Structural Studies of Nanocrystalline Nitrogen–Helium Solids by Raman Spectroscopy

Structural and thermal properties of nanocrystalline nitrogen–helium solids are studied by Raman spectroscopy at temperature range 1.8–41 K. The N₂ vibrational line possesses spectral structure very similar to what is observed in bulk solid nitrogen, indicating ordered structure inside the nanocrystallites. The spectral observations show that the structure of the solid is dependent on the N₂ content in the gas mixture used for sample preparation. Evidence for disordered nitrogen on the surfaces and interfaces of nanocrystallites can be extracted from the Raman spectra of the most diluted samples. Removing superfluid helium from the sample and annealing at 21 K did not affect the structure of the solid, whereas higher annealing temperatures yielded strong increase of density as evidenced by the up to ∼10-fold increase of Raman signal. The α-solid—β -solid transition, evidenced by a ∼0.8 cm⁻¹ shift in the peak position, was seen in the collapsed nitrogen samples approximately at the same temperature as in a solid nitrogen film.