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.
The ionization and ionic dissociation of the superexcited state of N20 are studied by using electron energy loss spectroscopy and positive ion time-of-flight mass spectroscopy at different momentum transfers; that is, 0 and 0.23 a.u. （atomic unit） . The transitions at 13.8 eV and 14.0 eV are reassigned as 3pπ（000） and 3pσ（000） converging to A^2∑＋, respectively. The competition between the main decay pathways of superexcited states at different momentum transfers is revealed. It is found that 3dσ converging to C^2∑＋ mainly decays into N2O^＋ while 4dσ can decay into both N2^O＋ and NO^＋.
