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Ices of CO 2 /H 2 O Mixtures. Reflection−Absorption IR Spectroscopy and Theoretical Calculations

2008; American Chemical Society; Volume: 112; Issue: 3 Linguagem: Inglês

10.1021/jp0769983

1520-5215

Belén Maté, Óscar Gálvez, Beatriz Martín-Llorente, M. Moreno, Vı́ctor J. Herrero, Rafael Escribano, Emilio Artacho,

Spectroscopy and Laser Applications

Ice mixtures of CO2 and H2O are studied using Fourier transform reflection−absorption infrared (RAIR) spectroscopy. Mixtures are prepared by sequential deposition or co-deposition of the two components from the gas phase onto an Al plate kept at 87 K inside a low-pressure chamber. Two CO2 structures are found in most experiments: a crystalline form similar to pure CO2, which evaporates when warming at 105 K, and a noncrystalline species which remains embedded in amorphous water ice after warming. Significant spectral variations are found depending on the deposition method and the thickness of the solid. Features characteristic of the RAIR technique appear in the spectral regions of the normal modes of the bending and asymmetric stretching CO2 vibrations. Simulations using Fresnel theory and Mie scattering are carried out with acceptable agreement with the experimental spectra of solids of variable thickness, from ∼1 μm to the limit of nanoparticles. Theoretical calculations of a pure CO2 crystal are performed. The relaxed geometry of the solid and its vibrational spectrum are determined and compared to the experimental results.