Complex dynamic behavior of fluctuating smectic-A films as studied by scattering with coherent X-rays

Coherent dynamic x-ray scattering has been used to study the thermally exci ted layer fluctuations in freely suspended smectic films of the compound 4O .8. Using 8-keV x rays and films with a thickness around 0.3 mu m we resolv e relaxation times down to a few mu s A combination of damped and oscillato ry behavior is observed for the layer undulations, which can be attributed to inertial effects. These are due to the surface contribution to the free energy which cannot be disregarded for thin films.