Nuclear spin conversion of molecular hydrogen on amorphous solid water in the presence of O2 traces

Nuclear spin conversion (NSC) of ortho- to para-H(2) and para- to ortho-D(2) has been investigated on an amorphous solid water (ASW) surface at 10 K, in the presence of co-adsorbed O(2). The dynamics of the nuclear spin conversion could be revealed by combination of resonance enhanced multiphoton ionization spectroscopy (REMPI) with temperature programmed desorption (TPD) experiments. The conversion rates are consistent with a diffusion of molecular hydrogen inducing a nuclear spin conversion enhanced in the vicinity of molecular oxygen. The conversion times were found to increase with decreasing O(2) and H(2) coverage. Finally, on oxygen free ASW surface, the extremely long conversion characteristic times measured showed that such surface is not an efficient catalyst for NSC, in contradiction with hypothesis commonly made for interstellar medium.     

Catalytic homogeneous hydrogenation of compounds containing X --> O semipolar bonds (X = N, S, P) with para-hydrogen as a promising route for preparation of para-water

The quantum-chemical simulation (DFT, PBE, TZ2p basis set) of the mechanism of catalytic hydrogenation of compounds containing R(n)X --> O semipolar bonds (R(n)X = N(2), Me(2)S, C(5)H(5)N, Ph(3)P) on the Wilkinson catalyst (Ph(3)P)(3)RhCl with para-hydrogen showed that this process proceeds with retention of proton nuclear spin correlation, which enables a principal possibility to synthesize para-H(2)O, i.e. the nuclear spin isomer of water with antiparallel proton spins, using this route.     

High resolution quantum cascade laser studies of the ν3 band of methyl fluoride in solid para-hydrogen

Spectra of solid para-H(2) doped with CH(3)F at 1.8 K are studied in the ν(3) region (~1040 cm(-1)) using a quantum cascade laser source. As shown previously, residual ortho-H(2) in the sample (~1000 ppm) gives rise to distinct spectral features due to clusters of the form CH(3)F-(ortho-H(2))(N), with N = 0, 1, 2, 3, etc. Brief annealing at 7 K is found to give narrower spectral lines (≥0.006 cm(-1)) than conventional (5 K) annealing, and causes the N = 3 and 4 lines to fragment into two or more components. The N = 3 line is observed to be particularly stable and persistent. The N = 0 line (no ortho-H(2) neighbors) is resolved into two closely spaced (≈0.007 cm(-1)) components which are assigned to the K = 0 and 1 states of CH(3)F rotating around its C(3v) symmetry axis (ortho- and para-CH(3)F, respectively). Similar K-structure is also evident for other lines. Weak but persistent features ("N = 1/2 lines") are observed mid way between N = 0 and 1.     

UV photolysis of C4N2 in water ices: new possible route of synthesis of ammonium bicarbonate and ammonium formate

Laboratory experiments involving ultraviolet (UV) irradiation of dicyanoacetylene (C(4)N(2)) trapped in water ice at 10 K have been conducted and monitored by infrared spectroscopy (FTIR). By the support of isotopic experiments and theoretical calculations, the irradiation of a DCA/H(2)O ice mixture at lambda > 230 nm has been found to be a possible source of NH(4)(+)HCO(3)(-) (ammonium bicarbonate) and NH(4)(+)HCOO(-) (ammonium formate). These latter compounds can arise from a proton-transfer reaction between H(2)O and the CN radical, which is issued from photolyzed C(4)N(2).     

Nuclear spin dependence of the reaction of H(3)+ with H2. II. Experimental measurements

The nuclear spin dependence of the chemical reaction H(3)(+)+ H(2) → H(2)?+ H(3)(+) has been studied in a hollow cathode plasma cell. Multipass infrared direct absorption spectroscopy has been employed to monitor the populations of several low-energy rotational levels of ortho- and para-H(3)(+) (o-H(3)(+) and p-H(3)(+)) in hydrogenic plasmas of varying para-H(2) (p-H(2)) enrichment. The ratio of the rates of the proton hop (k(H)) and hydrogen exchange (k(E)) reactions α ≡ k(H)/k(E) is inferred from the observed p-H(3)(+) fraction as a function of p-H(2) fraction using steady-state chemical models. Measurements have been performed both in uncooled (T(kin) ～ 350 K) and in liquid-nitrogen-cooled (T(kin) ～ 135 K) plasmas, marking the first time this reaction has been studied at low temperature. The value of α has been found to decrease from 1.6 ± 0.1 at 350 K to 0.5 ± 0.1 at 135 K.     

