Release of oxygen atoms and nitric oxide molecules from the ultraviolet photodissociation of nitrate adsorbed on water ice films at 100 K

Production of O((3)P(J), J = 2, 1, 0) atoms from the 295-320 nm photodissociation of NO(3)- adsorbed on water polycrystalline ice films at 100 K was directly confirmed using the resonance-enhanced multiphoton ionization technique. Detection of the O atom signals required an induction period after deposition of HNO3 onto the ice film held at 130 K due to the slow ionization rate of HNO(3) to H+ and NO(3)- with a rate constant of k = (5.3 +/- 0.2) x 10(-3)s(-1). Translational energy distributions of the O atoms were represented by a combination of two Maxwell-Boltzmann energy distributions with translational temperatures of 2000 and 100 K. Direct detection of NO from the secondary photodissociation process was also successful. On the atmospheric implications, the influence of the direct release of the oxygen atoms into the air from NO(3)- adsorbed on the natural snowpack was included in an atmospheric model calculation on the mixing ratios of ozone and nitric oxide at the South Pole, and the results compared favorably with the field data.     

Direct observation of chiral metal-organic complexes assembled on a Cu100 surface

We report single-molecule level STM observations of chiral complexes generated by the assembly of achiral components at a metal surface. Following co-deposition of iron atoms and 1,3,5-tricarboxylic benzoic acid (trimesic acid, TMA) on Cu(100) in ultrahigh vaccum, TMA molecules react with the metal centers, and metal-ligand interactions stabilize R and S chiral complexes which are clearly distinguished by STM.     

Thermodynamic and structural properties of water adsorbed film on MgO (100) ionic surface

We have investigated by adsorption isotherms and neutron diffraction measurements, respectively the thermodynamic and structural properties of water physisorbed film on MgO (100) powder. Thanks to a high temperature thermal treatment, under vacuum, our MgO powder samples are characterized by a highly homogeneous (100) MgO surface. We have determined the structure of the (2D) water film physisorbed on such an ionic surface. This one is a commensurate P(2×3) structure which is very similar to the (110) planes of ice-VII. Recall that ice VII, which is stable at very high pressure, is characterized by a quite large density (d = 1.6).     

Dewetting growth of crystalline water ice on a hydrogen saturated Rh(111) surface at 135 K

We investigated the water (D(2)O) adsorption at 135?K on a hydrogen pre-adsorbed Rh(111) surface using temperature programmed desorption and infrared reflection absorption spectroscopy (IRAS) in ultrahigh vacuum. With increasing the hydrogen coverage, the desorption temperature of water decreases. At the saturation coverage of hydrogen, dewetting growth of water ice was observed: large three-dimensional ice grains are formed. The activation energy of water desorption from the hydrogen-saturated Rh(111) surface is estimated to be 51 kJ/mol. The initial sticking probability of water decreases from 0.46 on the clean surface to 0.35 on the hydrogen-saturated surface. In IRAS measurements, D-down species were not observed on the hydrogen saturated surface. The present experimental results clearly show that a hydrophilic Rh(111) clean surface changes into a hydrophobic surface as a result of hydrogen adsorption.     

Direct kinetic study of the reaction of OH radicals with methyl-ethyl-ketone

The low-pressure discharge flow technique with resonance fluorescence monitoring of OH has been applied to study the kinetics of the overall reaction:

Direct identification of conformational isomers of adsorbed oligothiophene on Cu100

A direct conformational analysis using scanning tunneling microscopy (STM) has been performed for individual adsorbed alpha-octithiophene molecules on Cu(100). s-cis and s-trans conformational isomers are induced by the rotational flexibility of individual thiophene rings. By adding bulky N-silyl substituents to octithiophene, we successfully identify the s-cis and s-trans conformational isomers using STM. The obtained relative abundances of the s-cis and s-trans conformations are analyzed using ab initio molecular orbital calculations.     

