The decay pathways of the structured ionization region of oxygen at different momentum transfers,i.e.,0,0.23 a.u.（atomic unit）,and 0.91 a.u.,are studied by measuring the ion and the scattered electron coincidently.It is found that the dipole-forbidden superexcited states of（2σu）^-1（c^4Σu^-）npσu ^3Σg^-← X^3Σg^-decay into different channels according to the principal quantum number n.The broad ridge above 35 eV,which may be due to inner-valence excited states of（2σg）^-1nλ or multiply excited states,is observed both at small and large momentum transfers,and its decay channel of O＋＋ O is dominant.
Electron momentum distributions for 4a1 orbitals of serial freon molecules CFaC1, CF2Cl2, and CFCl3 （CFxC14-x, x=1-3） have been reanalyzed due to the severe discrepancies between theory and experiment in low momentum region. The tentative calculations using equilibrium geometries of molecular ions have exhibited a great improvement in agreement with the experimental data, which suggests that the molecular geometry distortion may be responsible for the observed high intensities at p〈0.5 a.u.. Further analyses show that the severe discrepancies at low momentum region mainly arise from the influence of molecular geometry distortion on C-Cl bonding electron density distributions.
