Eui-Seong Moon

Formation of Glycine on Ultraviolet-Irradiated Interstellar Ice-Analog Films and Implications for Interstellar Amino Acids

We report the synthesis of glycine on interstellar ice-analog films composed of water, methylamine (MA), and carbon dioxide under irradiation of ultraviolet (UV) photons. Analysis of the UV-irradiated ice films by in situ mass spectrometric methods revealed glycine and other isomers as photochemical products. Deuterium-labeling experiments were conducted to determine the structures of the photoproducts and to examine their formation pathways. The reactions occur via photocleavages of C-H and N-H bonds in MA, followed by subsequent reactions of the nascent H atom with CO2, leading to the formation of HOCO and then to glycine and carbamic acid. The photochemical synthesis of glycine occurs efficiently at the ice surfaces, and the competing photosynthesis and photodestruction processes can reach a steady-state kinetic balance at an extended UV exposure, maintaining a substantial population level of glycine. The observation suggests that interstellar amino acids can be created on ice grains, and that they can also be stored in the ices by maintaining a kinetic balance under interstellar UV irradiation. As such, the transport of amino acids in interstellar space may be possible without depleting the net abundance of amino acids in the ices but rather increasing the structural diversity of the molecules.

Direct Evidence for Ammonium Ion Formation in Ice through Ultraviolet-induced Acid-Base Reaction of NH3 with H3O+

We present direct evidence for ammonium ion (NH4 +) formation through ultraviolet (UV) photolysis of NH3-H2O mixture ice that does not contain acids. NH4 + forms by the reaction of NH3 with protonic defects (H3O+) in the UV-photolyzed ice. Our observations may explain the deficient counter-anions in interstellar ice relative to the abundance of NH4 +. Also, H3O+ may play an important role in the acid-base chemistry of interstellar ice in UV-irradiating environments. IR absorption results suggest that NH4 + is a potential contributor to the interstellar 6.85 μm band but is not a dominant component.

Energy barrier of proton transfer at ice surfaces.

We estimated the energy barrier of proton transfer on ice film surfaces through the measurement of the H/D exchange kinetics of H(2)O and D(2)O molecules. The isotopomeric populations of water molecules and hydronium ions on the surface were monitored by using the techniques of reactive ion scattering and low energy sputtering, respectively, along the progress of the H/D reaction. When hydronium ions were externally added onto an ice film at a temperature of 70 K, a proton was transferred from the hydronium ion mostly to an adjacent water molecule. The proton transfer distance and the H/D exchange rate increased as the temperature increased for 90-110 K. The activation energy of the proton transfer was estimated to be 10+/-3 kJ mol(-1) on a polycrystalline ice film grown at 135 K. The existence of a substantial energy barrier for proton transfer on the ice surface agreed with proton stabilization at the surface. We also examined the H/D exchange reaction on a pure ice film surface at temperatures of 110-130 K. The activation energy of the reaction was estimated to be 17+/-4 kJ mol(-1), which was contributed from the ion pair formation and proton transfer processes on the surface.

Generation of strong electric fields in an ice film capacitor

We present a capacitor-type device that can generate strong electrostatic field in condensed phase. The device comprises an ice film grown on a cold metal substrate in vacuum, and the film is charged by trapping Cs(+) ions on the ice surface with thermodynamic surface energy. Electric field within the charged film was monitored through measuring the film voltage using a Kelvin work function probe and the vibrational Stark effect of acetonitrile using IR spectroscopy. These measurements show that the electric field can be increased to ∼4 × 10(8) V m(-1), higher than that achievable by conventional metal plate capacitors. In addition, the present device may provide several advantages in studying the effects of electric field on molecules in condensed phase, such as the ability to control the sample composition and structure at molecular scale and the spectroscopic monitoring of the sample under electric field.

UV-induced protonation of molecules adsorbed on ice surfaces at low temperature

UV irradiation of ice films adsorbed with methylamine molecules induces protonation of the adsorbate molecules at low temperature (50-130 K). The observation indicates that long-lived protonic defects are created in the ice film by UV light, and they transfer protons to the adsorbate molecules via tunneling mechanism at low temperature. The methylammonium ion formed by proton transfer remains to be stable at the ice surface. It is suggested that this solid-phase protonation might play a significant role in the production of molecular ions in interstellar clouds.

Acidic Water Monolayer on Ruthenium(0001)