Kinetics of the reactions of OH radicals with n-alkanes at 299 ± 2 K

Relative rate constants for the reaction of OH radicals with a series of n-alkanes have been determined at 299 ± 2 K, using methyl nitrite photolysis in air as a source of OH radicals. Using a rate constant for the reaction of OH radicals with n-butane of 2.58 × 10^?12 cm³ molecule^?1s^?1, the rate constants obtained are (X10¹² cm³ molecule^?1 s^?1): propane 1.22 ± 0.05, n-pentane 4.13 ± 0.08, n-heptane 7.30 ± 0.17, n-octane 9.01 ± 0.19, n-nonane 10.7 ± 0.4, and n-decane 11.4 ± 0.6. The data for propane, n-pentane, and n-octane are in good agreement with literature values, while those for n-heptane, n-nonane, and n-decane are reported for the first time. These data show that the rate constant per secondary C—H bond is ∽40% higher for —CH₂— groups bonded to two other —CH₂— groups than for those bonded to a —CH₂— group and a —CH₃ group.     

Standard enthalpy of solution of ammonium nitrate in water at 298 K

We have made calorimetric measurements of the enthalpy of solution of NH₄NO₃(c, IV) in water at 298 K, where (c, IV) indicates the crystal form of amomonium nitrate that is stable from 256 to 305 K. Results of our measurements have been combined with enthalpy of dilution values from Parker to obtain the standard enthalpy of solution of NH₄NO₃ (c, IV) in water at 298.15 K to be ΔH^o = 25.41 kJ mol^?1.     

Experimental studies of surface reactions among OH radicals that yield H2O and CO2 at 40-60 K

We investigated the OH-related formation routes of two astrophysically important molecules, H(2)O and CO(2), under relatively warm astrophysical conditions. OH radicals, together with other neutral species such as H, O, H(2), and O(2), were produced in H(2)O microwave-discharge plasma and cooled to 100 K before being deposited on an Al substrate at 40-60 K. H(2)O formed at 40 and 50 K, but not at 60 K. Taking the experimental conditions into account, a possible route of H(2)O formation is via reactions involving OH + OH, which yield H(2)O(2) as the main reaction product. The present study is the first to show experimentally that surface reactions of two OH radicals can yield H(2)O at low temperatures. The products' branching ratio was 0.2 and 0.8 for H(2)O and H(2)O(2), respectively. When CO was co-deposited with neutral species that formed in the H(2)O plasma, CO(2) was formed at 40-60 K. H(2)CO(3) formed at 40 and 50 K. The present results may suggest that chemical reactions related to OH radicals are effective at yielding various molecules in relatively warm astrophysical environments, such as protostars.     

Surface behaviour of some phenol para-derivatives adsorbed at the free surface of water

Comparison is made between two methods for the determination of the average orientation angle of molecules adsorbed at the free surface of water; one based on electric surface potential measurements, the other on second harmonic generation. Assuming that the model of energetics of adsorbed molecules is similar to that of a system of non-interacting permanent dipoles in an external homogeneous field, the parameters of the probability density function in the form exp[ —a cos(0-0_o)], a =w/kT, w, being the absolute orientational energy, have been calculated for phenol, p-methyl-, p-nitro-, and p-bromophenol.     

Structure of CO2 adsorbed on the KCl(100) surface

The structure and dynamics of the adsorbate CO(2)/KCl(100) from a diluted phase to a saturated monolayer have been investigated with He atom scattering (HAS), low-energy electron diffraction (LEED), and polarization dependent infrared spectroscopy (PIRS). Two adsorbate phases with different CO(2) coverage have been found. The low-coverage phase is disordered at temperatures near 80 K and becomes at least partially ordered at lower temperatures, characterized by a (2√2×√2)R45° diffraction pattern. The saturated 2D phase has a high long-range order and exhibits (6√2×√2)R45° symmetry. Its isosteric heat of adsorption is 26 ± 4 kJ mol(-1). According to PIRS, the molecules are oriented nearly parallel to the surface, the average tilt angle in the saturated monolayer phase is 10° with respect to the surface plane. For both phases, structure models are proposed by means of potential calculations. For the saturated monolayer phase, a striped herringbone structure with 12 inequivalent molecules is deduced. The simulation of infrared spectra based on the proposed structures and the vibrational exciton approach gives reasonable agreement between experimental and simulated infrared spectra.     

