Hama and Watanabe examine the surface processes on interstellar amorphous solid water with regard to adsorption, diffusion, tunneling reactions and nuclear-spin conversion.
The anomalously low ortho-to-para ratios (OPRs) exhibited by gaseous water in space have been used to determine the formation temperature (< 50 kelvin) of ice on cold interstellar dust. This approach ...assumes that the OPR of water desorbed from ice is related to the ice formation temperature on the dust. However, we report that water desorbed from ice at 10 kelvin shows a statistical high-temperature OPR of 3, even when the ice is produced in situ by hydrogenation of O₂, a known formation process of interstellar water. This invalidates the assumed relation between OPR and temperature. The necessary reinterpretation of the low OPRs will help elucidate the chemical history of interstellar water from molecular clouds and processes in the early solar system, including comet formation.
Infrared multiple‐angle incidence resolution spectrometry (IR‐MAIRS) is a recently developed spectroscopic technique that combines oblique incidence transmission measurements and chemometrics ...(multivariate analysis) to obtain both pure in‐plane (IP) and out‐of‐plane (OP) vibration spectra for a thin sample. IR‐MAIRS is established for analyzing the molecular orientation of organic thin films at atmospheric pressure, but it should also be powerful for the structural characterization of vapor‐deposited thin samples prepared in a vacuum. The application of IR‐MAIRS to vapor‐deposited amorphous water is particularly interesting in the fields of physical and interstellar chemistry, because it is a representative model material for interstellar icy dust grains. We recently developed an experimental setup for in situ IR‐MAIRS under low‐temperature, ultrahigh‐vacuum conditions, which thus facilitates measurements of interstellar ice analogs such as vapor‐deposited amorphous water. This review considers the theoretical framework of IR‐MAIRS and our recent experimental results for vapor‐deposited amorphous water. We present spectroscopic signatures for the perpendicular orientation of dangling‐OH bonds for three‐coordinated water molecules at the surface of amorphous water at 90 K. The absolute absorption cross section of the three‐coordinated dangling‐OH bonds is quantitatively measured as 1.0 ± 0.2 × 10−18 cm2 at 3696 cm−1. As IR‐MAIRS can essentially be conducted using only a Fourier‐transform IR spectrometer and an angle‐controllable linear polarizer, it is a useful, low‐cost, and simple spectroscopic technique for studying laboratory analogs of interstellar ices including vapor‐deposited amorphous water.
Infrared multiple‐angle incidence resolution spectrometryfor vapor‐deposited amorphous water
We investigate the evolution of the ortho-to-para ratio of overall (gas + ice) via the nuclear spin conversion on grain surfaces coated with water ice under physical conditions that are relevant to ...star- and planet-forming regions. We utilize the rate equation model that considers adsorption of gaseous on grain surfaces, which have a variety of binding sites with a different potential energy depth, thermal hopping, desorption, and the nuclear spin conversion of adsorbed . It is found that the spin conversion efficiency depends on the gas density and the surface temperature. As a general trend, enhanced gas density reduces the efficiency, while the temperature dependence is not monotonic; there is a critical surface temperature at which the efficiency is the maximum. At low temperatures, the exchange of gaseous and icy is inefficient (i.e., adsorbed does not desorb and hinders another gaseous to be adsorbed), while at warm temperatures, the residence time of on surfaces is too short for the spin conversion. Additionally, the spin conversion becomes more efficient with lowering the activation barriers for thermal hopping. We discuss whether the spin conversion on surfaces can dominate over that in the gas phase in star- and planet-forming regions. Finally, we establish a simple, but accurate way to implement the spin conversion on grain surfaces in existing gas-ice astrochemical models.
Water has two nuclear-spin isomers: ortho- and para-H2O. Some observations of interstellar space and cometary comae have reported the existence of gaseous H2O molecules with anomalous ortho-to-para ...ratios (OPRs) less than the statistical value of three. This has been often used to estimate the formation temperature of ice on dust, which is inferred to be below 50 K. The relation between the nuclear-spin dynamics of H2O in ice at low temperatures and the OPR of gaseous H2O desorbed from the ice has yet to be explored in a laboratory. Consequently, the true meaning of the observed OPRs remains debated. We measure the OPR of H2O photodesorbed from ice made from para-H2O monomers at 11 K, which was prepared by the sublimation of Ne from a para-H2O/Ne matrix. The photodesorbed H2O molecules from the ice have the statistical OPR value of three, demonstrating the immediate nuclear-spin-state mixing of H2O toward the statistical value of ice even at 11 K. The OPR of H2O thermally desorbed from the ice also shows the expected statistical value. Our results indicate that the OPR of H2O desorbed from interstellar ice should be the statistical value regardless of the formation process of the ice, which cannot be used to deduce the ice-formation temperature. This study highlights the importance of interstellar gas-phase processes in understanding anomalous abundance ratios of nuclear-spin isomers of molecules in space.
