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.     

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.     

Interaction of CsF with multilayered water

The interaction of CsF with multilayered water has been investigated with metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy with HeI (UPS(HeI)). We have studied the emission from the ionization of H2O states 1b1, 3a1, and 1b2; of Cs5p and of F2p. We have prepared CsF-H2O interfaces, namely, CsF layers on thin films of multilayered water and vice versa; they were annealed between 80 and about 280 K. Up to about 100 K, a closed CsF layer can be deposited on H2O and vice versa; no interpenetration of the two components H2O and CsF could be observed. Above 110 K, CsF (H2O) layers deposited on thin H2O (CsF) films (stoichiometry CsF.1.5H2O) gradually transform into a mixed layer containing F, Cs, and H2O species. When annealing, H2O molecules can be detected up to 200 K from the mixed F-Cs-H2O layer (while for pure H2O desorption is essentially complete at 165 K); a water network is not formed under these conditions, and all H2O molecules are involved in bonding with Cs+ and F- ions. When CsF is deposited at 120 K on sufficiently thick water multilayers, full solvation of both F and Cs takes place, even for the species closest to the surface, as long as the stoichiometry remains CsF.(H2O)n with n > 3.     

Determination of the sticking coefficient and scattering dynamics of water on ice using molecular beam techniques

The sticking coefficient for D(2)O impinging on crystalline D(2)O ice was determined for incident translational energies between 0.3 and 0.7 eV and for H(2)O on crystalline H(2)O ice at 0.3 eV. These experiments were done using directed molecular beams, allowing for precise control of the incident angle and energy. Experiments were also performed to measure the intensity and energy of the scattered molecules as a function of scattering angle. These results show that the sticking coefficient was near unity, slightly increasing with decreasing incident energy. However, even at the lowest incident energy, some D(2)O did not stick and was scattered from the ice surface. We observe under these conditions that the sticking probability asymptotically approaches but does not reach unity for water sticking on water ice. We also present evidence that the scattered fraction is consistent with a binary collision; the molecules are scattered promptly. These results are especially relevant for condensation processes occurring under nonequilibrium conditions, such as those found in astrophysical systems.     

Interaction of acetic acid with solid water

The interaction of acetic acid (AA, CH(3)COOH), with solid water, deposited on metals, tungsten and gold, at 80 K, was investigated. We have prepared acid/water interfaces at 80 K, namely, acid layers on thin films of solid water and H(2)O adlayers on thin acid films; they were annealed between 80 and 200 K. Metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy UPS(HeII) were utilized to obtain information on the electronic structure of the outermost surface from the study of the electron emission from the weakest bound MOs of the acids, and of the molecular water. Temperature-programmed desorption (TPD) provided information on the desorption kinetics, and Fourier-transformed infrared spectroscopy (FTIR) provided information on the identification of the adsorbed species as well as on the water and acid crystallization. The results are compatible with the finding of ref 1 (preceding paper), made on the basis of DFT calculations, that AA adsorbs on ice as cyclic dimers. Above 120 K, a rearrangement of the AA dimers is suggested by a sharpening of the spectral features in the IR spectra and by spectral changes in MIES and UPS; this is attributed to the glass transition in AA around 130 K. Above 150 K the spectra transform into those characteristic for polycrystalline polymer chains. This structure is stable up to about 180 K; desorption of water takes place from underneath the AA film, and practically all water has desorbed through the AA film before AA desorption starts. There is no indication of water-induced deprotonation of the acid molecules. For the interaction of H(2)O molecules adsorbed on amorphous AA films, the comparison of MIES with the DFT results of ref 1 shows that the initial phase of exposure does not lead to the formation of a top-adsorbed closed water film at 80 K. Rather, the H(2)O molecules become attached to or incorporated into the preexisting AA network by H bonding; no water network is formed in the initial stage of the water adsorption. Also under these conditions no deprotonation of the acid can be detected.     

An infrared study of solid glycine in environments of astrophysical relevance

The conversion from neutral to zwitterionic glycine is studied using infrared spectroscopy from the point of view of the interactions of this molecule with polar (water) and non-polar (CO(2), CH(4)) surroundings. Such environments could be found on astronomical or astrophysical matter. The samples are prepared by vapour-deposition on a cold substrate (25 K), and then heated up to sublimation temperatures of the co-deposited species. At 25 K, the neutral species is favoured over the zwitterionic form in non-polar environments, whereas for pure glycine, or in glycine/water mixtures, the dominant species is the latter. The conversion is easily followed by the weakening of two infrared bands in the mid-IR region, associated to the neutral structure. Theoretical calculations are performed on crystalline glycine and on molecular glycine, both isolated and surrounded by water. Spectra predicted from these calculations are in reasonable agreement with the experimental spectra, and provide a basis to the assignments. Different spectral features are suggested as probes for the presence of glycine in astrophysical media, depending on its form (neutral or zwitterionic), their temperature and composition.     

