Photodissociation of polycrystalline and amorphous water ice films at 157 and 193nm

Chemical Reactivity of Strongly Interacting, Hydrogen-Bond-Forming Molecules Following 193 nm Photon Irradiation: Methanol in Amorphous Solid Water at Low Temperatures

Mixtures of methanol and amorphous solid water (ASW) ices are observed in the interstellar medium (ISM), where they are subject to irradiation by UV photons and bombardment by charged particles. The charged particles, if at high enough density, induce a local electric field in the ice film that potentially affects the photochemistry of these ices. When CD₃OD@ASW ices grown at 38 K on a Ru(0001) substrate are irradiated by 193 nm (6.4 eV) photons, products such as HD, D₂, CO, and CO₂ are formed in large abundances relative to the initial amount of CD₃OD. Other molecules such as D₂O, CD₄, acetaldehyde, and ethanol and/or dimethyl ether are also observed, but in smaller relative abundances. The reactivity cross sections range from (2.6 ± 0.3) × 10^-21 to (3.8 ± 0.3) × 10^-25 cm²/photon. The main products are formed through two competing mechanisms: direct photodissociation of methanol and water and dissociative electron attachment (DEA) by photoelectrons ejected from the Ru(0001) substrate. An electric field of 2 × 10⁸ V/m generated within the ASW film during Ne⁺ ions bombardment is apparently not strong enough to affect the relative abundances (selectivity) of the photochemical products observed in this study.

UV-Induced Formation of Ice XI Observed Using an Ultra-High Vacuum Cryogenic Transmission Electron Microscope and its Implications for Planetary Science

The occurrence of hydrogen atom-ordered form of ice Ih, ice XI, in the outer Solar System has been discussed based on laboratory experiments because its ferroelectricity influences the physical processes in the outer Solar System. However, the formation of ice XI in that region is still unknown due to a lack of formation conditions at temperatures higher than 72 K and the effect of UV-rays on the phase transition from ice I to ice XI. As a result, we observed the UV-irradiation process on ice Ih and ice Ic using a newly developed ultra-high vacuum cryogenic transmission electron microscope. We found that ice Ih transformed to ice XI at temperatures between 75 and 140 K with a relatively small UV dose. Although ice Ic partially transformed to ice XI at 83 K, the rate of transformation was slower than for ice Ih. These findings point to the formation of ice XI at temperatures greater than 72 K via UV irradiation of ice I crystals in the Solar System; icy grains and the surfaces of icy satellites in the Jovian and Saturnian regions.

Mechanism of Indirect Photon-Induced Desorption at the Water Ice Surface

Electronic excitations near the surface of water ice lead to the desorption of adsorbed molecules, through a so far debated mechanism. A systematic study of photon-induced indirect desorption, revealed by the spectral dependence of the desorption (7-13 eV), is conducted for Ar, Kr, N_{2}, and CO adsorbed on H_{2}O or D_{2}O amorphous ices. The mass and isotopic dependence and the increase of intrinsic desorption efficiency with photon energy all point to a mechanism of desorption induced by collisions between adsorbates and energetic H/D atoms, produced by photodissociation of water. This constitutes a direct and unambiguous experimental demonstration of the mechanism of indirect desorption of weakly adsorbed species on water ice, and sheds new light on the possibility of this mechanism in other systems. It also has implications for the description of photon-induced desorption in astrochemical models.

Hydrogen Dissociation Dynamics from Water Clusters on Triplet-State Energy Surfaces

The ice surface provides two-dimensional reaction fields for bimolecular collisions in interstellar space. As H₂O molecules on the surface are typically exposed to cosmic rays, H₂O in the excited state is easily dissociated into H + OH, where a H atom is released from the surface to the gas phase. In the present study, the reaction dynamics of small-sized water clusters on the triplet-state potential energy (T₁) surface, following the vertical electronic excitation from the ground state (S₀), were investigated using direct ab initio molecular dynamics to provide insights into the generation mechanism of H atoms from an irradiated ice surface. In all clusters, that is, (H₂O)_n (n = 2-6), the H atom was directly dissociated from one of the H₂O molecules in the clusters (direct dissociation), whereas the OH radical remained in the cluster. In the branched form of H₂O tetramer (n = 4) and the book form of H₂O hexamer (n = 6), the dissociated hydrogen atom (H') collided with the neighboring H₂O molecule, and the exchange of H atoms occurred as H' + H₂O → H'-H₂O (collision) → H'OH + H (hydrogen exchange). The translational energy of the exchanged H atom decreases significantly (by approximately 50%) because the kinetic energy of the H' atom is efficiently transferred to the vibrational modes of the cluster during the H-exchange reaction. The mechanism of H atom dissociation is discussed based on theoretical results.

Variation of optical spectra of water clusters with size from many-body Green's function theory

Water clusters are an important species in the environment and atmosphere and take part in various chemical and biological reactions. How their optical properties vary with size is still an open question. Using the GW method and Bethe-Salpeter equation within the ab initio many-body Green's function theory, we study the electronic excitations in a series of water clusters (H₂O)_n with n = 1-48. We find that their absorption peaks blueshift with increasing cluster size due to the reducing electron-hole binding energy which arises from the enhanced electronic screening and gradually delocalized excitonic spatial distribution. The position of the first absorption peak has a close relation to the average number of hydrogen bonds per molecule. Off-diagonal matrix elements of the self-energy operator have pronounced effects on the unoccupied electronic levels and optical absorption for small clusters with n ≤ 10 when using density functional theory as the starting point for GW calculations. Although the optical absorption is predominated by delocalized excitons, highly localized excitons on a single water molecule are always present on the cluster surface in the vicinity of the absorption edge. These localized excitons may facilitate the photodissociation of water molecules. This can provide inspiration on the excited-state dynamics and photolysis in water clusters.

