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 N2 and Ar-N2 clusters having from 100 to 400 particles per cluster. In the case of mixed Ar-N2 clusters, the w 1Δu→ X 1Σg+ transitions were observed to occur in single N2 molecules in an Ar environment, in very much the same way as in Ar-N2 bulk samples. In N2 clusters, however, a band series was detected which, to our knowledge, was never observed earlier. In the spectra of Ar-N2 clusters, this series coexisted with the ''bulk''w 1Δu→ X1Σg+ 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. 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.