HOT ETHENE DESORPTION FROM CU(111)

Irradiation of submonolayer and 4 layers of adsorbed 1,2-dichloroethan e (DCE) on Cu(lll) at 110 K with a pulsed 2 beV electron beam causes e thene desorption with translational temperatures of 2350+/-100 K (from a submonolayer), and 2540+/-200 K and 320+/-50 K (from a 4 layer surf ace). DCE physisorbed on the Cu(111) surface undergoes electron attach ment from the hot secondary electrons generated by the incident beam t o form a negative molecular ion. This then dissociates to form a short lived ClCH2CH2 . radical and chemisorbed chlorine. The radical then d ecomposes generating the high temperature (2350 and 2540 K) ethene and a second chemisorbed chlorine. The translational motion of the ethene is thought to come from part of the heat liberated in the formation o f the second Cu-CI bond as the radical decomposes. The 320 K peak is t hought to arise from radical decomposition at the surface of DCE cryst allites.