Interaction of formic acid with solid water

The interaction of formic acid (HCOOH) with solid water, deposited on tungsten at 80 K, was investigated. We have prepared and annealed formic acid (FA)/water interfaces (FA layers on thin films of solid water and H(2)O adlayers on thin FA films). Metastable impact electron spectroscopy and ultraviolet photoemission spectroscopy (He I and II) were utilized to study the electron emission from the 10a' to 6a' molecular orbitals (MOs) of FA, and the 1b(1), 3a(1), and 1b(2) MOs of H(2)O. These spectra were compared with results of density-functional theory calculations on FA-H(2)O complexes reported in Ref. 14 [A. Allouche, J. Chem. Phys. 122, 234703(2005), (preceding paper)]. Temperature programmed desorption was applied for information on the desorption kinetics. Initially, FA is adsorbed on top of the water film. The FA spectra are distorted with respect to those from FA monomers; it is concluded that a strong interaction exists between the adsorbates. Even though partial solvation of FA species takes place during annealing, FA remains in the top layer up to the desorption of the water film. When H(2)O molecules are offered to FA films at 80 K, no water network is formed during the initial stage of water exposure; H(2)O molecules interact individually via H bonds with the formic acid network. Experiment and theory agree that no water-induced deprotonation of the formic acid molecules takes place.     

Interaction of NaCl with solid water

The interaction of NaCl with solid water, deposited on tungsten at 80 K, was investigated with metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy (UPS) (He I). We have studied the ionization of Cl(3p) and the 1b(1), 3a(1), and 1b(2) bands of molecular water. The results are supplemented by first-principles density functional theory (DFT) calculations of the electronic structure of solvated Cl(-) ions. We have prepared NaCl/water interfaces at 80 K, NaCl layers on thin films of solid water, and H(2)O ad-layers on thin NaCl films; they were annealed between 80 and 300 K. At 80 K, closed layers of NaCl on H(2)O, and vice versa, are obtained; no interpenetration of the two components H(2)O and NaCl was observed. However, ionic dissociation of NaCl takes place when H(2)O and NaCl are in direct contact. Above 115 K solvation of the ionic species Cl(-) becomes significant. Our results are compatible with a transition of Cl(-) species from an interface site (Cl in direct contact with the NaCl lattice) to an energetically favored configuration, where Cl species are solvated. The DFT calculations show that Cl(-) species, surrounded by their solvation shell, are nevertheless by some extent accessed by MIES because the Cl(3p)-charge cloud extends through the solvation shell. Water desorption is noticeable around 145 K, but is not complete before 170 K, about 15 K higher than for pure solid water. Above 150 K the NaCl-induced modification of the water network gives rise to gas phase like structures in the water spectra. In particular, the 3a(1) emission turns into a well-defined peak. This suggests that under these conditions water molecules interact mainly with Cl(-) rather than among themselves. Above 170 K only Cl is detected on the surface and desorbs around 450 K.     

Hydration and dissociation of hydrogen fluoric acid (HF)

The hydration and dissociation phenomena of HF(H(2)O)(n)() (n < or = 10) clusters have been studied by using both the density functional theory with the 6-311++G[sp] basis set and the M?ller-Plesset second-order perturbation theory with the aug-cc-pVDZ+(2s2p/2s) basis set. The structures for n > or = 8 are first reported here. The dissociated form of the hydrogen-fluoric acid in HF(H(2)O)(n) clusters is found to be less stable at 0 K than the undissociated form until n = 10. HF may not be dissociated at 0 K solely by water molecules because the HF H bond is stronger than the OH H bond, against the expectation that the dissociated HF(H(2)O)(n) would be more stable than the undissociated one in the presence of a number of water molecules. The dissociation would be possible for only a fraction of a number of hydrated HF clusters by the Boltzmann distribution at finite temperatures. This is in sharp contrast to other hydrogen halide acids (HCl, HBr, HI) showing the dissociation phenomena at 0 K for n > or = 4. The IR spectra of dissociated and undissociated structures of HF(H(2)O)(n) are compared. The structures and binding energies of HF(H(2)O)(n) are found to be similar to those of (H(2)O)(n+1). It is interesting that HF(H(2)O)(n=5,6,10) are slightly less stable compared with other sizes of clusters, just like the fact that (H(2)O)(n=6,7,11) are slightly less stable. The present study would be useful for the experimental/spectroscopic investigation of not only the dissociation phenomena of HF but also the similarity of the HF-water clusters to the water clusters.     

