Photodissociation of water in crystalline ice: A molecular dynamics study

Probing Laser-Induced Plasma Generation in Liquid Water

Understanding photoexcitation dynamics in liquid water is of crucial significance for both fundamental scientific exploration and technological applications. Despite the observations of photoinduced macroscopic phenomena, the initial atomistic movements and associated energy transfer pathways immediately following laser irradiation are hard to track due to the extreme complexity of laser-water interaction and its ultrafast time scale. We explore the femtosecond evolution of liquid water upon intense photoexcitation based on nonadiabatic quantum dynamics simulations. Separate ionic and electronic dynamics were explicitly monitored with tremendous details unveiled on an unprecedented microscopic level. Water was found to undergo the two-step heating processes. The strong-field effects and electronic excitations dominate the first-stage heating and pressurization. Subsequent relaxation of ionic and electronic subsystems further increases the ionic temperature but releases the large internal pressure. The water molecules are stretched during the laser pulses, and the electronic excitations result in the proton transfers after laser pulses. Intense laser pulses violently excite liquid water, giving rise to severe molecular dissociation and plasma generation during the laser pulses. The laser-induced water plasma is characterized by a high fraction of free protons (∼50%), nonequilibrium ionic and electronic distributions, and a metallic electronic density of states.

Quantification of the Role of Chemical Desorption in Molecular Clouds

ConspectusDark molecular clouds have low temperatures of approximately 10 K and experience very little UV irradiation. These clouds are the birthplace of new stars and consist of gas and dust particles. The latter can act as a meeting place to facilitate surface chemistry to form saturated molecules such as formaldehyde, methyl formate, and dimethyl ether. These complex organic molecules or COMs become encapsulated in the ice that forms on the dust grains, and these ices are the precursor for cometary ices and other icy bodies. They likely played a role in bringing material to the early earth.Although these COMs are likely formed on the surfaces of dust grains, several of them have been detected in the gas phase. This means that they have desorbed from the grain under these cold, dark conditions where thermal desorption and photodesorption are negligible. It has been speculated that reactive, or chemical, desorption is responsible for the high gas-phase abundance. After a surface reaction, its products might be vibrationally, translationally, and/or rotationally excited. Dissipation of the excess energy to translational energy can briefly increase the desorption rate, leading to chemical desorption. Astrochemical modellers have added terms to their rate equations to account for this effect. These terms, however, have had little experimental or theoretical verification.In this Account, we use classical molecular dynamics (MD) simulations to give adsorbed molecules a fixed amount of energy as a proxy for excess energy and to record whether this leads to desorption. The excitation energy can be varied freely while keeping all other variables constant. This allows for the study of trends rather than being limited to a single reaction and a single system. The focus is on the dependence of the chemical desorption on the excitation energy, excitation type, and binding energy. Rotational and vibrational excitation was explicitly taken into account. An analytical expression for the chemical desorption probability was obtained in this way. It depends on the binding energy and reaction enthalpy. This expression was then implemented in a gas-grain astrochemical code to simulate the chemical evolution of a dark molecular cloud, and the results were compared against observational abundances of COMs in three different molecular clouds. The results with our new expression based on the MD simulations show good agreement for all species except H2CO, which has both gas-phase and surface-formation routes. This is a significant improvement over models without chemical desorption or with other expressions for chemical desorption, as frequently used by other authors. It is encouraging to see that a general description with a firmer theoretical basis leads to a significant improvement. Understanding chemical desorption can help to explain the unexpectedly high gas-phase abundance of some COMs, and chemical desorption also provides a link between the gas phase and the ice mantle, and its understanding might help in creating a diagnostic tool to learn more about the ice composition.

