Surface plasmon polaritons diffraction at the rectangular edge of a plane gold-dielectric interface was studied using monochromatic terahertz radiation of Novosibirsk free electron laser. Experimental results were compared with the exact solution of the two dimensional diffraction problem. For short distances from the sample edge experimental data reasonably agreed with theory. The field intensity distribution beyond the edge is not symmetrical regarding to the sample surface and extends mainly in the upper hemisphere. In the far zone the electromagnetic field forms a freely-propagating wave which has Lorentzian profile with the divergence depended on the surface impedance. The propagation length of surface plasmon polaritons along bare gold and gold covered with a submicron ZnS layers were measured and amounted to tens of millimeters
