  Diffraction of a surface wave on a rectangular wedge with impedance faces is
studied using the Sommerfeld-Malyuzhinets technique. An analog of Landau's
bypass rule in the theory of plasma waves is introduced for selection of a
correct branch of the Sommerfeld integral, and the exact solution is given in
terms of imaginary error function. The formula derived is valid both in the
near-field and far-wave zones. It is shown that a diffracted surface wave is
completely scattered into freely propagating electromagnetic waves and neither
reflected nor transmitted surface waves are generated in case of bare metals
which have positive real part of surface impedance. The scattered waves
propagate predominantly at a grazing angle along the direction of propagation
of the incident surface wave and mainly in the upper hemisphere regarding the
wedge face. The profile of radiated intensity is nonmonotonic and does not
resemble the surface wave profile which exponentially evanesces with the
distance from the wedge face.
  Comparison with experiments carried out in the terahertz spectral range at
Novosibirsk free electron laser has shown a good agreement of the theory and
the experiments.
