TOF measurements of He and D$_{2}$ molecules scattered from clean and H-covered Si(100) surfaces.

Angular distribution of He or H$_{2}$ (D$_{2})$ scattered from Si(100) surfaces has been found to be broad. In case of H$_{2}$, such a broad scattering was considered as evidence of physisorption. Our aim is to know whether light atoms and molecules such as He or D$_{2}$ are physisorbed upon collision with Si (100) surfaces. Using 300 K effusive beam, we measured Time-Of-Flight (TOF) distributions of He and D$_{2}$ molecules scattered from clean and H-terminated Si(100) surfaces at surface temperature T$_{s}$ = 300 and 600 K. We found that for T$_{s}$ = 300 K the scattered He atoms show a Maxwellian velocity distribution characterized with translational temperature of T$_{t}$=300 K. At T$_{s}$ = 600 K, on the other hand, the net increase in translational temperature was found to be very small, about 340 K. Similar results were also found on the H-terminated surfaces. These results indicate that the scattered atoms or molecules have not accommodated with the surface, suggesting physisorption does not take place.