Broadband spectroscopy of astrophysical ice analogues

IV. Optical constants of N₂ ice in the terahertz and mid-infrared ranges

¹ Max-Planck-Institut für Extraterrestrische Physik, Gießenbachstraße 1, Garching 85748, Germany
² Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
³ Prokhorov General Physics Institute of the Russian Academy of Sciences, 119991 Moscow, Russia
⁴ Institute for Theoretical Chemistry, University of Stuttgart, Pfaffenwaldring 55, Stuttgart, Germany
⁵ Department of Engineering, University of Perugia, Via Duranti 93, 06125 Perugia, Italy
⁶ Federal Institute of Education, Science and Technology of Rio de Janeiro (IFRJ), Nilópolis Campus, Rio de Janeiro, Brazil

^★★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 11 August 2025
Accepted: 29 December 2025

Abstract

Context. 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 N₂ ice introduces uncertainties in radiative transfer models, snow-line locations, and disk mass estimates.

Aims. We present direct measurements and analysis of the optical properties of N₂ 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 N₂ ice are supported by density functional theory (DFT) calculations. The consistency of our measurements and calculations with datasets from the literature is also assessed.

Methods. 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 N₂ 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.

Results. The complex refractive index of N₂ ice is quantified in the frequency range v = 0.3–16 THz (the wavelength range λ = 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_L = 1.47 and 2.13 THz; the damping constants of γ_L = 0.03 and 0.22 THz, respectively, are attributed to the well-known optically active phonons of the α-N₂ crystal.

Conclusions. We provide a complete set of 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.