The mechanism for the formation of OH radicals in condensed-phase water under ultraviolet irradiation

OH radicals can be produced via direct water photolysis through concerted proton and electron transfer.

Efficient electron-promoted desorption of benzene from water ice surfaces

We study the desorption of benzene from solid water surfaces during irradiation of ultrathin solid films with low energy electrons.

Excitation Energy Transfer and Low-Efficiency Photolytic Splitting of Water Ice by Vacuum UV Light

Experimental estimates of photolytic efficiency (yield per photon) for photodissociation and photodesorption from water ice range from about 10(-3) to 10(-1). However, in the case of photodissociation of water in the gas phase, it is close to unity. Exciton dynamics carried out by a quantum mechanical time-dependent propagator shows that in the eight most stable water hexamers, the excitation diffuses away from the initially excited molecule within a few femtoseconds. On the basis of these quantum dynamics simulations, it is hypothesized that the ultrafast exciton energy transfer process, which in general gives rise to a delocalized exciton within these clusters, may contribute to the low efficiency of photolytic processes in water ice. It is proposed that exciton diffusion inherently competes with the nuclear dynamics that drives the photodissociation process in the repulsive S1 state on the sub-10 fs time scale.

Surface abundance change in vacuum ultraviolet photodissociation of CO2 and H2O mixture ices

Photodissociation of amorphous ice films of carbon dioxide and water co-adsorbed at 90 K was carried out at 157 nm using oxygen-16 and -18 isotopomers with a time-of-flight photofragment mass spectrometer. O((3)P(J)) atoms, OH (v = 0) radicals, and CO (v = 0,1) molecules were detected as photofragments. CO is produced directly from the photodissociation of CO(2). Two different adsorption states of CO(2), i.e., physisorbed CO(2) on the surface of amorphous solid water and trapped CO(2) in the pores of the film, are clearly distinguished by the translational and internal energy distributions of the CO molecules. The O atom and OH radical are produced from the photodissociation of H(2)O. Since the absorption cross section of CO(2) is smaller than that of H(2)O at 157 nm, the CO(2) surface abundance is relatively increased after prolonged photoirradiation of the mixed ice film, resulting in the formation of a heterogeneously layered structure in the mixed ice at low temperatures. Astrophysical implications are discussed.

Some fundamental properties and reactions of ice surfaces at low temperatures

Ice surfaces offer a unique chemical environment in which reactions occur quite differently from those in liquid water or gas phases. In this article, we examine the basic properties of ice surfaces below the surface premelting temperature and discuss some of the recent investigations carried out on reactions at the ice surfaces. The static and dynamic properties of an ice surface as a reaction medium, such as its structure, molecule diffusion and proton transfer dynamics, and the surface preference of hydronium and hydroxide ions, are discussed in relation to the reactivity of the surface.

Formation mechanisms of oxygen atoms in the O((3)P(J)) state from the 157 nm photoirradiation of amorphous water ice at 90 K

Desorption of ground state O((3)P(J=2,1,0)) atoms following the vacuum ultraviolet photolysis of water ice in the first absorption band was directly measured with resonance-enhanced multiphoton ionization (REMPI) method. Based on their translational energy distributions and evolution behavior, two different formation mechanisms are proposed: One is exothermic recombination reaction of OH radicals, OH+OH-->H(2)O+O((3)P(J)) and the other is the photodissociation of OH radicals on the surface of amorphous solid water. The translational and internal energy distributions of OH radicals as well as the evolution behavior were also measured by REMPI to elucidate the roles of H(2)O(2) and OH in the O((3)P(J)) formation mechanisms.

Desorption of hydroxyl radicals in the vacuum ultraviolet photolysis of amorphous solid water at 90 K

We have studied the desorption dynamics of OH radicals from the 157 nm photodissociation of amorphous solid water (ASW) as well as H(2)O(2) deposited on an ASW surface at 90 K. The translational and internal energy distributions of OH were measured using resonance-enhanced multiphoton ionization methods. These distributions are compared to reported molecular dynamics calculations for the condensed phase photodissociation of water ice and also reported results for the gas phase photodissociation of H(2)O at 157 nm. We have confirmed that OH radicals are produced from two different mechanisms: one from primary photolysis of surface H(2)O of ASW, and the other being secondary photolysis of H(2)O(2) photoproducts on the ASW surface after prolonged irradiation at 157 nm.

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.

Excited state dynamics of liquid water: Insight from the dissociation reaction following two-photon excitation

The authors use transient absorption spectroscopy to monitor the ionization and dissociation products following two-photon excitation of pure liquid water. The primary decay mechanism changes from dissociation at an excitation energy of 8.3 eV to ionization at 12.4 eV. The two channels occur with similar yield for an excitation energy of 9.3 eV. For the lowest excitation energy, the transient absorption at 267 nm probes the geminate recombination kinetics of the H and OH fragments, providing a window on the dissociation dynamics. Modeling the OH geminate recombination indicates that the dissociating H atoms have enough kinetic energy to escape the solvent cage and one or two additional solvent shells. The average initial separation of H and OH fragments is 0.7+/-0.2 nm. Our observation suggests that the hydrogen bonding environment does not prevent direct dissociation of an O-H bond in the excited state. We discuss the implications of our measurement for the excited state dynamics of liquid water and explore the role of those dynamics in the ionization mechanism at low excitation energies.