Vibrational Relaxation of Highly Vibrationally Excited CO Scattered from Au(111): Evidence for CO^– Formation

Abstract

Electronically nonadiabatic dynamics can be important in collisions of molecules at surfaces; for example, when vibrational degrees of freedom of molecules are coupled to electron-hole-pair (EHP) excitation of a metal. Such dynamics have been inferred from a host of observations involving multiquantum relaxation of NO molecules scattered from metal surfaces. Electron transfer forming transient NO– is thought to be essential to the nonadiabatic coupling. The question remains: is this behavior usual? Here, we present final vibrational state distributions resulting from the scattering of CO­(v i = 17) from Au(111), which exhibits significantly less vibrational relaxation than NO­(v i = 16). We explain this observation in terms of the lower electron affinity of CO compared to NO, a result that is consistent with the formation of a transient CO– ion being important to CO vibrational relaxation.

Electron transfer mediates vibrational relaxation of CO in collisions with Ag(111)

“…When E v (r out ) À F B À3.5 eV, the required ICS is smaller, and the molecule need only approach to within 2.2 Å to reach the curve crossing for ET. Hence, the distance of the molecule's closest approach appears to be in the region of 1.75 and 2.2 Å-these values agree well with DFT calculations for the repulsive wall which give 1.9 Å for CO/Au(111), 33 2.0 Å for NO/Au(111), 33 2.2 Å for CO/Ag(111), 37 and 1.9 Å for NO/Ag(111). 37 Here, we estimate the position of the repulsive wall by calculating the molecule-surface distance at which the interaction energy reaches a value of 0.5 eV.…”

“…11 and CO(v i = 17)/Au(111). 33 For both CO and NO, the relaxation probability increases with v i . For similar v i , the NO relaxation probability is larger than that of CO. Molecule-surface systems with a relaxation probability between 0 and 1 exhibit a pronounced dependence on E i .…”

“…This trend reflects the distance of the molecule from the surface when ET occurs, as image charge interaction stabilizes the anion closer to the surface. 33 Fig. 8(b) shows the image charge stabilization (ICS) as a function of distance from the surface.…”

“…32 We have also employed P 3 D to study scattering of CO(v = 17) from Au(111). 33 The final vibrational state distribution indicates significantly less vibrational relaxation than is observed for NO scattered from Au(111). This observation is expected for an ET mediated process considering the reduced electron affinity of CO (E A = À1.5 eV) compared to NO (E A = 0.03 eV).…”

“…Whereas vibrational relaxation is close to absent on the chlorinated surface, it is stronger on Au(111) and strongest on Ag(111). For Au(111) 33 and chlorinated Ag(111), all molecules scatter into v Z 14. For Ag(111), about 60% of scattered CO molecules populate vibrational states below v = 14.…”