Magnesium monocationic complexes: a theoretical study of metal ion binding energies and gas-phase association kinetics

Bond dissociation energies (BDEs) for complexes of ground state Mg+ (2S) with several small oxygen- and nitrogen-containing ligands (H2O, CO, CO2, H2CO, CH3OH, HCOOH, H2CCO, CH3CHO, c-C2H4O, H2CCHOH, CH3CH2OH, CH3OCH3, NH3, HCN, H2CNH, CH3NH2, CH3CN, CH3CH2NH2, (CH3)2NH, H2NCN, and HCONH2) have been calculated at the CP-dG2thaw level of theory. These BDE values, as well as counterpoise-corrected MP2(thaw)/6-311+G(2df,p) calculations on the Mg+ complexes of several larger ligands, augment and complement existing experimental or theoretical determinations of gas-phase Mg+/ligand bond strengths. The reaction kinetics of complex formation are also investigated via variational transition state theory (VTST) calculations using the computed ligand and molecular ion parameters. Radiative association rate coefficients for most of these systems increase by approximately 1 order of magnitude with every 3-fold reduction in temperature from 300 to 10 K. Several of the largest molecules surveyed-notably, CH3COOH, (CH3)2CO, and CH3CH2CN-exhibit comparatively efficient radiative association with Mg+ (k(RA) > or = 1.0 x 10(-10) cm3 molecule(-1) s(-1)) at temperatures as high as 100 K, implying that these processes may have a considerable influence on the metal ion chemistry of warm molecular astrophysical environments known to contain these potential ligands. Our calculations also identify the infrared chromophoric brightness of various functional groups as a significant factor influencing the efficiency of the radiative association process.     

Dissolution nature of cesium fluoride by water molecules

The structures, stabilities, thermodynamic quantities, dissociation energies, infrared spectra, and electronic properties of CsF hydrated by water molecules are investigated by using density functional theory, M?ller-Plesset second-order perturbation theory (MP2), coupled cluster theory with singles, doubles, and perturbative triples excitations (CCSD(T)), and ab initio molecular dynamic (AIMD) simulations. It is revealed that at 0 K three water molecules (as a global minimum structure) begin to half-dissociate the Cs-F, and six water molecules (though not a global minimum energy structure) can dissociate it. By the combination of the accurate CCSD(T) conformational energies for Cs(H2O)6 at 0 K with the AIMD thermal energy contribution, it reveals that the half-dissociated structure is the most stable at 0 K, but this structure (which is still the most stable) changes to the dissociated structure above 50 K. The spectra of CsF(H2O)(1-6) from MP2 calculations and the power spectra of CsF(H2O)6 from 50 and 100 K AIMD simulations are also reported.     

Variations in the strength of the infrared forbidden 2328.2 cm-1 fundamental of solid N2 in binary mixtures.

We present the 2335-2325 cm-1 infrared spectra and band positions, profiles and strengths (A values) of solid nitrogen and binary mixtures of N2 with other molecules at 12 K. The data demonstrate that the strength of the infrared forbidden N2 fundamental near 2328 cm-1 is moderately enhanced in the presence of NH3, strongly enhanced in the presence of H2O and very strongly enhanced (by over a factor of 1000) in the presence of CO2, but is not significantly affected by CO, CH4, or O2. The mechanisms for the enhancements in N2-NH3 and N2-H2O mixtures are fundamentally different from those proposed for N2-CO2 mixtures. In the first case, interactions involving hydrogen-bonding are likely the cause. In the latter, a resonant exchange between the N2 stretching fundamental and the 18O = 12C asymmetric stretch of 18O12C16O is indicated. The implications of these results for several astrophysical issues are briefly discussed.     

