  A fluorescent image analysis procedure to determine the distribution of
species concentration and density in a gas flow is proposed. The fluorescent
emission is due to the excitation of atoms/molecules of a gas that is
intercepted by an electron blade. The intensity of the fluorescent light is
proportional to the local number density of the gas. When the gas flow is a
mixture of different species, this proportionality can be exploited to extract
the contribution associated to the species from the spectral superposition
acquired by a digital camera. This yields a method that simultaneously reveals
species concentrations and mass density of the mixture. The procedure is
applied to two under-expanded sonic jets discharged into a different gas
ambient - Helium into Argon and Argon into Helium - to measure the
concentration and density distribution along the jet axis and across it. A
comparison with experimental and numerical results obtained by other authors
when observing under-expanded jets at different Mach numbers is made with the
density distribution along the axis of the jet. This density distribution
appears to be self-similar.
