Reaction dynamics at a metal surface; halogenation of Cu(110)

Abstract

Scanning Tunnelling Microscopy (STM) is opening up a new field of reaction dynamics, followed one-molecule-at-a-time, only recently applied to reaction at a metal surface. Here we combine experiment with theory in studying the motions involved in the successive breaking by electron-induced reaction of the two carbon-halogen bonds, C-Cl or C-I, in physisorbed p-dihalobenzene, to form chemisorbed halogen-atoms and organic residue on Cu(110) at 4.6 K. We characterize the geometry of the physisorbed initial state, p-dichlorobenzene (pDCB) and p-diiodobenzene (pDIB), at the copper surface, as well as the successive final states of both chemisorbed reaction products: electron #1 giving rise to the first halogen-atom and a chemisorbed halophenyl and electron #2 giving a second halogen-atom and a chemisorbed phenylene. The major findings reported are (a) the distance and angular distributions of the chemisorbed reaction products relative to the physisorbed reagent molecule, (b) an approximate ab initio calculation, coupled with classical molecular dynamics (MD), of the repulsion between the products on the excited potential-energy surfaces, pes*, following excitation by electrons #1 or #2, and subsequently MD on the ground-state pes with inclusion of inelastic surface-interaction as a means to understanding the above, (c) observation of the changing dynamics with the chemistry of the halogen-atom, and (d) characterization of the effects of secondary encounters among the reaction products in the constrained space of the more highly localized reaction of pDIB. Item (d) shows clear evidence of high reactivity in surface-aligned collisions with restricted impact parameter, termed Surface Aligned Reaction, SAR, characterized by STM.

Bond selectivity in electron-induced reaction due to directed recoil on an anisotropic substrate

“…As in previous studies of the electron-induced reaction of aryl iodides by ∼1 eV electrons on Cu(110)2627283132, we ascribe the reaction to adsorbate electronic excitation. The computed projected density of states is shown in Fig.…”

“…As in previous work252627283132 we used the ‘Impulsive Two-State’ (I2S) model to simulate the MD of the electron-induced reaction. In this two-electronic state model, the MD was first followed for the 192-atom system on an approximate anionic repulsive potential-energy surface (PES) obtained by the transfer of an electron to the valence shell of a halogen atom for a period of femtoseconds (a time t *).…”

Catalyzed Surface-Aligned Reaction, H(ad) + H2(ad) = H2(g) + H(ad) on Coinage Metals

“…Figure 1 exemplifies for the case of Ag(100) three stages of reaction, IS (Initial State), TS (Transition State) and FS (Final State) computed for Surface Aligned Reaction (SAR) [22] with the attacking atom treated as recoiling from HBr(ad).The three pictures constitute frames from a classical dynamics movie of the reactive event on Ag(100), at the times indicated. Excitation of the H-halogen bond to its anti-bonding anionic state (as in previous experimental and theoretical work; [22][23][24]29]) at t = 0 causes H to recoil along the Br-H bond direction. The HBr(ad) is in a physisorbed location at a fourfold hollow.…”

“…Van de Waals interaction was considered with the method of Grimme [28]. The methodology is similar to the previous work [22][23][24]29]. To facilitate H(ad) + H 2 (ad) exchange at a metal surface, we employed a model in which HBr and H 2 were co-adsorbed at the metal, Cu/Ag/Au(100), surfaces.…”

“…The scenario for achieving bimolecular reaction is that of 'Surface Aligned Reaction' (SAR). The concept of SAR derives from the common observation of preferred atomic recoil directions in adsorbate dissociation at surfaces [22][23][24]. Reproducible observation of SAR has recently been reported for the reaction of atomic iodine on Cu(110), observed with atomic resolution by Scanning Tunneling Microscopy (STM) [23].…”