Broadband Spectroscopy Of Astrophysical Ice Analogues: IV. Optical Constants Of N2 Ice In The Terahertz And Mid-infrared Ranges

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 (N₂), 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 N₂ ice introduces uncertainties in radiative transfer models, snowline locations, and disk mass estimates.

Aims. We present direct measurements of the optical properties of N₂ 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. N₂ 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 N2 ice is quantified from ν=0.3–16~THz (λ=1~mm–18.75 μm). Resonant absorption peaks at νL=1.47 and 2.13~THz with damping constants γL=0.03 and 0.22~THz are attributed to optically active phonons of the α-N₂ crystal.

Conclusions. We provide a complete set of the THz–IR optical constants for 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.

F. Kruczkiewicz, A.A. Gavdush, F. Ribeiro, D. Campisi, A. Vyjidak, B.M. Giuliano, G.A. Komandin, S.V. Garnov, T. Grassi, P. Theulé, K.I. Zaytsev. A.V. Ivlev, Paola Caselli

Comments: Accepted for publication in A&A, 9 pages, 3 figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA); Instrumentation and Methods for Astrophysics (astro-ph.IM)
Cite as: arXiv:2601.03951 [astro-ph.EP] (or arXiv:2601.03951v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2601.03951
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Submission history
From: Franciele Kruczkiewicz
[v1] Wed, 7 Jan 2026 14:05:48 UTC (4,710 KB)
https://arxiv.org/abs/2601.03951
Astrobiology, Astrochemistry,