Bremsstrahlung Isochromat Study of Si(111) by 6.2 eV Photons

We present the first experimental bremsstraldung isochromats at an extremal low photon energy of 6.2 eV measured by using a grating spectrometer placed in the air outside the ultra-high-vacuum chamber. Isochromats of the Si(111) cleaved surface have been measured for normal incidence of electrons. We have found that a free electron approximation for the initial electron state is quite good for the interpretation of bulk interband traηsitions occurring in the 6.2 eV isochromat of Si(111). A strong sensitivity for surface roughness of the 6.2 eV isochromat is observed.