The air–water interface plays central roles in “on-droplet” synthesis, living systems, and the atmosphere; however, what makes reactions at the interface specific is largely unknown. Here, we ...examined carbocationic reactions of monoterpene (C10H16 isomer) on an acidic water microjet by using spray ionization mass spectrometry. Gaseous monoterpenes are trapped in the uppermost layers of a water surface via proton transfer and then undergo a chain-propagation reaction. The oligomerization pathway of β-pinene (β-P), which showed prompt chain-propagation, is examined by simultaneous exposure to camphene (CMP). (CMP)H+ is the most stable isomer formed via rearrangement of (β-P)H+ in the gas phase; however, no co-oligomerization was observed. This indicates that the oligomerization of (β-P)H+ proceeded via ring-opening isomerization. Quantum chemical calculations for carbocation–(H2O) n=1,2 complexes revealed that the ring-opened isomer is stabilized by hydrogen−π bonds. We propose that partial hydration is a key factor that makes the interfacial reaction unique.
Abstract Infrared (IR) spectra of vapor-deposited amorphous water at low temperatures show two weak peaks at around 3720 and 3696 cm −1 assigned to free-OH stretching modes of two- and ...three-coordinated water molecules (so-called “dangling” OH bonds), respectively, on the ice surface. A recent JWST observation first succeeded in detection of a potential dangling OH feature at 3664 cm −1 for ices in molecular clouds, highlighting the importance of dangling OH bonds in interstellar ice chemistry. A lack of band strengths of these features at low temperatures restricts the quantification of dangling OH bonds from IR spectra, hindering development of a molecular-level understanding of the surface structure and chemistry of ice. Using IR multiple-angle incidence resolution spectrometry, we quantified the band strengths of two- and three-coordinated dangling OH features in amorphous water at 20 K as being 4.6 ± 1.6 × 10 −18 and 9.1 ± 1.0 × 10 −18 cm molecule −1 , respectively. These values are more than an order of magnitude lower than band strengths of bulk-water molecules in ice and liquid water and are similar to those of H 2 O monomers confined in solid matrices. Adsorption of carbon monoxide with dangling OH bonds results in the appearance of a new broad dangling OH feature at 3680–3620 cm −1 , with a band strength of 1.8 ± 0.1 × 10 −17 cm molecule −1 . The band strengths of dangling OH features determined in this study advance our understanding of the surface structure of interstellar ice analogs and recent IR observations of the JWST.
Amphiphilic organic compounds that accumulate naturally on the outermost layers of aqueous aerosols totally change the mechanisms involved in the uptake and reactions of volatile organic compounds ...adsorbed on the surfaces. By means of mass spectrometry of aqueous microjets, we examined how quaternary alkylammonium cations Me(CH2) n−1N+Me3 (n = 1, 4, 8, 10, 12, and 14) and nonionic octan-1-ol influence acid-catalyzed oligomerization of gaseous isoprene (ISO) at the air–water interface. The oligomerization is initiated by proton-transfer (PT) reaction from an interfacial hydronium ion to isoprene to form a (ISO)H+ and is subsequently followed by chain-propagation (CP) reaction to form (ISO) m≥2H+. We found that although the total mass spectral signals of (ISO) m H+ products were suppressed by the presence of either the alkylammonium cations or octan-1-ol, the suppression by the former surfactants was much larger than that of the latter. The suppression was observed even at aqueous microjets of 5 μM equimolar solution of the alkylammonium cations at which the air–water interface is only sparsely occupied by the alkylammonium cations. We propose that electrostatic repulsions between the alkylammonium cation and the interfacial H3O+ cause the hindrance of the PT reaction. In contrast, the subsequent CP reactions were not suppressed by the presence of either the alkylammonium cations or octan-1-ol. We suggest that the presence of long-chain organic surfactants hinders the initial PT reaction but hardly suppresses the subsequent CP reaction. Our study has revealed hitherto unrecognized interface-specific roles of surfactants that affect multiphase chemistry in the atmosphere.
Abstract
The absolute absorption cross section of dangling OH bonds in water ice, a free OH stretch mode by three-coordinated surface water molecules, is derived experimentally as 1.0 ± 0.2 × 10
−18
...cm
2
at 3696 cm
−1
for amorphous water at 90 K using infrared multiple-angle incidence resolution spectrometry (IR–MAIRS). The integrated absorption cross section (band strength) of the dangling OH bond at 90 K (1.4 ± 0.3 × 10
−17
cm molecule
−1
at 3710–3680 cm
−1
) is found to be more than 1 order of magnitude smaller than those in bulk ice or liquid water. This indicates that a lack of hydrogen-bonding significantly decreases the band strength of dangling OH bonds. The present study also provides average molecular orientations of dangling OH bonds at 10 K and 90 K, because both the surface-parallel (in-plane) and surface-perpendicular (out-of-plane) vibration spectra of dangling OH bonds are quantitatively measured by IR–MAIRS. The intensity ratio of the dangling-OH peaks between in-plane to out-of-plane spectra shows the isotropic nature (random orientation) of the two- and three-coordinated dangling OH bonds in microporous amorphous water prepared at 10 K; however, the three-coordinated dangling OH bonds in nonporous amorphous water prepared at 90 K are dominantly located at the top ice surface and oriented perpendicular to it. These findings provide fundamental insights into the relationship between the structure and optical properties of ice surfaces, and aid quantitative understanding of the surface structure of interstellar ices and their laboratory analogs.
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