Terahertz Time-Domain Spectroscopy of Astrophysical Ices

This presentation will be on the status of the project “terahertz time-domain spectroscopy (THz-TDS) of astrophysical ices”, being carried at University of Alabama at Birmingham. This project aims at generating a laboratory-based library of THz absorptions in ices, which is useful to analyze the data returns from THz telescopes Herschel, SOFIA, and ALMA. The first part of the presentation will focus on the THz-TDS of Nitrogen ices in 10 K-30 K and the second part focuses on the high frequency resolution THz-TDS of layered materials. A THz-TDS setup using ZnTe crystals operating in 0.2 THz-2 THz was built for this project. Nitrogen ices were grown on a sapphire substrate in a cryostat by vapor deposition at 10K. The Nitrogen ices showedabsorption features around 1.46 THz at 10 K. ATheory of phonons suggests that the absorption could be due to the lower frequency Tu symmetric transverse optical phonon of fcc crystal of Nitrogen ice. We have fitted the temperature dependent absorption central frequency to a theory and obtained the Lennard-Jones parameters of the intermolecular potential of Nitrogen ice. For the demonstration of the high frequency resolution THz-TDS, “air-teflon-air-quartz-air” as sample system and “nitrogengas-teflon-nitrogengas-quartz-nitrogengas” as reference system were chosen. The characteristic matrix formulation was used to model the THz transmissions through the sample and reference systems.  A function involving the experimental transmission coefficient and theoretical transmission coefficient was optimized to obtain the high frequency resolution THz optical constants of air. The measured THz absorption central frequencies and widthswere consistent with the literature indicating the validity of characteristic matrix method for high resolution THz-TDS of layered materials.