Infrared Absorption Of Solid/Liquid Molecular Nitrogen At Near-triple Point Temperatures

Nitrogen molecules（N₂） are the most in the atmosphere gas on the earth. N₂ is one of an idea research objects for investigating molecular system due to its simplicity in theoretical calculations and the stability of triple bonds between two N atoms in it. In case of the homonuclear diatomic molecule such as N₂, a transition between internal vibrations is rigorously forbidden at standard condition under the pressure of 1 bar, at room temperature, and with gaseous state due to its high symmetry. That is to say N₂ is infrared inactive. In the condensed-matter state at low temperature, however, the infrared activity of N₂ can be induced by intermolecular interaction such as quadrupole moments or electron overlaps, which allows us to obtain the infrared absorption spectra of N₂ for investigating their physical properties and exploring new condensed-matter physical phenomena. The study on the infrared absorption of solid/liquid molecular nitrogen at near-triple point temperatures not only deepens our understanding of the physical properties of condensed N₂, but also allows us to extend and explore the application of infrared absorption of N₂. According to literatures, for example, a high-energy-density material may be acquired if the single-bond nitrogen in the high pressure can be metastably decompressed to ambient pressure.All experiments involved in this thesis are based on the home-made instruments including the low-temperature solid/liquid molecular preparation system and the infrared measurement apparatus designed and made at research center of laser fusion（LFRC） of China academy of engineering physics（CAEP）. The main works in this thesis are as follows:（1） Preparation and infrared absorption of liquid N₂. Liquid N₂ samples was prepared first and the phase transition process from gas to liquid for N₂ was observed and studied by the backlit shadowgraphy technology. The infrared absorption spectra of samples at the low temperature were in-situ measured at different temperatures. The spectra show that the absorption intensity is increase with decreasing temperature due to an increase in the density of liquid N₂.（2） Preparation and infrared absorption of solid N₂. The preparation process of smooth, uniform, and transparent N₂ solids were grown out of the liquid phase at temperatures near the N₂ triple point. The infrared absorption spectra of the samples were measured and a broad absorption band was observed from 2222 to 2439 cm-1 with the strongest peak at 2293 cm-1. This has been well explained theoretically on the basis of the ground-state vibration and rotation as well as their coupling at low temperatures within the framework of anharmonic approximation.（3） Vibrational properties of solid-state N₂. The vibrational frequencies of an isolated N₂ and solid N₂ with the α-N₂ crystal structures were calculated and compared. The results indicate that the vibrational frequencies of N₂ in the solid phase are higher than the one of an isolated N₂ due to the collective effect. The N₂ vibrational frequencies inside the N₂ solid are always higher than those of N₂ at the surfaces of the N₂ solid. This results mainly from the differently local coordination environments. Moreover, the bond forces between molecules at the solid surfaces are weaker due to a smaller number of molecules interacting with each other, which results in a lower vibrational frequency than those inside the solid.