Amphiphilic guest sorption of K2[Cr3O(OOCC2H5)6(H2O)3]2[alpha-SiW12O40] ionic crystal

An ionic crystal K2[Cr3O(OOCC2H5)6(H2O)3]2[alpha-SiW12O40] x 3H2O (1a) is synthesized by the complexation of a Keggin-type polyoxometalate of [alpha-SiW12O40]4- with K+ and a macrocation of [Cr3O(OOCC2H5)6(H2O)3]+. Compound 1a possesses both hydrophilic and hydrophobic channels in the crystal lattice. The 3 mol mol(-1) of the water of crystallization in 1a resides in the hydrophilic channel. The water of crystallization is removed by the evacuation at 303 K to form the guest-free phase 1b with small changes in the lattice lengths (+/-0.2 A). The water sorption profile is reproduced by the single rate constant. Therefore, the water sorbed probably resides in the hydrophilic channel. Compound 1b sorbs various kinds of polar organic molecules, and the amounts of < or = C3 alcohols are comparable to or larger than that of water, while chlorocarbons with no hydrogen-bonding ability and nonpolar molecules are excluded. Thus, 1b showed the amphiphilic sorption property. The states of the polar organic molecules sorbed in 1b have been quantitatively investigated using ethanol as a probe molecule. The IR, NMR, and single-crystal X-ray diffraction studies combined with the sorption kinetics reveal that ethanol molecules are mainly sorbed into the hydrophilic channel at P/P0 < or = 0.5, while the sorption into the hydrophobic channel is dominant at P/P0 > or = 0.6. Thus, it is demonstrated that ethanol molecules enter both hydrophilic and hydrophobic channels of 1b.     

Clathrate hydrate formation after CO2-H2O vapour deposition

We study vapour condensation of carbon dioxide and water at 77 K in a high-vacuum apparatus, transfer the sample to a piston-cylinder apparatus kept at 77 K and subsequently heat it at 20 MPa to 200 K. Samples are monitored by in situ volumetric experiments and after quench-recovery to 77 K and 1 bar by powder X-ray diffraction. At 77 K a heterogeneous mixture of amorphous solid water (ASW) and crystalline carbon dioxide is produced, both by co-deposition and sequential deposition of CO(2) and H(2)O. This heterogeneous mixture transforms to a mixture of cubic structure I carbon dioxide clathrate and crystalline carbon dioxide in the temperature range 160-200 K at 20 MPa. However, no crystalline ice is detected. This is, to the best of our knowledge, the first report of CO(2) clathrate hydrate formation from co-deposits of ASW and CO(2). The presence of external CO(2) vapour pressure in the annealing stage is not necessary for clathrate formation. The solid-solid transformation is accompanied by a density increase. Desorption of crystalline CO(2) atop the ASW sample is inhibited by applying 20 MPa in a piston-cylinder apparatus, and ultimately the clathrate is stabilized inside layers of crystalline CO(2) rather than in cubic or hexagonal ice. The vapour pressure of carbon dioxide needed for clathrate hydrate formation is lower by a few orders of magnitude compared to other known routes of CO(2) clathrate formation. The route described here is, thus, of relevance for understanding formation of CO(2) clathrate hydrates in astrophysical environments.     

Two-dimensional water and ice layers: neutron diffraction studies at 278, 263, and 20 k

Neutron diffraction elucidates the structures of two-dimensional (2D) water layers (278 K) or 2D ice layers confined in an organic slit-shaped nanospace. The two-dimensional ice phases reported here consist of individual eight-membered rings or folded-chain segments (263 K) and condensed twelve-membered irregular rings (20 K). This is quite different from bulk or other 2D ice structures; the latter usually form hexagonal honeycomb lattices. Both low-temperature structures typically feature water molecules which are surrounded by two or three other water molecules. Neutron diffraction and thermochemical studies indicate a liquid-solid-phase transition around 277 K for two-dimensional D2O layers. A further solid-solid-phase transition occurs between 263 and 20 K.     

Molecular dynamics study of tryptophylglycine: a dipeptide nanotube with confined water

To investigate the mechanism of structural changes of a peptide nanotube and water confined inside the channel, the helical peptide tryptophylglycine monohydrate (WG.H2O) was studied by molecular dynamics (MD) simulations using the three-dimension parallel MD program ddgmq (software package) and a consistent force field. Simulations were performed on both the water-containing system and a model system without water molecules. The details of the structural behavior with temperature are investigated for the entire simulated temperature range. Phase transitions were obtained at 115, 245, 270, 310, and 385 K, due to the contributions of both the peptide and the confined water subsystems. The crystalline, amorphous, liquidlike, liquid, and superheated phases of water were observed in the temperature ranges 40-115, 115-245, 245-310, 310-385, and >385 K, respectively. At 300 K, the diffusion constant of the confined water is 0.46 x 10-5 cm2 s-1, a value comparable to that of other peptide nanotubes. The empty peptide system melts at 440 K. Mechanisms of the negative thermal expansion (NTE) along the tube axis were investigated for different temperature ranges. The contraction of the crystalline water (or amorphous water) draws also the tube walls in and leads to NTE below 245 K. The other NTEs appear to be connected to the collapse of the ice network or the solid peptide network between 245 K and room temperature or from 310 to 440 K, respectively.     

