Quantum dynamics and spectroscopy of photodetachment in Cl-...H2O and Cl-...D2O complexes

We have modeled Zero Electron Kinetic Energy (ZEKE) spectra of Cl-…H2O and Cl-…D2O complexes using 3D quantum dynamical simulations on the three low-lying electronic states of the nascent neutral systems. Time-dependent quantum simulations combined with anionic and neutral stationary-state calculations by imaginary time propagation allowed for a detailed interpretation of the spectral features in terms of the underlying dynamics. Because of large differences between the anionic and neutral potential surfaces, the systems are found after electron photodetachment primarily high above the dissociation threshold. Nevertheless, pronounced long-lived resonances are observed, particularly for the lowest neutral state, reflecting the fact that a significant portion of the excess energy is initially deposited into nondissociative modes, that is, to (hindered) water rotation. These resonances form bands corresponding to water rotational states with a fine structure due to intermolecular stretch progressions. Comparison is made to experimental zero electron kinetic energy (ZEKE) spectra of the I-…H2O complex, where analogous anharmonic vibrational progressions have been observed

Preference of Cluster Isomers as a Result of Quantum Delocalization: Potential Energy Surfaces and Intermolecular Vibrational States of Ne...HBr, Ne...HI, and HI(Ar)n (n=1-6)

Intermolecular vibrational states are calculated for Ne…HBr, Ne…HI, and HI(Ar)n (n=1-6) complexes using potential energy surfaces constructed by accurate ab initio methods. Potentials of rare gas-hydrogen halide clusters exhibit two collinear minima, one corresponding to hydrogen lying between the heavy atoms, and the other to hydrogen facing away from the rare gas atom. The relative depths of the two minima are a result of a subtle balance between polarization and dispersion interactions. Moreover, due to a large quantum delocalization in the hydrogen bending (librational) motion the relevance of a particular stationary point on the potential energy surface is only limited. It is more appropriate to discuss the isomers in terms of vibrationally averaged structures. For Ne…HBr the potential minimum and the vibrationally averaged structure correspond to the same isomer with hydrogen between neon and bromide. However, for Ne…HI the global minimum corresponds to the Ne-IH collinear geometry, while the vibrationally averaged structure has hydrogen between the heavy atoms. In the case of HI(Ar)n we show that one can flip between the two isomers by adding argon atoms, which reconciles the seemingly contradictory experimental results obtained for the photodissociation of HI…Ar on one side, and of large HI(Ar)n clusters on the other side

The ice-vapor interface and the melting point of ice Ih for the polarizable POL3 water model

International audienceWe use molecular dynamics simulations to determine the melting point of ice Ih for the polarizable POL3 water force field (Dang, L. X. J. Chem. Phys.1992, 97, 2659). Simulations are performed on a slab of ice Ih with two free surfaces at several different temperatures. The analysis of the time evolution of the total energy in the course of the simulations at the set of temperatures yields the melting point of the POL3 model to be Tm = 180 ± 10 K. Moreover, the results of the simulations show that the degree of hydrogen-bond disorder occurring in the bulk of POL3 ice is larger (at the corresponding degree of undercooling) than in ice modeled by nonpolarizable water models. These results demonstrate that the POL3 water force field is rather a poor model for studying ice and ice−liquid or ice−vapor interfaces. While a number of polarizable water models have been developed over the past years, little is known about their performance in simulations of supercooled water and ice. This study thus highlights the need for testing of the existing polarizable water models over a broad range of temperatures, pressures, and phases, and developing a new polarizable water force field, reliable over larger areas of the phase diagram

OXIDATION OF NaBr AEROSOL BY OZONE: IMPORTANCE OF A SURFACE REACTION

$^{a}$Support by NSF Collaborative Research in Chemistry grant is gratefully acknowledged.Author Institution: School of Physical Sciences, Department of Chemistry, University of California; School of Physical Sciences, Department of Chemistry, EPRI; The Henry Samueli School of Engineering, Department of Mechanical and Aerospace Engineering, University of California; School of Physical Sciences, Department of Chemistry, University of CaliforniaThe release of Br atoms from photolyzable bromine species, such as $Br_{2}$, is responsible for the almost complete destruction of ground-level ozone observed in the Arctic after polar sunrise, and is likely to play a role in the partial destruction of ozone observed in the marine boundary layer at mid-latitudes. To investigate the mechanism of the reactions of $O_{3}$ with NaBr aerosol, experiments were carried out at room temperature and atmospheric pressure in a 561 L aerosol chamber above the deliquescence point of NaBr aerosol. Fourier transform infrared spectroscopy was used to measure the concentrations of $O_{3}$ and molecular bromine concentrations were monitored using atmospheric pressure chemical ionization mass spectrometry. A computer kinetics model, including gas and aqueous phase chemical reactions, gas and aqueous phase diffusion, and mass transfer between the liquid aerosol droplets and the gas phase, was used to evaluate the mechanism for bromine production. Experimental results are not reproduced well by known gas phase and aqueous phase bromine chemistry alone, and thus, a reaction occurring at the air-water interface between gaseous ozone and aqueous bromide ion to produce $Br_{2}$ via an $O_{3}\cdots Br^{-}$ surface complex is proposed. With the inclusion of this interface reaction, the model satisfactorily reproduces experimental results. While there is no direct spectroscopic evidence of the surface complex, molecular dynamics simulations provide further support for the proposed heterogeneous reaction mechanism. They show that $O_{3}$ strongly prefers to reside at the interface rather than in the bulk solution, and that it makes frequent and long enough contacts with bromide ion for the surface reaction to be feasible. The atmospheric implications will be discussed

A review of air-ice chemical and physical interactions (AICI): liquids, quasi liquids, and solids in snow

Snow in the environment acts as a host to rich chemistry and provides a matrix for physical exchange of contaminants within the ecosystem. The goal of this review is to summarise the current state of knowledge of physical processes and chemical reactivity in surface snow with relevance to polar regions. It focuses on a description of impurities in distinct compartments present in surface snow, such as snow crystals, grain boundaries, crystal surfaces, and liquid parts. It emphasises the microscopic description of the ice surface and its link with the environment. Distinct differences between the disordered air–ice interface, often termed quasi-liquid layer, and a liquid phase are highlighted. The reactivity in these different compartments of surface snow is discussed using many experimental studies, simulations, and selected snow models from the molecular to the macro-scale. Although new experimental techniques have extended our knowledge of the surface properties of ice and their impact on some single reactions and processes, others occurring on, at or within snow grains remain unquantified. The presence of liquid or liquid-like compartments either due to the formation of brine or disorder at surfaces of snow crystals below the freezing point may strongly modify reaction rates. Therefore, future experiments should include a detailed characterisation of the surface properties of the ice matrices. A further point that remains largely unresolved is the distribution of impurities between the different domains of the condensed phase inside the snowpack, i.e. in the bulk solid, in liquid at the surface or trapped in confined pockets within or between grains, or at the surface. While surface-sensitive laboratory techniques may in the future help to resolve this point for equilibrium conditions, additional uncertainty for the environmental snowpack may be caused by the highly dynamic nature of the snowpack due to the fast metamorphism occurring under certain environmental conditions. Due to these gaps in knowledge the first snow chemistry models have attempted to reproduce certain processes like the long-term incorporation of volatile compounds in snow and firn or the release of reactive species from the snowpack. Although so far none of the models offers a coupled approach of physical and chemical processes or a detailed representation of the different compartments, they have successfully been used to reproduce some field experiments. A fully coupled snow chemistry and physics model remains to be developed