In situ FTIR study on the formation and adsorption of CO on alumina-supported noble metal catalysts from H2 and CO2 in the presence of water vapor at high pressures

The formation and adsorption of CO from CO(2) and H(2) at high pressures were studied over alumina-supported noble metal catalysts (Pt, Pd, Rh, Ru) by in situ FTIR measurements. To examine the effects of surface structure of supported metal particles and water vapor on the CO adsorption, FTIR spectra were collected at 323 K with untreated and heat (673 K) treated catalysts in the absence and presence of water (H(2)O, D(2)O). It was observed that the adsorption of CO occurred on all the metal catalysts at high pressures, some CO species still remained adsorbed under ambient conditions after the high pressure FTIR measurements, and the frequencies of the adsorbed CO species were lower either for the heat treated samples or in the presence of water vapor. It is assumed that the CO absorption bands on atomically smoother surfaces appear at lower frequencies and that water molecules are adsorbed more preferentially on atomically rough surfaces rather than CO species.     

Can ortho-para transitions for water be observed?

The spectrum of water can be considered as the juxtaposition of the spectra of two molecules, with different total nuclear spin: ortho-H2O, and para-H2O. No transitions have ever been observed between the two different nuclear-spin isotopomers. The interconversion time is unknown and it is widely assumed that interconversion is forbidden without some other intervention. However, weak nuclear spin-rotation interaction occurs and can drive ortho to para transitions. Ab initio calculations show that the hyperfine nuclear spin-rotational coupling constants are about 30 kHz. These constants are used to explore the whole vibration-rotation spectrum with special emphasis on the coupling between nearby levels. Predictions are made for different spectral regions where the strongest transitions between ortho and para levels of water could be experimentally observed.     

Energy partitioning in the reaction O(1D) + H2O → OH + OH. V. Rotational relaxation of OH(X2Π, ν″, J″)

The rotational relaxation of OH(X²Π, ν″, J″) in ν″, = 0, 1, and 2 produced from the reaction of O (¹D) with H₂O has been studied as a function of H₂O vapor pressure and added argon. Water molecules are extremely efficient in bringing about relaxation and the experiments performed indicate that, on the average, the high temperature distribution is relaxed to nearly room temperature at a gas kinetic rate. This observation is rationalized by assumming a collision complex between OH and H₂O having a quasichemical interaction similar to weak hydrogen bonding. The nascent OH internal energy distribution does not depend upon the translational energy of the O(¹D) reactant. Translational relaxation of the nascent OH product by H₂O is fast, as fast as rotational relaxation.     

Influence of monolayer amounts of HNO3 on the evaporation rate of H2O over ice in the range 179 to 208 K: a quartz crystal microbalance study

