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.

Pradeep Thalappil

Department Of Chemistry

Chemical process

CHEMICAL SPUTTERING

COLLISION EXPERIMENTS

HYPERTHERMAL ENERGIES

ION ENERGIES

Low energies

MOLECULAR MATERIALS

MOLECULAR SOLID

MOLECULAR SURFACES

Secondary ion mass spectrometry

Experiments

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IIT Madras

Chemical Reviews

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.

Journal of Physical Chemistry C

Interaction of Acetonitrile with Alcohols at Cryogenic Temperatures

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.

Diffusion and Crystallization of Dichloromethane within the Pores of Amorphous Solid Water

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). © 2015 American Chemical Society.

Fulltext

Interaction of acetonitrile with water-ice: An infrared spectroscopic study

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.

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/surface collisions in the ultralow- to low-energy (1-100-eV) window represent an excellent technique for investigation of the properties of condensed molecular solids at low temperatures. For example, this technique has revealed the unique physical and chemical processes that occur on the surface of ice, versus the liquid and vapor phases of water. Such instrument-dependent research, which is usually performed with spectroscopy and mass spectrometry, has led to new directions in studies of molecular materials. In this review, we discuss some interesting results and highlight recent developments in the area. We hope that access to the study of molecular solids with extreme surface specificity, as described here, will encourage investigators to explore new areas of research, some of which are outlined in this review. Copyright © 2013 by Annual Reviews.

Annual Review of Analytical Chemistry

Probing molecular solids with low-energy ions

The molecular ion of dihydrogen (H2+) is produced by 1 eV collisions of protons (H+) on amorphous water ice surfaces. The reaction is also observed on crystalline ice surfaces, but with lower efficiency. Collisions of D+ on amorphous H2O and D 2O ices yield D2+ on the former, subsequent to isotope exchange on the H2O surface. Ultra-low-energy collision-induced dihydrogen ion production is also observed from alkanol surfaces, with decreasing efficiency as the alkyl chain length increases. There is no corresponding reaction on solid hexane. This endothermic reaction, with implications for interstellar chemistry and plasma etching processes, is proposed to occur as a result of stabilization of the other reaction product, a hydroxyl radical (OH•), on water surfaces through hydrogen-bonding interactions with the surface. These results point to an interesting chemistry involving ultra-low-energy ions on molecular solids. © 2011 American Chemical Society.

Formation of H2+ by ultra-low-energy collisions of protons with water ice surfaces

We have investigated the interaction of n-butanol (NBA) with thin layers of water ice prepared in ultra high vacuum in the temperature range of 110-150 K. From the mass spectra of the chemically sputtered species, created upon the collision of low energy (≥30 eV) Ar+ ions, we study the process of diffusive mixing of NBA with water ice, at the molecular level. The results show that NBA undergoes diffusive mixing with H2O. Even after depositing 1000 monolayers (MLs) of amorphous solid water (ASW) over NBA, both the species are observed on the surface. However, when NBA is deposited over ASW, no water is seen on the surface above 3-5 MLs of NBA. This could be interpreted as the absence of diffusive mixing in this system or surface segregation of NBA, in view of its lower surface energy just as in the case of liquid alcohols. An isomeric alcohol, namely, tert-butyl alcohol (TBA), also behaves similarly. Although the presence of NBA and TBA is detected, in the presence of ASW, they undergo selective ionization, giving specific peaks in the mass spectrum. D2O behaves in a manner similar to that of H 2O. Preliminary experiments with other alcohols; namely, methanol, ethanol, and propanol were also done, and the results suggest that incomplete diffusion or surface segregation begins with propanol. © 2009 American Chemical Society.

Low energy ion scattering investigations of n-butanol-ice system in the temperature range of 110-150 K

In this paper, we investigated the interaction of simple carboxylic acids, formic acid (FA), acetic acid (AA), and propionic acid (PA) with thin layers of water ice in the temperature range of 110-190 K in ultrahigh vacuum. The focus, however, is on the AA-ice system. Molecularly thin layers of these systems were prepared on a pre-cooled polycrystalline copper substrate. The interactions and phase changes in the system were monitored with chemical sputtering using low-energy (≤30 eV) Ar+, which probes the topmost surface layers. At 110 K, the deposited AA exists as dimers in its amorphous solid form. At the same temperature, in the presence of water ice, this dimeric form gets converted to chainlike oligomers. Chemical sputtering spectra show distinct features for these two surface species. The data suggest that ion formation reflects the surface structure, implying a unique mechanism for its formation. Detailed studies have been made with amorphous solid water (ASW) and crystalline water (CW) to get a complete understanding of the system. Experiments carried out with AA-D2O ice confirmed the proton-transfer mechanism during chemical sputtering. Other studies were conducted with AA-CH3OH and AA-CCl4 systems. Detailed investigations performed to understand the effect of thickness of AA and ice overlayers suggest that the extent of water molecules required to effect the structural transformation in the acid is dependent on the amount of the latter. Dimeric-to-oligomeric transformation does not occur for the PA-ice system. Detection of a structural transition at the very top of ice in molecularly thin films adds additional capabilities to the low-energy ion scattering technique. © 2008 American Chemical Society.

Interaction of carboxylic acids and water ice probed by argon ion induced chemical sputtering

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 effect of ice substrate on the temperature dependent structural changes of formic acid. Infrared spectra of HCOOH@KBr, HCOOH@H 2O amorphous and HCOOH@H 2O crystalline were measured in the temperature range of 18-180 K (X@Y represents X deposited on Y). At 18 K formic acid deposited over ice exists principally as its dimer. Upon increase of temperature, the dimer gets converted to the polymeric crystalline form. In the case of amorphous ice surface, this conversion starts at a lower temperature of 37 K. For KBr and crystalline ice surfaces, this occurs at 98 and 83 K, respectively. The dangling -OH found on amorphous ice surfaces disappears completely upon formic acid deposition. The greater mobility of formic acid molecules on amorphous ice aids in the early onset of its crystallization. © 2004 Elsevier B.V. All rights reserved.

Journal	Chemical Reviews
ISSN	00092665
Open Access	No