Inducing non-adiabatic effects through coadsorption: CO + NO on iridium

We compare the response of the intramolecular C–O stretch of carbon monoxide alone and coadsorbed with nitric oxide on Ir{1 1 1} following femtosecond laser heating with the help of time-resolved vibrational sum frequency (SF) spectroscopy. The C–O stretch of a pure CO layer couples anharmonically to the CO frustrated translation and its frequency adiabatically follows the temperature of the iridium surface. In a mixed CO/NO layer, the C–O frequency exhibits non-adiabatic coupling to the hot iridium electrons with a friction coefficient that depends on the electron temperature and the CO:NO ratio. Two possible scenarios emerge: NO causes a static tilt of the CO with a tilt angle depending on the relative coverage. This increases the degree of bonding of the CO 2π^* orbital to the iridium surface, which in turn increases the degree of non-adiabatic coupling. Alternatively, the C–O frequency reflects transient changes in the bonding configuration of the neighboring NO. The latter interaction could be the primary step in the direct reduction of NO by CO to form CO₂.