  Structural and optical properties of MBE-grown GaAs(001) surface have been
studied by reflection high-energy electron diffraction and single-wavelength
ellipsometry under dynamic conditions of ramp heating after desorption of
passivating As-cap-layer with and without As4 beam applied to the surface. For
a number metastable reconstruction transitions a clear correlation is
stablished between diffraction and optical data. Boundary lines for
transitional superstructures are determined as a function of As flux and
corresponding activation energies are estimated. For the first time it is shown
ellipsometrically that optical response of the surface is drastically different
for transitions of the order->order and order->disorder type.
