in space at the instant of the collision [3]. In the case of dissociative ionization of H2, we obtained, for the first time in a collision experiment, molecular-frame angular distributions of emitted electrons within the axial recoil approximation [4]. The experimental data for different channels of dissociative ionization of CO2 by 6 MeV protons have been compared with the predictions of a CDW-EIS calculation for molecular orbitals [5]. A good qualitative agreement is observed even though the vibrational motion of the molecule is not taken into account in the model. The low-energy electron spectra show a rich structure which may be attributed to the presence of molecular excitation channels, which undergo radiationless decay, via autoionization and also via predissociation. This interpretation is supported by photoionization studies of CO2 [6]. References [1] C. Dimopoulou et al., Phys. Rev. Lett. 93, 123203 (2004)[2] N. Stoltherfoht et al., Phys. Rev. Lett. 87, 023201 (2001) [3] G. Laurent et al., J. Phys. B 35, L495 (2002) [4] C. Dimopoulou et al., J. Phys. B 38, 593 (2005) [5] M.E. Galassi et al., Phys. Rev. A 66, 052705 (2002) [6] C. Dimopoulou et al., J. Phys. B 38, 3173 (2005)