  Supersonic-jet luminescence spectroscopy was applied to study vibronic
transitions in icosahedral N2 and Ar-N2 clusters having from 100 to 400
particles per cluster. In the case of mixed Ar-N2 clusters, the w->X
transitions were observed to occur in single N2 molecules in an Ar environment,
that is very much in the same way as in Ar-N2 bulk samples. In N2 clusters,
however, a band series was detected which was never observed earlier. In the
spectra of Ar-N2 clusters, this new series coexisted with the 'bulk' w->X
transitions. Our analysis demonstrated that the series should be assigned to
emission of van der Waals (N2)2 dimers from inside clusters. Earlier, such
dimers were only observed in molecular beams and gaseous nitrogen, this paper
reports their observation in the solid phase of nitrogen suggesting that they
may be the basic building blocks of small N2 clusters. Our results can be of
interest from the viewpoint of producing polymeric nitrogen since (N2)2 dimers
can be considered to be a starting species for its synthesis.