Mimicking solvent shells in the gas phase. II. Solvation of K+

The observed gas-phase coordination number of K+ in K+(H2O)m clusters is smaller than that observed in bulk solution, where the coordination number has been reported to be between 6 and 8. Both theoretical and gas-phase studies of K+(H2O)m cluster ions point to a coordination number closer to 4. In the gas phase, the coordination number is determined by a variety of factors-the most critical being the magnitude of the K+...ligand pairwise interaction. Decreasing the magnitude of the ion...ligand interaction allows more ligands to directly interact with the cation. One method for decreasing the ion...ligand interaction in K+(H2O)m clusters is to systematically substitute weakly bound ligands for the more strongly bound water molecules. The systematic introduction of para-difluorobenzene (DFB) to K+(H2O)m clusters was monitored using infrared photodissociation spectroscopy in the OH stretching region. By varying the ratio of DFB molecules to water molecules present in K+(H2O)m(DFB)n clusters, the observed coordination number of gas-phase K+ was increased to 8, similar to that reported for bulk solution.     

Confocal laser microspectroscopic Rabi-flopping study of an iron oxide emitter surface used for Rydberg matter generation

Rydberg matter (RM) is a novel metal-like material in the form of electronically excited clusters of atoms (e.g. K and H) or molecules (e.g. H(2)). It is used as the inverted laser medium for IR in the RM laser. RM has recently been formed in its lowest state, which is proposed to be metallic hydrogen [Energy and Fuels 19 (2005) 2235]. An emitter material (K-doped iron oxide catalyst) that forms RM is studied by a specialized spectroscopic method, needed to detect the Rydberg states on the emitter surface. The spectroscopic method is phase-delay Rabi-flopping; it gives spectra from the time delay due to the periodic motion of the optical nutation vector. The formation of Rydberg species in the form of complexes K*-M (M a general small molecule) and (K-M)* is studied. So-called avoided transitions in K(+) ions are detected, of the same type as observed as transitions in the RM laser by stimulated emission. The formation and detection of Rydberg complexes containing H and H(2) is of great interest for metallic hydrogen production. Complexes with M=CH(2), H(2)O (or OH), CHO, H(2) and M'H are observed. Avoided transitions in RM clusters K(N)(*) are also identified. The identification of H containing Rydberg complexes on the surface indicates that metallic hydrogen is formed by the same cluster desorption route as other RM clusters.     

Thermally induced mixing of water dominated interstellar ices

Despite considerable attention in the literature being given to the desorption behaviour of smaller volatiles, the thermal properties of complex organics, such as ethanol (C(2)H(5)OH), which are predicted to be formed within interstellar ices, have yet to be characterized. With this in mind, reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD) have been used to probe the adsorption and desorption of C(2)H(5)OH deposited on top of water (H(2)O) films of various thicknesses grown on highly oriented pyrolytic graphite (HOPG) at 98 K. Unlike many other molecules detected within interstellar ices, C(2)H(5)OH has a comparable sublimation temperature to H(2)O and therefore gives rise to a complicated desorption profile. RAIRS and TPD show that C(2)H(5)OH is incorporated into the underlying ASW film during heating, due to a morphology change in both the C(2)H(5)OH and H(2)O ices. Desorption peaks assigned to C(2)H(5)OH co-desorption with amorphous, crystalline (CI) and hexagonal H(2)O-ice phases, in addition to C(2)H(5)OH multilayer desorption are observed in the TPD. When C(2)H(5)OH is deposited beneath ASW films, or is co-deposited as a mixture with H(2)O, complete co-desorption is observed, providing further evidence of thermally induced mixing between the ices. C(2)H(5)OH is also shown to modify the desorption of H(2)O at the ASW-CI phase transition. This behaviour has not been previously reported for more commonly studied volatiles found within astrophysical ices. These results are consistent with astronomical observations, which suggest that gas-phase C(2)H(5)OH is localized in hotter regions of the ISM, such as hot cores.