Spectroscopic observation of (N2)2 dimers in free icosahedral N2 and Ar-N2 clusters

Supersonic-jet luminescence spectroscopy was applied to study vibronic transitions in icosahedral N-2 and Ar-N-2 clusters having from 100 to 400 particles per cluster. In the case of mixed Ar-N-2 clusters, the omega(1)Delta(u) -> X-1 Sigma(+)(g) transitions were observed to occur in single N-2 molecules in an Ar environment, in very much the same way as in Ar-N-2 bulk samples. In N-2 clusters, however, a band series was detected which, to our knowledge, was never observed earlier. In the spectra of Ar-N-2 clusters, this series coexisted with the "bulk" omega(1)Delta(u) -> X-1 Sigma(+)(g) transitions. Our analysis demonstrated that the series should be assigned to emission of van der Waals (N-2)(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. Our results can be of interest from the viewpoint of producing polymeric nitrogen since (N-2)(2) dimers can be considered to be a starting species for its synthesis.