A molecular beam study of ethane on Si(111)7×7: Energy accommodation and trapping

The interaction of a molecular beam of ethane with the Si(111)7×7 surface has been studied using electron energy loss spectroscopy (EELS), low energy electron reflectivity (LEER), and temperature programmed desorption (TPD). Dissociative adsorption of ethane is not observed on this surface at <90 K for incident energies up to 1.5 eV and nozzle temperature of 1000 K. Ethane adsorbs molecularly on Si(111)7×7 at <80 K in submonolayer coverages and forms multilayers at temperatures below 66 K. The monolayer is found to desorb with first order kinetics with an activation energy of 0.25 eV and pre-exponential factor of 1014±1 s−1. At normal incidence, the probability of trapping is found to decrease with increasing translational energy, falling by a factor of 10 as the incident energy is increased from 0.1 to 1.5 eV. The data are fit by a hard cube model with an effective surface mass of 100 amu, which is between three and four times the mass of a silicon atom. For impact energies ≳0.8 eV, trapping is observed to be higher than would be expected from the hard cube model. Possible explanations for this deviation are discussed. The trapping probability is found to obey total energy scaling. Changes in the internal energy of the ethane due to variations in the nozzle temperature from 300 to 1000 K are observed to produce negligible effects on the trapping probability.