  The absence of optical afterglow at some gamma-ray bursts (so called dark
bursts) requires analyses of physical features of this phenomenon. It is shown
that such singularity can be connected with multiphoton processes of
frequencies summation in the Rayleigh- Jeans part of spectra, their pumping
into higher frequencies. It can be registered most probably on young objects
with still thin plasma coating, without further thermalization, i.e. soon after
a prompt beginning of the explosive activity.
