抄録
Offer Organization: Japan Society for the Promotion of Science, System Name: Grants-in-Aid for Scientific Research, Category: Grant-in-Aid for Scientific Research (B), Fund Type: -, Overall Grant Amount: - (direct: 10900000, indirect: -)
We studied the quantum reflection of the metastable helium atoms on a flat polished silicon surface. We measured the reflectivity as a function of the normal incident velocity component between 3 and 30 cm/s, and confirm that on a flat surface the reflection of atoms is caused by the attractive van der Waals potential. In our case the atoms are mainly reflected in a region where the retardation effect leads to a potential of the approximate form, U(z)=-C_4/z^4 where z is the distance from the surface. We also studied the reflection and diffraction on surface structures consisting of parallel narrow ridges. The structure consists of wall-like ridges at a distance of L=5μm and with a width at the top of l=100nm. We observed on this structure an increase of the reflectivity by two orders of magnitude, exceeding 10% for normal incident velocities below about 25cm/s. At grazing incidence even fast atoms of 100m/s can be reflected very efficiently at such a surface structure.
In order to quantitatively describe our data, we have developed a theoretical description of the reflection and scattering of atomic waves from our surface structures. As a first approximation we can assume that the van der Waals constant is reduced by a factor l/L, where l is the size at the top of the ridges and L the distance between the ridges. The reflectivity on the ridges structures then follows immediately from the scaling law of the quantum reflection. Deviations appear when the ratio l/L is very small. In the limiting case of l/L→0, i.e. of idealized thin ridges, a new interesting effect can be observed: The atomic wave is reflected due to the Fresnel diffraction at the array of ridges.