We studied the dissociation of CO and N-2 gases on a W(110)-oriented tip by
an atom probe (AP) technique. We paid attention to the effect of electric
field induced dissociation as well as the temperature dependence of dissoci
ation probabilities. We first show that the AP can discriminate between phy
sisorbed and trapped states based on the difference in desorption field str
ength. Then we show that field-induced dissociation can be properly measure
d. Finally, we conclude the activation energy required for the conversion f
rom the trapped state to the dissociated state to be 30 meV for CO gas and
22 meV for N-2 gas. (C) 1999 American Vacuum Society. [S0734-2101(99)17004-
0].