PROBING HARPOONING AND DISSOCIATION IN GAS-SURFACE REACTIONS BY EXOEMISSION

The reaction of electronegative molecules with low work function surfa ces may proceed nonadiabatically. As a consequence ''exoemission'' of negative ions or electrons can be observed. The reactions of O-2 and N 2O with Cs are discussed. If the molecules interact with a metal surfa ce the nonadiabaticity is triggered by harpooning, i.e. resonant ioniz ation and acceleration in the image field, and subsequent dissociation . From recent experiments with state-selected N2O molecules on Cs [1] the range of harpooning reactions is extended. From exoemission of N2O in the first excited bending mode and its suppression in the ground s tate, it is concluded that a molecular affinity level may be lowered b y more than 3.5 eV in front of a metallic Cs surface. This is a hint t hat harpooning is more common than expected.