The evaporation flux J(ev) of H2O from thin H2O ice films containing between 0.5 and 7 monolayers of HNO3 has been measured in the range 179 to 208 K under both molecular and stirred flow conditions in isothermal experiments. FTIR absorption of the HNO3/H2O condensate revealed the formation of metastable alpha-NAT (HNO(3).3H2O) converting to stable beta-NAT at 205 K. After deposition of HNO3 for 16-80 s on a 1 mum thick pure ice film at a deposition rate in the range (6-60) x 10(12) molecules s(-1) the initial evaporative flux J(ev)(H2O) was always that of pure ice. J(ev)(H2O) gradually decreased with the evaporation of H2O and the concomitant increase of the average mole fraction of HNO3, chi(HNO3), indicating the presence of an amorphous mixture of H2O/HNO3 that is called complexed or (c)-ice whose vapor pressure is that of pure ice. The final value of J(ev) was smaller by factors varying from 2.7 to 65 relative to pure ice. Depending on the doping conditions and temperature of the ice film the pure ice thickness d(D) of the ice film for which J(ev) < 0.85J(ev)(pure ice) varied between 130 and 700 nm compared to the 1000 nm thick original ice film at 208 and 191 K, respectively, in what seems to be an inverse temperature dependence. There exist three different types of H2O molecules under the present experimental conditions, namely (a) free H2O corresponding to pure ice, (b) complexed H2O or c-ice, and (c) H2O molecules originating from the breakup of NAT or amorphous H2O/HNO3 mixtures. The significant decrease of J(ev)(H2O) with increasing chi(HNO3) leads to an increase of the evaporative lifetime of atmospheric ice particles in the presence of HNO3 and may help explain the occurrence of persistent and/or large contaminated ice particles at certain atmospheric conditions.     

Radiolysis of astrophysical ice analogs by energetic ions: the effect of projectile mass and ice temperature

An experimental study of the interaction of highly charged, energetic ions (52 MeV (58)Ni(13+) and 15.7 MeV (16)O(5+)) with mixed H(2)O : C(18)O(2) astrophysical ice analogs at two different temperatures is presented. This analysis aims to simulate the chemical and the physicochemical interactions induced by cosmic rays inside dense, cold astrophysical environments, such as molecular clouds or protostellar clouds as well at the surface of outer solar system bodies. The measurements were performed at the heavy ion accelerator GANIL (Grand Accelerateur National d'Ions Lourds) in Caen, France. The gas samples were deposited onto a CsI substrate at 13 K and 80 K. In situ analysis was performed by a Fourier transform infrared (FTIR) spectrometer at different fluences. Radiolysis yields of the produced species were quantified. The dissociation cross section at 13 K of both H(2)O and CO(2) is about 3-4 times smaller when O ions are employed. The ice temperature seems to affect differently each species when the same projectile was employed. The formation cross section at 13 K of molecules such as C(18)O, CO (with oxygen from water), and H(2)O(2) increases when Ni ions are employed. The formation of organic compounds seems to be enhanced by the oxygen projectiles and at lower temperatures. In addition, because the organic production at 13 K is at least 4 times higher than the value at 80 K, we also expect that interstellar ices are more organic-rich than the surfaces of outer solar system bodies.     

Infrared photochromism in a quantum crystal: laser pumping and probing in solid para-H2 doped with CH3F

Solid para-H(2) doped with CH(3)F at 1.8 K is studied in the ν(3) region (～1040 cm(-1)) using a quantum cascade laser source. Residual ortho-H(2) gives rise to distinct spectral features due to CH(3)F-(ortho-H(2))(N) clusters with N = 0, 1, 2, etc. The laser power (～30 mW) is sufficient to significantly affect the sample, enabling novel photochromism experiments to be performed on a solid para-H(2) matrix-isolated sample. It is found that population can be reversibly transferred between the N = 1 line and satellite features close to the N = 0 line.     

Bending modes of the water molecules and the MO stretching modes in the series MF2 · 4H2O (M = Fe,Co, Ni,Zn)

The infrared spectra of the series MF₂·4X₂O (M = Fe, Co, Ni, Zn; X = H, D) are reported in the frequency ranges of the bending vibrations of the water molecules (ν₂) at 296 and ∼100 K and the MO lattice vibrations (ν_MO) at 296 K. Four ν_MO vibrations consisting of two doublets are identified using deuterium substitution. The various ν_ZnO vibrations correlate well with the metal-oxygen distances R(ZnO), and this correlation is further used to calculate R(MO)'s of the remainder of the series and to refine R(ZnO). Four ν₂(H₂O, HDO, D₂O) vibrations, consisting of two sharp overlapping bands flanked by two broad shoulders, are identified. The number of ν₂(H₂O) components, the sequence of ν₂ in the series and the correlation with R(MO) suggest that the ν₂ frequencies are mainly determined by R(MO). Using this assignment the two types of ν₂ bands are assigned to the two types of crystallographically distinct water molecules found in the MF₂·4H₂O structure.     