The ubiquity of water in natural environments makes the interaction of water with solid surfaces an important subject of study in a wide variety of scientific disciplines and technologies. One of the most intensively investigated systems for the interaction of water with metal surfaces is water on Ru(0001), which has become a test system for our understanding of this scientific field. Numerous experimental and theoretical studies conducted during the past decade have greatly improved our understanding of the structure and dynamics of water adsorption on Ru(0001). These studies have reached the consensus that water adsorption leads to the formation of an intact molecularwater layer on the surface at low temperature (less than about 155 K). As the surface is heated, H2O partially dissociates to form amixedOH+H2O+H adsorption layer, in competition with desorption of H2O. On the other hand, D2O does not dissociate on the surface and desorbs intact due to a kinetic isotope effect. Despite the wealth of theoretical and experimental research on this system, there are still many open questions, in particular concerning the acid–base properties of adsorbed water. This information is fundamentally important to heterogeneous catalysis, corrosion, and electrochemistry because it determines the proton transfer and acid–base characteristics of the water–solid interface. Therefore, it is highly desirable to investigate these properties for adsorbed water using a systematic surface science approach; this could be one way to unravel the intricacies of the acid–base chemistry at water–solid interfaces. In the present work, we study the proton-transfer ability of water molecules adsorbed on a Ru(0001) surface by using surface spectroscopic measurements and ammonia adsorption experiments. The study shows that the first monolayer of water is much more acidic than bulk water, with the ability to spontaneously transfer a proton to an ammonia molecule. We prepared a water layer on Ru(0001) in ultrahigh vacuum (UHV) by the adsorption of H2O vapor at 140 K to a monolayer saturation coverage, a condition that is known to produce an intact molecular-water layer on the surface. Then, NH3 was adsorbed onto the H2O monolayer surface for a small coverage [0.04 ML; 1 ML= 1.14 10 moleculescm 2 corresponding to the monolayer density of water on Ru(0001)]. The NH3 molecules served as a probe for the surface acidity. Figure 1 shows the results of low-energy sputtering (LES) and reactive-ion scattering (RIS) measurements for the H2O monolayer before and after the adsorption of NH3. The RIS and LES methods measure neutral and ionic species, respectively, on the surface. For a layer of pure H2O, spectrum a in Figure 1 shows the RIS signal of CsH2O + (m/z= 151 amu/charge), which was produced by the pickup of surface H2O molecules by scattering Cs + projectiles. The peak of elastically scattered Cs ions appeared at m/z= 133. After NH3 adsorption (spectrum b in Figure 1), a CsNH3 + (m/ z= 150) signal appeared with a small intensity, indicating the presence of neutral NH3 adsorbates on the surface. In addition, LES signals appeared for NH4 + (m/z= 18) and NH4(H2O) + (m/z= 36), indicating the presence of NH4 + and its hydrated species. These ammonium signals indicated that protons were transferred from the water monolayer to NH3 adsorbates to form NH4 . In additional experiments, we observed that the ammonium signals exhibited the following features. First, the NH4 + and NH4(H2O) + signals did not appear when NH3 was adsorbed onto a multilayer ice film grown on Ru(0001). This result showed that the first water monolayer was the proton donor to NH3. Second, in order to check if the ammonium signals originated from preformed ions on the surface, we measured the appearance threshold of the NH4 + signal as a function of Cs impact energy for the water monolayer (where the NH4 + was formed by proton transfer) and for the multilayer ice film (where only neutral NH3 was present). The two surfaces showed well-distinguished characteristics for the threshold energy and intensity of NH4 + emission. The NH4 + signal from the water monolayer exhibited a lower threshold energy (20–25 eV) and stronger intensity than that from the multilayer film, which is characteristic for the low-energy sputtering of preformed NH4 + species. On the other hand, on the multilayer ice film, Figure 1. LES and RIS mass spectra of positive ions obtained from a) H2O monolayer formed on Ru(0001) at 140 K, and b) after NH3 adsorption (ca. 0.04 ML) on surface at 80 K. The LES and RIS measurements were conducted at 80 K with a Cs beam energy of 25 eV.

Metastable hydronium ions in UV-irradiated ice.

We show that the irradiation of UV light (10-11 eV) onto an ice film produces metastable hydronium (H(3)O(+)) ions in the ice at low temperatures (53-140 K). Evidence of the presence of metastable hydronium ions was obtained by experiments involving adsorption of methylamine onto UV-irradiated ice films and hydrogen-deuterium (H∕D) isotopic exchange reaction. The methylamine adsorption experiments showed that photogenerated H(3)O(+) species transferred a proton to the methylamine arriving at the ice surface, thus producing the methyl ammonium ion, which was detected by low energy sputtering method. The H(3)O(+) species induced the H∕D exchange of water, which was monitored through the detection of water isotopomers on the surface by using the Cs(+) reactive ion scattering method. Thermal and temporal stabilities of H(3)O(+) and its proton migration activity were examined. The lifetime of the hydronium ions in the amorphized ice was greater than 1 h at ∼53 K and decreased to ∼5 min at 140 K. Interestingly, a small portion of hydronium ions survived for an extraordinarily long time in the ice, even at 140 K. The average migration distance of protons released from H(3)O(+) in the ice was estimated to be about two water molecules at ∼54 K and about six molecules at 100 K. These results indicate that UV-generated hydronium ions can be efficiently stabilized in low-temperature ice. Such metastable hydronium ions may play a significant role in the acid-base chemistry of ice particles in interstellar clouds.

Spectroscopic Monitoring of the Acidity of Water Films on Ru(0001): Orientation-Specific Acidity of Adsorbed Water

We examined the acid–base properties of water films adsorbed onto a Ru(0001) substrate by using surface spectroscopic methods in vacuum environments. Ammonia adsorption experiments combined with low-energy sputtering (LES), reactive ion scattering (RIS), reflection–absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) measurements showed that the adsorbed water is acidic enough to transfer protons to ammonia. Only the water molecules in an intact water monolayer and water clusters larger than the hexamer exhibit such acidity, whereas small clusters, a thick ice film or a partially dissociated water monolayer that contains OH, H2O and H species are not acidic. The observations indicate the orientation-specific acidity of adsorbed water. The acidity stems from water molecules with H-down adsorption geometry present in the monolayer. However, the dissociation of water into H and OH on the surface does not promote but rather suppresses the proton transfer to ammonia.