COHERENT RAMAN-SPECTROSCOPY OF NITROGEN MOLECULES AND CLUSTERS IN SUPERSONIC JETS

Coherent Stokes and anti-Stokes Raman scattering CSRS and CARS have been employed to study the spectroscopy of nitrogen molecules and clusters in the expansion of a supersonic jet. In the vibrational spectrum, at strong stagnation conditions, an intense redshifted peak is observed which can be assigned to the intramolecular vibrations in large N2 clusters having adopted the beta-phase structure. Another weak feature is assigned to nitrogen clusters in the alpha-phase. In the rotational region of the spectrum only monomer features have been observed. The failing to observe librational motions is consistent with the finding that the nitrogen clusters are predominantly in the orientationally unordered beta-phase. The low rotational temperature suggests supercooling of the beta-phase.