Crystalline ice growth on Pt(111) and Pd(111): nonwetting growth on a hydrophobic water monolayer

The growth of crystalline ice films on Pt(111) and Pd(111) is investigated using temperature programed desorption of the water films and of rare gases adsorbed on the water films. The water monolayer wets both Pt(111) and Pd(111) at all temperatures investigated [e.g., 20-155 K for Pt(111)]. However, crystalline ice films grown at higher temperatures (e.g., T>135 K) do not wet the monolayer. Similar results are obtained for crystalline ice films of D2O and H2O. Amorphous water films, which initially wet the surface, crystallize and dewet, exposing the water monolayer when they are annealed at higher temperatures. Thinner films crystallize and dewet at lower temperatures than thicker films. For samples sputtered with energetic Xe atoms to prepare ice crystallites surrounded by bare Pt(111), subsequent annealing of the films causes water molecules to diffuse off the ice crystallites to reform the water monolayer. A simple model suggests that, for crystalline films grown at high temperatures, the ice crystallites are initially widely separated with typical distances between crystallites of approximately 14 nm or more. The experimental results are consistent with recent theory and experiments suggesting that the molecules in the water monolayer form a surface with no dangling OH bonds or lone pair electrons, giving rise to a hydrophobic water monolayer on both Pt(111) and Pd(111).     

Hydrogen atom formation from the photodissociation of water ice at 193 nm

The TOF spectra of photofragment hydrogen atoms from the 193 nm photodissociation of amorphous ice at 90-140 K have been measured. The spectra consist of both a fast and a slow components that are characterized by average translational energies of 2k(B)T(trans)=0.39+/-0.04 eV (2300+/-200 K) and 0.02 eV (120+/-20 K), respectively. The incident laser power dependency of the hydrogen atom production suggests one-photon process. The electronic excitation energy of a branched cluster, (H(2)O)(6+1), has been theoretically calculated, where (H(2)O)(6+1) is a (H(2)O)(6) cyclic cluster attached by a water molecule with the hydrogen bond. The photoabsorption of this branched cluster is expected to appear at around 200 nm. The source of the hydrogen atoms is attributed to the photodissociation of the ice surface that is attached by water molecules with the hydrogen bond. Atmospheric implications are estimated for the photodissociation of the ice particles (Noctilucent clouds) at 190-230 nm in the region between 80 and 85 km altitude.     

Two-dimensional infrared spectroscopy of neat ice Ih

The OH stretch line shape of ice Ih exhibits distinct peaks, the assignment of which remains controversial. We address this longstanding question using two dimensional infrared (2D IR) spectroscopy of the OH stretch of H(2)O and the OD stretch of D(2)O of ice Ih at T = 80 K. The isotropic response is dominated by a 2D line shape component which does not depend on the pump pulse frequency. The decay time of the component that does depend on the pump frequency is calculated using singular value decomposition (bi-exponential decay H(2)O: 30 fs, 490 fs; D(2)O: 40 fs, 690 fs). The anisotropic contribution exhibits on-diagonal peaks, which decay on a very fast timescale (H(2)O: 85 fs; D(2)O: 65 fs), with no corresponding anisotropic cross-peaks. Both isotropic and anisotropic results indicate that randomization of excited dipoles occurs with a very rapid rate, just like in neat liquid water. We conclude that the underlying mechanism relates to the complex interplay between exciton migration and exciton-phonon coupling.     

