Scattering of highly vibrationally excited NO from Ag(111) and Cs/Ag(111): electronically adiabatic or not?

The goal of this proposal is to investigate the validity of the BO-approximation for the reactive and vibrationally inelastic scattering of highly vibrationally excited NO from Ag(111) and cesiated Ag(111) The NO molecule is selected because its initial vibrational state can be varied over a wide range of quantum numbers using infrared overtone pumping and stimulated emission pumping. The NO + Ag(111) system is selected because it is expected to exhibit a high barrier to dissociative chemisorption (about 2 eV). The barrier is expected to be late, and in the electronically adiabatic picture putting vibrational energy into NO should then promote reaction. If this is indeed predicted by the theory and experiment would not confirm this, that would indicate that the molecules vibrational energy would be lost to electron-hole pair excitations in a non-adiabatic mechanism. It is therefore a good test system: comparison to experiment will show wether the reaction is adiabatic or not. The NO + Cs/Ag(111) system is chosen because it it is this type of system (with low work function) for which electron emission was measured, and because Cs/Ag(111) surfaces can be prepared in highly ordered structures for a range of Cs-coverages. In contrast , the Cs/Au(111) surface used in the original experiments on electron emission induced by highly vibrationally excited NO cannot be prepared in ordered structures, making the original experiments less well defined. Comparison of our DFT-based predictions for the competition between reaction (dissociative chemisorption) and multi-quantum vibrational relaxation in these systems with experiments that will be performed in the group of Prof. A.M. Wodtke (UCSB) will clearly show whether these processes can be modelled with electronically adiabatic theory. For Cs/Au(111), electron emission was only observed in 2% of the collisions with highly vibrationally excited NO. Because a large fraction of the remaining collisions may well proceed mostly adiabatically, an electronically adiabatic approach could well be successful at modelling vibrationally inelastic scattering and reaction of NO from/on the surface. Furthermore, it is not clear why an adiabatic approach should not be successful at modelling multi-quantum vibrational relaxation of NO on a noble metal surface like Ag(111), as has been suggested. In the mechanism proposed (the Antoniewicz mechanism), NO picks up an electron when it is close to the surface and its bond length is extended. The efficient multi-quantum vibrational relaxation is then explained from the resulting coupling between the NO-vibration and the molecule-surface motion, which will however also arise if NO picks up the electron from the surface and gives it back adiabatically. The proposed calculations will help with establishing which classes of molecule-surface reactions require a non-adiabatic treatment, which is currently a topic of intense international debate.