Beatriz Martín-Llorente

A study of the interaction of CO2 with water ice

Aims. We studied the interaction between CO2 (guest) and H2O (host) molecular ices. Methods. Ices of CO2 and H2O are prepared by four di erent deposition techniques: sequential deposition (amorphous water ice followed by addition of CO2), co-deposition (both gases added simultaneously), inverse sequential deposition (carbon dioxide ice fol- lowed by addition of water) and crystalline sequential deposition (crystalline water ice is prepared first and CO2 is added afterwards). Samples are deposited at 80 K and are studied by temperature programmed desorption and transmission infrared spectroscopy. Results. Two slightly di erent varieties of association of CO2 and H2O are revealed from the di erent spectroscopic properties of the asymmetric stretching band of 12 CO2 and 13 CO2. The two varieties are found to co-exist in some of the samples at 80 K, whereas only the so-called internal CO2 remains after heating at 105 K. At 80 K carbon dioxide is able to adhere to a crystalline water ice surface. Activation energies for the desorption of CO2 from amorphous (Ed = 20:7 2 kJ mol 1 ) and crystalline (Ed = 19:9 2 kJ mol 1 ) water ice are derived from measurements of the sticking of CO2 as a function of ice temperature. Conclusions. These findings may have implications for the study of icy bodies of the Solar System.

Phases of Solid Methanol

The solid phases of methanol were investigated using IR spectroscopy and numerical calculations with the SIESTA method. Improved spectra are reported of amorphous methanol at 90 K, and in particular of the alpha and beta phases at 130 and 165 K, respectively, with assignments of bands not previously measured. The main features of the spectra of each phase are discussed and compared. A study of spectral changes with temperature leads to the conclusion that the metastable phase previously reported might be a mixture of the two known stable phases. Such a mixture could explain all spectral features observed in this investigation. The theoretical calculations provide reasonable agreement with the experimental data for most of the parameters, but predict H-bondings stronger than those observed. Differences between the spectra of the alpha and beta phases are predicted with similar characteristics to the experimental results.

Ices of CO2/H2O Mixtures. Reflection−Absorption IR Spectroscopy and Theoretical Calculations

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 approximately 1 microm 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.

Theoretical Study on Hydrogen-Bond Effects in IR Spectra of High- and Low-Temperature Phases of Nitric Acid Dihydrate

The low- and high-temperature phases (alpha and beta, respectively) of solid nitric acid dihydrate (NAD) are studied in depth by DFT methods. Each phase contains two types of complex structures (H(3)O(+)) x (H(2)O), designated A and B, with different hydrogen-bonding (HB) characteristics. The theoretical study reveals that type A complexes are weakly bound and could be described as (H(3)O)(+) and H(2)O aggregates, with decoupled vibrational modes, whereas in type B structures the proton is situated close to the centre of the O...O bond and induces strong vibrational coupling. The proton-transfer mode is predicted at quite different wavenumbers in each complex, which provides an important differentiating spectral feature, together with splitting of some bands in beta-NAD. Theoretical spectra are estimated by using two GGA parameterizations, namely, PBE and BLYP. The potential-energy surface for each type of HB in NAD is also studied, as is the spectral influence of displacement of the shared H atom along the O-O bond. The results are compared to literature infrared spectra recorded by different techniques, namely, transmission and reflection-absorption, with both normal and tilted incident radiation. This work provides a thorough assignment of the observed spectra, and predictions for some spectra not yet available. The usefulness of high-level theoretical calculations as performed herein to discriminate between two phases of a solid crystal is thus evidenced.