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

TL;DR

This study provides the first direct measurements of the optical constants of N$_2$ ice across a broad THz to IR range, crucial for modeling astrophysical environments like protoplanetary disks.

Contribution

We present comprehensive optical constants of N$_2$ ice from 0.3 to 16 THz using TPS and FTIR, supported by DFT calculations, filling a key data gap for astrophysical modeling.

Findings

Identified resonant absorption peaks at 1.47 and 2.13 THz.

Quantified the complex refractive index of N$_2$ ice.

Validated optical response with DFT calculations.

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.