Infrared spectra of N2O-(ortho-D2)N and N2O-(HD)N clusters trapped in bulk solid parahydrogen

High-resolution infrared spectra of the clusters N2O-(ortho-D2)N and N2O-(HD)N, N=1-4, isolated in bulk solid parahydrogen at liquid helium temperatures are studied in the 2225 cm-1 region of the nu3 antisymmetric stretch of N2O. The clusters form during vapor deposition of separate gas streams of a precooled hydrogen mixture (ortho-D2para-H2 or HDpara-H2) and N2O onto a BaF2 optical substrate held at approximately 2.5 K in a sample-in-vacuum liquid helium cryostat. The cluster spectra reveal the N2O nu3 vibrational frequency shifts to higher energy as a function of N, and the shifts are larger for ortho-D2 compared to HD. These vibrational shifts result from the reduced translational zero-point energy for N2O solvated by the heavier hydrogen isotopomers. These spectra allow the N=0 peak at 2221.634 cm-1, corresponding to the nu3 vibrational frequency of N2O isolated in pure solid parahydrogen, to be assigned. The intensity of the N=0 absorption feature displays a strong temperature dependence, suggesting that significant structural changes occur in the parahydrogen solvation environment of N2O in the 1.8-4.9 K temperature range studied.     

四氢呋喃/水体系中以蒸气相传输法合成高硅FER沸石

以X射线衍射(XRD), 红外光谱(FT-IR), 扫描电镜(SEM), 低温氮吸附, ²⁹Si固体核磁共振(MAS NMR)等研究了含FER晶种的Na₂O-SiO₂-Al₂O₃干胶(SDG)在四氢呋喃(THF)/水(H₂O)混合蒸气相中的结晶行为, 同时研究了体系中THF分子和[SiO₄], [AlO₄]基团在结晶前后状态的变化. 结果表明, 在THF/H₂O混合蒸气中以蒸气相传输法(VPT)可合成结晶度较高、结构完美且孔道开放的FER沸石. ¹³C交叉极化固体核磁共振(CPMASNMR)和差热分析(TG-DTG-DTA)等研究证明THF分子作为模板剂, 位于FER笼内. FER晶种和水能促进FER沸石的结晶.     

Infrared spectra of seeded hydrogen clusters: (para-H2)N-N2O and (ortho-H2)N-N2O, N = 2-13

High-resolution infrared spectra of clusters containing para-H2 and/or ortho-H2 and a single nitrous oxide molecule are studied in the 2225-cm(-1) region of the upsilon1 fundamental band of N2O. The clusters are formed in pulsed supersonic jet expansions from a cooled nozzle and probed using a tunable infrared diode laser spectrometer. The simple symmetric rotor-type spectra generally show no resolved K structure, with prominent Q-branch features for ortho-H2 but not para-H2 clusters. The observed vibrational shifts and rotational constants are reported. There is no obvious indication of superfluid effects for para-H2 clusters up to N=13. Sharp transitions due to even larger clusters are observed, but no definite assignments are possible. Mixed (para-H2)N-(ortho-H2)M-N2O cluster line positions can be well predicted by linear interpolation between the corresponding transitions of the pure clusters.     

FTIR study of CO2 and H2O/CO2 nanoparticles and their temporal evolution at 80 K

Fourier transform infrared (FTIR) spectroscopy combined with a long-path collisional cooling cell was used to investigate the temporal evolution of CO2 nanoparticles and binary H2O/CO2 nanocomposites in the aerosol phase at 80 K. The experimental conditions for the formation of different CO2 particle shapes as slab, shell, sphere, cube, and needle have been studied by comparison with calculated data from the literature. The H2O/CO2 nanoparticles were generated with a newly developed multiple-pulse injection technique and with the simpler flow-in technique. The carbon dioxide nu3-vibration band at 2360 cm(-1) and the water ice OH-dangling band at 3700 cm(-1) were used to study the evolution of structure, shape, and contact area of the nanocomposites over 150 s. Different stages of binary nanocomposites with primary water ice cores were identified dependent on the injected CO2 portion: (a) disordered (amorphous) CO2 slabs on water particle surfaces, (b) globular crystalline CO2 humps sticking on the water cores, and (c) water cores being completely enclosed in bigger predominantly crystalline CO2 nanoparticles. However, regular CO2 shell structures on primary water particles showing both longitudinal (LO) and transverse (TO) optical mode features of the nu3-vibration band could not be observed. Experiments with reversed nucleation order indicate that H2O/CO2 composite particles with different initial structures evolve toward similar molecular nanocomposites with separated CO2 and H2O regions.