  The "Impulsive Stimulated Raman Scattering" (ISRS), is generally performed
using ultrashort laser pulses. It shifts the frequency of the laser beam
without any change of the wave surfaces and any appearance of extra lines. ISRS
works with the pulses which make the usual incoherent light provided that the
interactive medium is a low pressure gas having low frequency Raman
transitions, that is an hyperfine spectrum. ISRS has no intensity threshold,
but at low intensity level it becomes a linear effect, so that the relative
frequency shift $\Delta\nu/\nu$ is nearly constant in the spectrum. This linear
effect named "Incoherent Light, Coherent Raman Scattering" (ILCRS) may be
confused with a Doppler effect in astrophysics; it explains all optical
properties of the quasars, including the debatable ones, in particular the
width of the BAL lines, the infrared thermal radiation attributed to hot dust,
and the spectral discrepancies attributed to a variation of the hyperfine
constant.
