Figure 1: Schematic illustration of the experimental approach.

Monochromatic light, emitted by a light-emitting diode, shines on a semitransparent mirror, which is used to split the beam allowing simultaneous measurements of the transmission through the sample and the intensity of the source. A diffusor is used to homogenize the illumination, before the entry window of the ultrahigh vacuum chamber. The transmitted light is registered by a CCD camera. The sample is covered with Al2O3 to prevent hydrogen from entering and leaving, except for a small region at one end of the sample.