Metal-surface mapping by means of soft-x-ray laser interferometry

We present an investigation of perturbed surface states performed by using an X-UV laser (λ = 21.2nm). A dc electric field, which takes increasing values from 0 to 37 MV/m, is continuously applied to the surface of a niobium sample. The surface is irradiated by the pulsed 21.2-nanometer radiation of the X-UV laser, without breaking off the E field. A Fresnel bimirror interferometer supplies surface interferograms, which show the emergence of growing perturbations for an E field about 14 MV/m, even though no electric microbreakdown has yet been recorded between the anode and the Nb sample. The perturbed surface has been observed in a 0.3×9-nm area during hours, before and after an only electric breakdown which occurred at 37 MV/m. Surface maps, obtained by holographic reconstruction from interferograms, show local “vertical” surface shifts of 10-19 nm, which are more or less aligned on constant E-field lines. Comparison with “post-mortem” surface photographs obtained by the standard scanning electron microscopy method shows perturbation traces only in the small electric breakdown zone, which suggests the extreme-surface structure observed by X-UV laser interferometry to slowly vanish after the end of E-field application.