SignificanceFormation of clathrate hydrate (CH) requires high pressures and moderate temperatures, which enable their existence in marine sediments and the permafrost region of earth. The presence of CHs in interstellar medium (ISM) is still in question due to the extreme high vacuum and ultracold conditions present there. Here, we conclusively identified methane and carbon dioxide hydrates in conditions analogous to ISM. We found that molecular mobility and interactions play crucial roles in the formation of CHs, even though there is no external pressure to force cage formation. Various chemical processes on these hydrates in ISM may lead to relevant prebiotic molecules.

Interaction of water-ice and acetonitrile has been studied at low temperatures in their codeposited mixtures, in ultrahigh vacuum conditions. They interact strongly at low temperatures (in the temperature range of 40−110 K), which was confirmed from the new features manifested in the reflection absorption infrared spectra of the mixtures. This interaction was attributed to strong hydrogen bonding which weakens upon warming as the acetonitrile molecules phase segregate from water-ice. Complete phase separation was observed at 130 K prior to desorption of acetonitrile from the water-ice matrix. Such a hydrogen-bonded structure is not observed when both the molecular solids are deposited as water on acetonitrile or acetonitrile on water overlayers. A quantitative analysis shows that in a 1:1 codeposited mixture, more than 50% acetonitrile molecules are hydrogen bonded with water-ice at low temperatures (40−110 K).

Extremely surface specific information, limited to the first atomic layer of molecular surfaces, is essential to understand the chemistry and physics in upper atmospheric and interstellar environments. Ultra low energy ion scattering in the 1-10 eV window with mass selected ions can reveal extremely surface specific information which when coupled with reflection absorption infrared (RAIR) and temperature programmed desorption (TPD) spectroscopies, diverse chemical and physical properties of molecular species at surfaces could be derived. These experiments have to be performed at cryogenic temperatures and at ultra high vacuum conditions without the possibility of collisions of neutrals and background deposition in view of the poor ion intensities and consequent need for longer exposure times. Here we combine a highly optimized low energy ion optical system designed for such studies coupled with RAIR and TPD and its initial characterization. Despite the ultralow collision energies and long ion path lengths employed, the ion intensities at 1 eV have been significant to collect a scattered ion spectrum of 1000 counts/s for mass selected CH2(+).

Dichloromethane (CH2Cl2) thin films deposited on Ru(0001) at low temperatures (∼80 K or lower) undergo a phase transition at ∼95 K, manifested by the splitting of its wagging mode at 1265 cm–1, due to factor group splitting. This splitting occurs at relatively higher temperatures (∼100 K) when amorphous solid water (ASW) is deposited over it, with a significant reduction in intensity of the high-wavenumber component (of the split peaks). Control experiments showed that the intensity of the higher wavenumber peak is dependent on the thickness of the water overlayer. It is proposed that diffusion of CH2Cl2 into ASW occurs and it crystallizes within the pores of ASW, which increases the transition temperature. However, the dimensions of the CH2Cl2 crystallites get smaller with increasing thickness of ASW with concomitant change in the intensity of the factor group split peak. Control experiments support this suggestion. We propose that the peak intensities can be correlated with the porosity of the ice film. Diffusion of CH2Cl2 has been supported by low-energy Cs+ scattering and temperature-programmed desorption spectroscopies.

The phase transition of solid propane and a propane-water mixture under ultrahigh vacuum has been investigated using reflection absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption mass spectrometry (TPD-MS). Here, the investigation is divided into two sections: the phase transition of pure propane and the interaction of propane with water. RAIR spectra of pure propane reveal an unknown crystalline phase at 50 K (phase I), which gradually converts to a known crystalline phase (phase II) at higher temperature. This conversion is associated with certain kinetics. Co-deposition of water and propane restricts the amorphous to crystalline phase transition, while sequential deposition (HO@CH; propane over predeposited water) does not hinder it. For an alternative sequential deposition (CH@HO; water over predeposited propane), the phase transition is hindered due to diffusional mixing within the given experimental time, which is attributed to the reason behind the restricted phase transition.