  Cathodoluminescence VUV spectra of free pure nitrogen clusters produced by
condensation of gas mixtures in supersonic jets expanding into vacuum were
studied. The clusters were of icosahedral structure, as evidenced by our
electron diffraction measurements, i.e., quasicrystals, and contained about
1000 atoms. The cluster temperature was about 40 K. The luminescence spectra
measured in the range of 45,000-75,000 cm^-1 clearly showed three vibronic w ->
X series of transitions from the excited 'w' to the ground 'X' state, which are
intrinsic to nitrogen molecules. All the transitions revealed a strong
dependence of the matrix shift of their peak positions on vibration quantum
number, while for bulk samples of N2 the matrix shift is known to remain
unchanged. Our analysis showed that in clusters the vibrational anharmonicity
parameter changes drastically for both the upper and lower states, while the
change in the vibrational frequencies is much less prominent. The results
obtained provide information about an abnormally strong influence of cluster
environment on the nonlinear contribution to the vibrations of nitrogen
molecules.
