State-resolved investigation of the dynamics of scattering and formation of NO at Ge surfaces

Abstract

A combination of resonantly enhanced two-photon ionization and time-of-flight (REMPI-TOF) spectroscopy was applied to determine the translational energy distributions of NO molecules in individual rotational states coming off clean or oxidized Ge surfaces, either as a consequence of scattering of a NO molecular beam or of formation in the surface reaction NO 2 + Ge → NO + GeO. The incident supersonic molecular beam exhibited a narrow translational energy distribution with E kin,i > being variable between 0.1 and 0.8 eV and was rotationally cold so that the scattering experiments represent a good approximation to true state selectivity. The two channels of trapping/desorption and direct-inelastic scattering can be clearly resolved from each other in the TOF spectra if the incident kinetic energy is greater than 700 meV. The thermal desorption channel exhibits Boltzmann distributions for translation and rotation, with E trans,f > independent of E rot . At a surface temperature of T s - 550 K the translational energy of the desorbing particles amounts only to about 80% of T s . This effect of “translational cooling in desorption” supplements previous observations in the same system of a similar effect, “rotational cooling”. In direct-inelastic scattering the rotational state population exhibits a pronounced “rotational rainbow” effect. Those molecules which are rotationally excited in the collision transfer less of their kinetic energy to the solid. Semi-quantitative description of this effect is achieved by application of a simple classical model of non-spherical molecules bouncing off a surface. NO molecules formed in the exothermic decomposition of NO 2 differ to some extent with respect to their rotational and translational energy distributions from thermally desorbing particles, but there is no indication that a substantial fraction of the large excess energy ( ~ 2 eV) is carried away into the gas phase which is instead transferred to the heat bath of the solid.