Vibrational Energy Transfer upon the Collision of NO with VO₂ Thin Films across the Insulator-to-Metal Transition

The mechanism and consequently the magnitude of vibrational relaxation of molecules on surfaces differ significantly between insulators and metals, making the vibrational energy transfer at the NO/metal versus the NO/insulator interface a canonical example in the field. We report the influence of the surface temperature, the initial vibrational state, and the incident translational energy on the vibrational relaxation probability of vibrationally excited NO(v_I = 3 and v_I = 11) undergoing a direct scattering from thin films of vanadium dioxide (VO₂) across the Mott transition at 68 °C. At that temperature, thin-film VO₂ transforms from the insulating to the metallic phase, exhibiting ∼4 orders of magnitude drop in resistivity. As VO₂ undergoes the Mott transition, at T > 68 °C, we observe a surprisingly small, yet measurable enhancement in the relaxation probability of NO(v_I = 3 and v_I = 11) due to the metallic phase of VO₂. The magnitudes of vibrational relaxation for NO(v_I = 3)/VO₂ and NO(v_I = 11)/VO₂ are ∼2 and ∼20%, respectively─considerably lower than expected, based on the S-shaped dependence of vibrational relaxation probability on the asymptotic affinity level, observed for diatomic molecules on coinage metal surfaces. By analyzing the distinct dynamic features of NO scattering, including the dependence of vibrational relaxation on the initial vibrational state and on the incidence energy, as well as the relationship between rotational excitation and vibrational inelasticity, we explain the low magnitude of vibrational relaxation of NO on VO₂ using the electron transfer model.