Rotational excitation from a molecular mirror: NO scattering from Ru(0001)-(1 × 1)H

The rotational state distributions of NO molecules scattered off the (1 × 1)H-covered Ru(0001) surface have been investigated by resonance enhanced multi-photon ionisation (REMPI). Angular and energy distributions reported earlier for this scattering channel, show an angular spread of only ≃8° (θ i  = 60°, E i  = 2.1 eV), combined with small translational energy loss in the collision (E f /E i  = 0.91). These observations suggest scattering from a very weakly corrugated surface, a ‘molecular mirror’, and a high probability to yield cold rotational distributions. The present experiments, however, reveal substantial rotational excitation, accompanied by rotational rainbows for higher quantum numbers (J > 20). The rotational temperatures increase from  = 450 K at an incidence energy E i  = 0.32 eV, to  = 950 K at E i  = 1.50 eV. The kinetic to rotational energy transfer efficacy, appears to be slightly higher than for NO scattering of the fairly inert Ag(111) surface. This system has much wider angular spread and larger translational energy losses. Assuming parallel momentum conservation in the collision the observed rotational excitation would lead to an angular spread that is considerably smaller than that observed for an incoming angle of θ i  = 15°.