Polarization dependence in non-resonant photo-triple-ionization of CO <inf>2</inf>

We investigate the angular dependence of the triple fragmentation of CO3+2, formed by the absorption of linearly polarized 200 eV photons. Complete momentum mapping of all fragment ions (C+, O+, O+) enables us to determine the plane of fragmentation and its orientation w.r.t. the polarization axis. While the C+ ions are found to be preferentially ejected perpendicular to the polarization axis, the plane spanned by the velocity vectors of the three fragments is found to contain the polarization axis. The distribution of the angles between the velocity vectors is mapped to the O = C = O bond angle of the precursor under the Coulomb explosion model. The mapping shows that the triple ionization at this energy proceeds from the normal state of CO2. In the light of this we are led to the surprising conclusion that non-resonant photo-triple-ionization preferentially occurs in those CO2 molecules whose C2v symmetry axis is nearly perpendicular to the polarization axis. © 2009 IOP Publishing Ltd.