Broadband spectroscopy of astrophysical ice analogues: IV. Optical constants of N$_2$ ice in the terahertz and mid-infrared ranges

Abstract

Context. Understanding the optical properties of astrophysical ices is crucial for modeling dust continuum emission and radiative transfer in cold, dense interstellar environments. Molecular nitrogen (N$_2$), a major nitrogen reservoir in protoplanetary disks, plays a key role in nitrogen chemistry, yet the lack of direct terahertz (THz)--infrared (IR) optical constants for N$_2$ ice introduces uncertainties in radiative transfer models, snowline locations, and disk mass estimates. Aims. We present direct measurements of the optical properties of N$_2$ ice over a broad THz--IR spectral range using terahertz pulsed spectroscopy (TPS) and Fourier-transform infrared spectroscopy (FTIR), supported by density functional theory (DFT) calculations and comparison with literature data. Methods. N$_2$ ice was grown at cryogenic temperatures by gas-phase deposition onto a cold silicon window. The THz complex refractive index was directly reconstructed from TPS data, while the IR response was derived from FTIR measurements using Kramers--Kronig relations. The optical response was parameterized with a Lorentz dielectric model and validated by DFT calculations. Results. The complex refractive index of N$_2$ ice is quantified from $\nu = 0.3$--$16$~THz ($\lambda = 1$~mm--$18.75~\mu$m). Resonant absorption peaks at $\nu_\mathrm{L} = 1.47$ and $2.13$~THz with damping constants $\gamma_\mathrm{L} = 0.03$ and $0.22$~THz are attributed to optically active phonons of the $\alpha$-N$_2$ crystal. Conclusions. We provide a complete set of the THz--IR optical constants for \ce{N2} ice by combining TPS and FTIR spectroscopy. Our results have implications for future observational and modeling studies of protoplanetary disk evolution and planet formation.