Infrared spectra of ice mixtures: Methane/Ethylene
Germán Molpeceres1(*), Miguel Angel Satorre2, Juan Ortigoso1, Carlos Millán2, Rafael
Escribano1, Víctor Herrero1, Isabel Tanarro1, Ramón Luna2, Belén Maté11

Instituto de Estructura de la Materia, IEM-CSIC, Serrano 123, 28006 Madrid, Spain
2
Escuela Politécnica Superior de Alcoy, UPV, 03801 Alicante, Spain)

Ethylene, a product of energetic processing of methane ices [1] is known to be an astromoleculepresent in our solar system [2]. In addition, methane is thought to be the most abundant ofhydrocarbons among those present in the surface of transneptunian objects (TNOs) [3]. Recently, theNew Horizons mission has provided abundant data of Pluto’s surface, and assignment of several
spectroscopic features associated with planetary ices is required.
In the present work, the second of a series dedicated to astrophysical relevant mixtures[4], a full setof spectroscopic data from 6000 cm-1 (1.6m) to 800 cm-1 (12.5m) has been obtained of ice mixturesat 30K of CH4/C2H4 in several proportions, namely 100/0, 75/25, 50/50, 25/75 and 0/100. Band
displacements, band strengths and imaginary and real components of the complex refractive index
have been determined for these materials in the spectral window under consideration. Along with thespectroscopic features, density and visible refractive index of these ices have also been measured.
For the study of the subtle spectroscopic effects arising from the mixtures with respect to the purespecies, we have built several theoretical models at the DFT level using the CASTEP code [5].
Simulation cells are randomly built using Montecarlo simulations and then, molecules in the cell arerelaxed until a minimum in the potential energy surface of the system is reached. Infrared spectra arecomputed at this minimum.
From our experiments we observe a deviation in several factors such as band positions and bandstrengths with respect to the pure species, as expected from previous works [6]. Band displacements
follow the trend expected for methane, in which a dilution in a non polar environment displaces themethane vibrations to lower wavenumbers with an increase in the wavenumbers of the second
molecule. An abrupt decrease of the density of ethylene in comparison with previous diffractometricmeasurements has also been observed [7]. Band strengths for methane also behave as in our previous
work, leading to decays in the total band strength with the dilution. However, for ethylene,
strengthening and weakening have been observed for different bands, leading to a complex
interpretation in which atomistic calculations play an important role.

References
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