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. © 2016 American Chemical Society.

Pradeep Thalappil

Department Of Chemistry

Radha Gobinda Bhuin

Amorphous materials

Dichloromethane

TEMPERATURE PROGRAMMED DESORPTION

AMORPHOUS SOLID WATER

Control experiments

Low temperatures

Peak intensity

SPLIT PEAK

TEMPERATURE PROGRAMMED DESORPTION SPECTROSCOPY

WAGGING MODES

WAVE NUMBERS

Diffusion

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IIT Madras

Journal of Physical Chemistry C

Clathrate hydrates (CHs) typically nucleate under high-pressure conditions, but their existence in ultrahigh vacuum (UHV) is an open question. Here, we report the formation of tetrahydrofuran (THF) hydrate in UHV, using reflection absorption infrared spectroscopy (RAIRS). Annealing both sequentially and co-deposited mixtures of THF and H2O to 130 K for adequate time, originally prepared at 10 K, led to the formation of THF hydrate, at 10-10 mbar. Nucleation of THF hydrate was associated with the crystallization of amorphous solid water. Crystallization kinetics was examined through isothermal kinetic measurements using RAIRS in the temperature range of 120-130 K. The kinetic measurements revealed that the THF hydrate formation was a diffusion-controlled process and the overall activation energy for the process was found to be ∼23.12 kJ mol-1. This considerably lower activation energy as compared to that for the crystallization of pure ice established the spontaneity of the process. The results provide valuable insights into the low-pressure characteristics of CHs and associated thermodynamics. © 2019 American Chemical Society.

Spontaneous Formation of Tetrahydrofuran Hydrate in Ultrahigh Vacuum

Temperature-dependent interaction of acetonitrile with methanol and ethanol, as codeposited and sequentially deposited films, was studied in the 10-130 K window, in ultra high vacuum. Films in the range of 50-100 monolayers were investigated using temperature-dependent reflection-absorption infrared spectroscopy (RAIRS), Cs+ ion scattering mass spectrometry, and temperature-programmed desorption (TPD). Acetonitrile interacts with methanol and ethanol through intermolecular hydrogen bonding. When a codeposited system was annealed, acetonitrile underwent a phase segregation at 110 K, and large changes in the infrared spectrum were observed. The OH stretching of methanol gave two peaks characteristic of the change to the α-phase of methanol, while ethanol gave three peaks at the same temperature. The surface composition of the systems probed by 40 eV Cs+ scattering showed that both the alcohols and the acetonitrile were of equal intensity below 110 K, while above 110 K the intensity of the latter went down substantially. We find that the presence of acetonitrile does not allow ethanol to undergo complete phase transition prior to desorption, while methanol could do so. This behavior is explained on the basis of the size, extent of hydrogen bonding, and phase transition temperature, of the two alcohols. Additional peaks in the hydroxyl region observed in alcohols in the 110-130 K window may be used as a signature of the presence of acetonitrile mixed with alcohol, especially ethanol, and hence this may be used in observational studies of such molecular environments. © 2017 American Chemical Society.

Interaction of Acetonitrile with Alcohols at Cryogenic Temperatures

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 CH 2+. © 2014 AIP Publishing LLC.

Fulltext

Review of Scientific Instruments

Development of ultralow energy (1-10 eV) ion scattering spectrometry coupled with reflection absorption infrared spectroscopy and temperature programmed desorption for the investigation of molecular solids

Ion scattering using ultralow energy projectiles is considered to be a unique method to probe the nature of molecular surfaces because of its capacity to probe the very top, atomically thin layers. Here, we examine one of the most studied molecular solids, water-ice, using this technique. When ice surface undergoes the amorphous to crystalline transition, an ultralow energy reactive projectile identifies the change through the reaction product formed. It is shown that ultralow energy (2, 3, 4, 5, 6, and 7 eV) CH2+ (or CD2+) collision on amorphous D2O (or H 2O) ice makes CHD+, while crystalline ice does not. The projectile undergoes H/D exchange with the dangling -OD (-OH) bond present on amorphous ice surfaces. It is also shown that H/D exchange product disappears when amorphous ice is annealed to the crystalline phase. The H/D exchange reaction is shown to be sensitive only to the surface layers of ice as it disappears when the surface is covered with long chain alcohols like 1-pentanol as the ice surfaces become inaccessible for the incoming projectile. This article shows that ultralow energy reactive ion collision is a novel method to distinguish phase transitions in molecular solids. © 2013 American Chemical Society.

Distinguishing amorphous and crystalline ice by ultralow energy collisions of reactive ions

A study was conducted to demonstrate low-energy ionic collisions at molecular solids. The investigations were limited to molecular surfaces and associated chemical processes occurring during impact of a hyperthermal energy ion. The term 'molecular solid' comprised the wide range of molecular materials that were used in ion/surface collision experiments. The energy regime considered in the investigations lied in an intermediate range. Primary ion energies of several kiloelectronvolts used in a secondary ion mass spectrometry (SIMS) experiment was considered in the case of condensed molecular solids to allow correlation with the results of chemical sputtering (CS) or other low-energy sputtering (LES) experiments.

Chemical Reviews

Low-energy ionic collisions at molecular solids

In this work, we have examined the diffusive mixing of chloromethanes in different molecular solids H2O, D2O, and CH3OH by monitoring their chemical sputtering spectra due to the impact of Ar + ions in the collision energy range of 3-60 eV, focusing on amorphous solid water. The chemical sputtering spectra have been monitored over the temperature window accessible by liquid nitrogen, and the coverages of the molecules of interest and ice have been varied from one to several hundred monolayers. Instrumentation and sensitivity of the technique have been discussed. It is found that while the diffusion of CCl4 in the molecular solids investigated is hindered, other choloromethanes such as CHCl3 and CH2Cl2 undergo diffusive mixing over the same temperature range. Quantitatively, while ∼4 monolayers (ML) of ice are found to block CCl4 diffusion, the numbers are ∼250 and ∼600 ML for CHCl3 and CH2Cl2, respectively. Crystallinity of ice does not have any effect on the diffusivity of water molecules when it is deposited below the chloromethanes. The effect of substrate was insignificant, and the rise in temperature increased diffusive mixing wherever the process was observed at a lower temperature. © 2007 American Chemical Society.

Probing difference in diffusivity of chloromethanes through water ice in the temperature range of 110-150 K

We have investigated the mobility of bromoform (CHBr3) and chloroform (CHCl3) on amorphous solid water and crystalline ice surfaces, by monitoring their adsorption and desorption behavior using temperature programmed desorption spectroscopy and reflection absorption infrared spectroscopy. Up to its desorption temperature, of 140 K, CHCl 3 does not diffuse over the crystalline ice surface, whereas CHBr3 is found to be mobile at temperatures as low as 85 K. The results demonstrate distinct differences between the surface mobility of structurally similar haloform molecules on crystalline ice surfaces, which may have implications to the halocarbon chemistry occurring on atmospheric ice particles. © 2004 Elsevier B.V. All rights reserved.

Journal	Data powered by TypesetJournal of Physical Chemistry C
Publisher	Data powered by TypesetAmerican Chemical Society
ISSN	19327447
Open Access	No