Abstract
A method to predict the near-infrared spectra of amorphous solids by means of ab initio molecular dynamics is presented. These solids can simulate molecular ices. To test the method, mixtures of methane, water and nitrogen are generated as amorphous samples of various concentrations. The full theoretical treatment includes as a first step, the optimization of their geometrical structure for a range of densities, after which, the most stable systems are taken as initial structures for molecular dynamics, performed at 200 K in trajectories of 4 ps duration with a 0.2 fs time step. All the dynamics are carried out using the first principles method, solving the quantum problem for the electrons using density-functional theory (DFT), and integrating the DFT forces, following the Born-Oppenheimer dynamics. After the dynamics, near-IR spectra are predicted by the Fourier transform of the macroscopic polarization autocorrelation function. The calculated spectra are compared with the experimental spectra of ice mixtures of CH4 and H2O recorded in our laboratory, and with some spectra recorded by the New Horizons mission on Pluto.
Highlights
The near-IR, covering wavelengths from 0.8 to 2.5 mm approximately, corresponds to the absorption of overtones and combination bands of molecular vibrations
We explore in this paper the application of ab initio molecular dynamics (AIMD) calculations to predict the spectra of molecular ices in the near-IR region
Since we were mainly interested in dealing with weak spectral features arising from some 2nd order effects like overtones or combination bands, we chose ab initio molecular dynamics (AIMD) to try to use a better technique to follow the variation of interactions along the dynamics, without the constraint of parameterization
Summary
The near-IR, covering wavelengths from 0.8 to 2.5 mm approximately, corresponds to the absorption of overtones and combination bands of molecular vibrations. This is a interesting IR region for many reasons, one of them being its extended use for the observation of astrophysical ices in the solar system. Individual molecular components in this kind of ice formed by water plus non polar molecules are held together mainly by hydrogen bond interactions among the water molecules. They build up a sort of skeleton to which non polar components attach. The very nature of the interactions determines the methodological approach needed to describe the electron structure of the ices in the simulation of the spectra
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