Electrochemical reduction of NAD+ and pyridinium cations adsorbed at the mercury/water interface: Electrochemical behavior of adsorbed pyridinyl radicals

Neutralization of the positive charge of the pyridinium cation (Pyr⁺)_ads adsorbed at the mercury/water interface following electrochemical reduction is likely to provoke a very fast flat-to-perpendicular reorientation of the adsorbed species, thus rendering the perpendicularly adsorbed radical (Pyr)_ads electrochemically inactive. Therefore, it cannot be ascertained by means of cyclic voltammetry whether dimerization of the (Pyr)_ads radicals, which occurs in solution, also occurs at the mercury/water interface.     

Catalytic decomposition of azomethane and reactions of adsorbed methyl radicals on the molybdenum surface

The methods of temperature-programmed reaction/desorption (TPR/TPD) are used to study azomethane (CH³N=NCH³) decomposition and the reactions of the products of its pyrolysis (CH ₃ ^* radicals and N₂) on the polycrystalline molybdenum surface. A TPR spectrum of adsorbed azomethane decomposition shows mainly N₂, H₂, and unreacted azomethane. Upon preliminary adsorption of azomethane pyrolysis products on a catalyst sample, a TPR spectrum shows N₂, H₂, and CH₄ in comparable amounts. The difference in the composition of desorption products found for these two types of experiments shows that, in the decomposition of adsorbed azomethane, surface methyl moieties are not formed. The rate constants were calculated for the dissociation of adsorbed CH₃, CH₂, and CH, recombination of hydrogen atoms with each other and with CH₃ and CH₂, and the recombinative desorption of nitrogen atoms. Deceased.     

Rate coefficients for the gas‐phase reactions of OH radicals with methylbutenols at 298 K

The relative‐rate method has been used to determine the rate coefficients for the reactions of OH radicals with three C₅ biogenic alcohols, 2‐methyl‐3‐buten‐2‐ol (k₁), 3‐methyl‐3‐buten‐1‐ol (k₂), and 3‐methyl‐2‐buten‐1‐ol (k₃), in the gas phase. OH radicals were produced by the photolysis of CH₃ONO in the presence of NO. Di‐n‐butyl ether and propene were used as the reference compounds. The absolute rate coefficients obtained with the two reference compounds agreed well with each other. The O₃ and O‐atom reactions with the target alcohols were confirmed to have a negligible contribution to their total losses by using two kinds of light sources with different relative rates of CH₃ONO and NO₂ photolysis. The absolute rate coefficients were obtained as the weighted mean values for the two reference compound systems and were k₁ = (6.6 ± 0.5) × 10^?11, k₂ = (9.7 ± 0.7) × 10^?11, and k₃ = (1.5 ± 0.1) × 10^?10 cm³ molecule^?1 s^?1 at 298 ± 2 K and 760 torr of air. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 379–385 2004     

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.     

Radiation yield of oxygen-based radicals in hyperquenched glassy water gamma-irradiated at 77 K

Hyperquenching of liquid water with cooling rates of 10⁶–10⁷ K s⁻¹ yields glassy water. Upon γ-irradiation at 77 K, the only paramagnetic species accumulating in hyperquenched glassy water are the hydroxyl and hydroperoxyl radicals. There are no hydrogen atoms or electrons seen by the ESR technique. For irradiation doses up to about 70 kGy, the relative contributions of hydroxyl and hydroperoxyl radicals to the total amount of paramagnetic species remain virtually constant. The total amount of paramagnetic species, n, is sublinear in dose, d, well approximated by n=8.55×10¹⁶d^0.8 for n in spin g⁻¹ and d in kGy.     

Absolute rate constants for the gas-phase reaction of OH radicals with cyclohexane and ethane at 295 K

The absolute rate constants for the gas-phase H-atom abstraction by hydroxyl radicals from cyclohexane and ethane have been determined at room temperature. OH radicals were produced by pulse radiolysis of an H₂O-Ar mixture, and the decay of OH was followed by monitoring the transient light absorption around 309 nm. The rate constants were found to be k = (5.24±0.36) × 10⁻¹² and (2.98±0.21) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹ for cyclohexane and ethane, res- pectively. These results are compared with literature data.