Molecular Dynamics of the Electron-Induced Reaction of Diiodomethane on Cu(110)

Diiodomethane is used to generate C-1 fragments at surfaces, en route to higher hydrocarbons. Here scanning tunneling microscopy was employed to examine the interaction of diiodomethane, CH2I2, with a Cu(110) surface, from 4.6 to 8.8 K. In this temperature range unexpectedly rapid thermal reaction resulted in the rupture of two CI bonds, yielding pairs of I atoms recoiling in opposite directions. Approximately 65% of the carbene, CH2, product from this highly exothermic (4.1 eV) thermal reaction remained chemisorbed. Two stable physisorbed configurations of diiodomethane were found, vertical (75%) and horizontal (25%). Electron-induced reaction of these intact adsorbates led to single-electron dissociation of both the CI bonds, with a minor path leading to single bond breaking to form CH2I. Directed recoil of chemisorbed carbene was observed in approximately half the electron-induced reactive events. Simulation of the electron-induced reaction by the impulsive two-state (I2S) model consistently predicted delayed dissociation of the second CI bond, due to vibrational excitation of the CH2I radical product. Theory and experiment agreed in evidencing long-range recoil for the CH2 along the [11(_)0] direction of the copper. This recoiling diradical was shown by the I2S model to undergo migration by a novel process of walking along a pair of adjacent copper rows.