Ultrafast photochemistry of methyl hydroperoxide on ice particles

Simulations show that photodissociation of methyl hydroperoxide, CH(3)OOH, on water clusters produces a surprisingly wide range of products on a subpicosecond time scale, pointing to the possibility of complex photodegradation pathways for organic peroxides on aerosols and water droplets. Dynamics are computed at several excitation energies at 50 K using a semiempirical PM3 potential surface. CH(3)OOH is found to prefer the exterior of the cluster, with the CH(3)O group sticking out and the OH group immersed within the cluster. At atmospherically relevant photodissociation wavelengths the OH and CH(3)O photofragments remain at the surface of the cluster or embedded within it. However, none of the 25 completed trajectories carried out at the atmospherically relevant photodissociation energies led to recombination of OH and CH(3)O to form CH(3)OOH. Within the limited statistics of the available trajectories the predicted yield for the recombination is zero. Instead, various reactions involving the initial fragments and water promptly form a wide range of stable molecular products such as CH(2)O, H(2)O, H(2), CO, CH(3)OH, and H(2)O(2).

Excited state properties of liquid water

In this paper, we give an overview of the state of the art in calculations of the electronic band structure and absorption spectra of water. After an introduction to the main theoretical and computational schemes used, we present results for the electronic and optical excitations of water. We focus mainly on liquid water, but spectroscopic properties of ice and vapor phase are also described. The applicability and the accuracy of first-principles methods are discussed, and results are critically presented.

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.

Molecular-dynamics study of photodissociation of water in crystalline and amorphous ices

We present the results of classical dynamics calculations performed to study the photodissociation of water in crystalline and amorphous ice surfaces at a surface temperature of 10 K. A modified form of a recently developed potential model for the photodissociation of a water molecule in ice [S. Andersson et al., Chem. Phys. Lett. 408, 415 (2005)] is used. Dissociation in the top six monolayers is considered. Desorption of H(2)O has a low probability (less than 0.5% yield per absorbed photon) for both types of ice. The final outcome strongly depends on the original position of the photodissociated molecule. For molecules in the first bilayer of crystalline ice and the corresponding layers in amorphous ice, desorption of H atoms dominates. In the second bilayer H atom desorption, trapping of the H and OH fragments in the ice, and recombination of H and OH are of roughly equal importance. Deeper into the ice H atom desorption becomes less important and trapping and recombination dominate. Motion of the photofragments is somewhat more restricted in amorphous ice. The distribution of distances traveled by H atoms in the ice peaks at 6-7 Angstroms with a tail going to about 60 Angstroms for both types of ice. The mobility of OH radicals is low within the ice with most probable distances traveled of 2 and 1 Angstrom for crystalline and amorphous ices, respectively. OH is, however, quite mobile on top of the surface, where it has been found to travel more than 80 Angstroms. Simulated absorption spectra of crystalline ice, amorphous ice, and liquid water are found to be in very good agreement with the experiments. The outcomes of photodissociation in crystalline and amorphous ices are overall similar, but with some intriguing differences in detail. The probability of H atoms desorbing is 40% higher from amorphous than from crystalline ice and the kinetic-energy distribution of the H atoms is on average 30% hotter for amorphous ice. In contrast, the probability of desorption of OH radicals from crystalline ice is much higher than that from amorphous ice.

Stretching of hydrogen-bonded OH in the lowest singlet excited electronic state of water dimer

The lowest singlet excited electronic state of water monomer in the gas phase is strictly dissociative along a OH stretch coordinate but changes its nature when the stretched OH moiety is hydrogen bonded to a neighboring water molecule. This work extends previous exploration of the water dimer excited singlet potential-energy surface, using computational methods that are reliable even at geometries well removed from the ground-state equilibrium. First, the hydrogen-bonded OH moiety is stretched far enough to establish the existence of a barrier that is sufficient to support a quasibound vibrational state of the OH oscillator near the Franck-Condon region. Second, the constraint of an icelike structure is relaxed, and it is found that a substantial fraction of liquidlike structures also supports a quasibound vibrational state. These potential-energy explorations on stretching of the hydrogen-bonded OH moiety in a water dimer are discussed as a model for understanding the initial dynamics upon excitation into the lowest excited singlet state of condensed water. The possibility is raised that the excited-state lifetime may be long enough to allow for exciton migration, which would provide a mechanism for energy transport in condensed water phases.