arXiv subjects

A. A. Gavdush

Publications and source records attributed to A. A. Gavdush.

5 recordsLinked to original sources

arXiv2026

Optical constants of Ih, Ic, and amorphous H$_2$O ices in the THz and IR ranges

Direct measurements of optical constants in the THz spectral region for astrophysically relevant H$_2$O ice samples are scarce. Extrapolation of optical properties in the THz spectral region from IR data can introduce uncertainties into astrophysical models. We measured the optical properties of water ice samples in the Ih and Ic forms as well as amorphous solid water (ASW) in the THz region in order to derive broad optical constants using literature and experimental data in the THz-IR range. In our experiments, the Ih, Ic, and ASW ices were grown by vapour deposition onto a cold substrate and measured by THz pulsed spectroscopy. Their THz optical properties were retrieved, compared with the THz-IR literature data, and approximated using the multiple-Lorentz model. From the existing literature data on the Ih, Ic, and ASW ices, we selected samples with the highest optical constants and classified them as compact. Their optical properties were merged in the frequency range of $ν= 0.3$-$120$~THz (the wavelength range of $λ= 1$~mm-2.5$~μ$m). The underlying absorption bands were attributed to vibrational modes and approximated using the multiple-Lorentz model while accounting for anharmonicity. Discrepancies primarily arising in low-absorption regions between the experimental data and broadband models were attributed to factors such as the model's complexity and the baseline-subtraction procedure. The THz response of all ices is formed by the low-frequency wings of the IR bands and the single broad low-intense THz peak around $1.8$~THz, which is very similar for all phases. The opacity calculation for dust grains covered by H$_2$O ice mantles based on experimental data shows discrepancies with data derived by extrapolation. The inferred THz-IR optical constants of water ice are important for future observations and modelling of cold clouds and protoplanetary disks.

astro-ph.GA↗

arXiv2026

Broadband spectroscopy of astrophysical ice analogues: IV. Optical constants of N$_2$ 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 (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 $ν= 0.3$--$16$~THz ($λ= 1$~mm--$18.75~μ$m). Resonant absorption peaks at $ν_\mathrm{L} = 1.47$ and $2.13$~THz with damping constants $γ_\mathrm{L} = 0.03$ and $0.22$~THz are attributed to optically active phonons of the $α$-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.

astro-ph.EP↗

arXiv2025

Broadband spectroscopy of astrophysical ice analogues: III. Scattering properties and porosity of CO and CO$_2$ ices

$Context.$ The quantification of the terahertz (THz) and IR optical properties of astrophysical ice analogs, which have different molecular compositions, phases, and structural properties, is required to model both the continuum emission by the dust grains covered with thick icy mantles and the radiative transfer in the dense cold regions of the interstellar medium. $Aims.$ We developed a model to define a relationship between the THz$-$IR response and the ice porosity. It includes the reduced effective optical properties of porous ices and the additional wave extinction due to scattering on pores. The model is applied to analyze the measured THz$-$IR response of CO and CO$_2$ laboratory ices and to estimate their scattering properties and porosity. $Methods.$ Our model combines the Bruggeman effective medium theory, the Lorentz-Mie and Rayleigh scattering theories, and the radiative transfer theory to analyze the measured THz$-$IR optical properties of laboratory ices. $Results.$ We apply this model to show that the electromagnetic-wave scattering in studied laboratory ices occurs mainly in the Rayleigh regime at frequencies below 32 THz. We conclude that pores of different shapes and dimensions can be approximated by spheres of effective radius. By comparing the measured broadband response of our laboratory ices with those of reportedly compact ices from earlier studies, we quantify the scattering properties of our CO and CO$_2$ ice samples. Their porosity is shown to be as high as 15% and 22%, respectively. Underestimating the ice porosity in the data analysis leads to a proportional relative underestimate of the THz$-$IR optical constants. $Conclusions.$ The scattering properties and porosity of ices have to be quantified along with their THz$-$IR response in order to adequately interpret astrophysical observations.

astro-ph.IM↗

arXiv2022

Broadband spectroscopy of astrophysical ice analogues: II. Optical constants of CO and CO$_2$ ices in the terahertz and infrared ranges

Context: Broadband optical constants of astrophysical ice analogues in the infrared (IR) and terahertz (THz) ranges are required for modeling the dust continuum emission and radiative transfer in dense and cold regions, where thick icy mantles are formed on the surface of dust grains. Aims: In this paper, the THz time-domain spectroscopy (TDS) and the Fourier-transform IR spectroscopy (FTIR) are combined to study optical constants of CO and CO$_2$ ices in the broad THz-IR spectral range. Methods: The measured ices are grown at cryogenic temperatures by gas deposition on a cold Si window. A method to quantify the broadband THz-IR optical constants of ices is developed based on the direct reconstruction of the complex refractive index of ices in the THz range from the TDS data, and the use of the Kramers-Kronig relation in the IR range for the reconstruction from the FTIR data. Uncertainties of the Kramers-Kronig relation are eliminated by merging the THz and IR spectra. The reconstructed THz-IR response is then analyzed using classical models of complex dielectric permittivity. Results: The complex refractive index of CO and CO$_2$ ices deposited at the temperature of $28$ K is obtained in the range of 0.3--12.0 THz. Based on the measured dielectric constants, opacities of the astrophysical dust with CO and CO$_2$ icy mantles are computed. Conclusions: The developed method can be used for a model-independent reconstruction of optical constants of various astrophysical ice analogs in a broad THz-IR range. Such data can provide important benchmarks to interpret the broadband observations from the existing and future ground-based facilities and space telescopes. The reported results will be useful to model sources that show a drastic molecular freeze-out, such as central regions of prestellar cores and mid-planes of protoplanetary disks, as well as CO and CO$_2$ snow lines in disks.

astro-ph.SR↗

arXiv2019

Broadband spectroscopy of astrophysical ice analogs. I. Direct measurement of complex refractive index of CO ice using terahertz time-domain spectroscopy

Context: Reliable, directly measured optical properties of astrophysical ice analogs in the infrared (IR) and terahertz (THz) range are missing. These parameters are of great importance to model the dust continuum radiative transfer in dense and cold regions, here thick ice mantles are present, and are necessary for the interpretation of future observations planned in the far-IR region. Aims: Coherent THz radiation allows direct measurement of the complex dielectric function (refractive index) of astrophysically relevant ice species in the THz range. Methods: The time-domain waveforms and the frequency-domain spectra of reference samples of CO ice, deposited at a temperature of 28.5 K and annealed to 33 K at different thicknesses, have been recorded. A new algorithm is developed to reconstruct the real and imaginary parts of the refractive index from the time-domain THz data. Results: The complex refractive index in the wavelength range of 1 mm - 150 $μ$m (0.3 - 2.0 THz) has been determined for the studied ice samples, and compared with available data found in the literature. Conclusions: The developed algorithm of reconstructing the real and imaginary parts of the refractive index from the time-domain THz data enables, for the first time, the determination of optical properties of astrophysical ice analogs without using the Kramers-Kronig relations. The obtained data provide a benchmark to interpret the observational data from current ground based facilities as well as future space telescope missions, and have been used to estimate the opacities of the dust grains in presence of CO ice mantles.

astro-ph.IM↗