Thickening of a smectic membrane in an evanescent X-ray beam.

We report an unusual thickening of smectic membranes under the influence of X-ray irradiation below the critical angle. In the case of a four-layer film the thickness was found to grow at the footprint of the beam, reaching within minutes tens of layers. The effect is attributed to the localized energy dissipation of the evanescent wave. The "island" thus created is not connected to the meniscus and after the beam is switched off the film returns to its original state. A possible explanation is given in terms of a local disrupture of the tension of the smectic membrane.