Broadband spectroscopy of astrophysical ice analogues. IV. Optical constants of N_2 ice in the terahertz and mid-infrared ranges

Understanding the optical properties of astrophysical ices is crucial for modeling dust continuum emission and radiative transfer in dense, cold interstellar environments. Molecular nitrogen, a primary carrier of N in protoplanetary disks, plays a key role in the formation of nitrogen-bearing species. However, the lack of direct measurements of the terahertz (THz) to infrared (IR) optical constants of ice introduces uncertainties in radiative transfer models, snow-line locations, and disk mass estimates. N2 We present direct measurements and analysis of the optical properties of ice across a broad THz--IR spectral range by combining THz pulsed spectroscopy (TPS) and Fourier-transform IR (FTIR) spectroscopy. The observed optically active THz vibrational modes of ice are supported by density functional theory (DFT) calculations. The consistency of our measurements and calculations with datasets from the literature is also assessed. N2 N2 N2 ice was grown at cryogenic temperatures via gas-phase deposition onto a cold silicon window. The optical properties of the ice samples were quantified using our earlier-reported method: it involves the direct reconstruction of the THz complex refractive index from the TPS data, combined with the derivation of the IR response from the FTIR data using the Kramers-Kronig relations. The ice response was parameterized using the Lorentz model of complex dielectric permittivity, which was verified with our DFT calculations and compared with the literature data. N2 The complex refractive index of ice is quantified in the frequency range ν = 0.3--16 THz (the wavelength range łambda = 1 mm--18.75 μm), and was compared with the DFT results as well as with the available literature data. The observed resonant absorption peaks at ν_ = 1.47 and 2.13 THz; the damping constants of γ_ = 0.03 and 0.22 THz, respectively, are attributed to the well-known optically active phonons of the α- crystal. N2 L L N2 We provide a complete set of THz--IR optical constants for ice by combining TPS and FTIR spectroscopy. Our results have implications for future observational and modeling studies of protoplanetary disk evolution and planet formation. N2