Electronic Excitation Dynamics in Liquid Water under Proton Irradiation

Molecular behaviour of liquid water under proton irradiation is of great importance to a number of technological and medical applications. The highly energetic proton generates a time-varying field that is highly localized and heterogeneous at the molecular scale, and massive electronic excitations are produced as a result of the field-matter interaction. Using first-principles quantum dynamics simulations, we reveal details of how electrons are dynamically excited through non-equilibrium energy transfer from highly energetic protons in liquid water on the atto/femto-second time scale. Water molecules along the path of the energetic proton undergo ionization at individual molecular level, and the excitation primarily derives from lone pair electrons on the oxygen atom of water molecules. A reduced charge state on the energetic proton in the condensed phase of water results in the strongly suppressed electronic response when compared to water molecules in the gas phase. These molecular-level findings provide important insights into understanding the water radiolysis process under proton irradiation

QUANTUM DYNAMICS OF EXCITED ELECTRONS FROM FIRST PRINCIPLES: HOT CARRIER RELAXATION AND ELECTRONIC STOPPING

An understanding of dynamical properties of matter is often essential to developing a deeper understanding of systems and their behavior. Many important examples of complex electron dynamics are a result of excited systems such as in the photoexcitation of photosynthetic complexes or hot carrier relaxation in photovoltaic devices. Applying first-principles computational methods to describe these systems and their dynamics would be a valuable tool to gain a deeper understanding of the relationship between atomic structure and the non-equilibrium electron dynamics. Approaches based on the Born-Oppenheimer adiabatic approximation fail, however, as the separation of electron and nuclear motion is no longer valid in the presence of excited electrons. First-principles simulations that incorporate non-adiabatic effects represent an approach to properly simulating the quantum dynamics of excited electrons while maintaining predictive power. In this work, we investigate two non-adiabatic phenomena while at the same time maintaining atomistic-level detail. These two phenomena are hot-carrier relaxation in silicon quantum dots and electronic stopping power in liquid water. We conclude that in nanoscale systems, excited electron relaxation dynamics are sensitive to the surface passivation. Using a fewest-switches surface hopping approach, we identified a unique electronic state that appears in nanocrystalline sillicon when passivated by fluorine that acts as a “shuttle state” to extend the iv electron relaxation rate, by a factor of five compared to an identical system passivated by hydrogen. To simulate electronic stopping in water, we use real-time time-dependent density functional theory simulations to capture the energy transfer process from high-energy protons and alpha-particles (1MeV-10MeV) into liquid water. We report the first ab initio electronic stopping power curve for liquid water. Additionally, we use the real-time electron density to offer a qualitative interpretation of the electron dynamics. We observe a greater contribution of lone pair electrons compared to electrons in the OH bond during the excitations of individual molecules involved in the electronic stopping process. Finally, we conclude that the effective charge of energetic ions in liquid phases result in a significant suppression of excitations by an order of magnitude compared to the excitations observed in the gas phase.Doctor of Philosoph

Electronic stopping power in liquid water for protons and α particles from first principles

Atomistic calculations of the electronic stopping power in liquid water for protons and α particles from first principles are demonstrated without relying on linear response theory. The computational approach is based on nonequilibrium simulation of the electronic response using real-time time-dependent density functional theory. By quantifying the velocity dependence of the steady-state charge of the projectile proton and α particle from nonequilibrium electron densities, we examine the extent to which linear response theory is applicable. We further assess the influence of the exchange-correlation approximation in real-time time-dependent density functional theory on the stopping power with range-separated and regular hybrid functionals with exact exchange

Constraining quenching timescales in galaxy clusters by forward-modelling stellar ages and quiescent fractions in projected phase space

We forward-model mass-weighted stellar ages (MWAs) and quiescent fractions in projected phase space (PPS), using data from the Sloan Digital Sky Survey, to jointly constrain an infall quenching model for galaxies in $\log(M_{\mathrm{vir}}/\mathrm{M}_{\odot})>14$ galaxy clusters at $z\sim 0$. We find the average deviation in MWA from the MWA-$M_\star$ relation depends on position in PPS, with a maximum difference between the inner cluster and infalling interloper galaxies of $\sim 1$ Gyr. Our model employs infall information from N-body simulations and stochastic star-formation histories from the UniverseMachine model. We find total quenching times of $t_\mathrm{Q}=3.7\pm 0.4$ Gyr and $t_\mathrm{Q}=4.0\pm 0.2$ Gyr after first pericentre, for $9<\log(M_{\star}/\mathrm{M}_{\odot})<10$ and $10<\log(M_{\star}/\mathrm{M}_{\odot})<10.5$ galaxies, respectively. By using MWAs, we break the degeneracy in time of quenching onset and timescale of star formation rate (SFR) decline. We find that time of quenching onset relative to pericentre is $t_{\mathrm{delay}}=3.5^{+0.6}_{-0.9}$ Gyr and $t_{\mathrm{delay}}=-0.3^{+0.8}_{-1.0}$ Gyr for our lower and higher stellar mass bins, respectively, and exponential SFR suppression timescales are $\tau_{\mathrm{env}}\leq 1.0$ Gyr and $\tau_{\mathrm{env}}\sim 2.3$ Gyr for our lower and higher stellar mass bins, respectively. Stochastic star formation histories remove the need for rapid infall quenching to maintain the bimodality in the SFR of cluster galaxies; the depth of the green valley prefers quenching onsets close to first pericentre and a longer quenching envelope, in slight tension with the MWA-driven results. Taken together these results suggest that quenching begins close to, or just after pericentre, but the timescale for quenching to be fully complete is much longer and therefore ram-pressure stripping is not complete on first pericentric passage.Comment: 21 pages, 13 figures, submitted to MNRA

Recruitment Facilitation and Spatial Pattern Formation in Soft-Bottom Mussel Beds

Mussels (Mytilus edulis) build massive, spatially complex, biogenic structures that alter the biotic and abiotic environment and provide a variety of ecosystem services. Unlike rocky shores, where mussels can attach to the primary substrate, soft sediments are unsuitable for mussel attachment. We used a simple lattice model, field sampling, and field and laboratory experiments to examine facilitation of recruitment (i.e., preferential larval, juvenile, and adult attachment to mussel biogenic structure) and its role in the development of power-law spatial patterns observed in Maine, USA, soft-bottom mussel beds. The model demonstrated that recruitment facilitation produces power-law spatial structure similar to that in natural beds. Field results provided strong evidence for facilitation of recruitment to other mussels—they do not simply map onto a hard-substrate template of gravel and shell hash. Mussels were spatially decoupled from non-mussel hard substrates to which they can potentially recruit. Recent larval recruits were positively correlated with adult mussels, but not with other hard substrates. Mussels made byssal thread attachments to other mussels in much higher proportions than to other hard substrates. In a field experiment, mussel recruitment was highest to live mussels, followed by mussel shell hash and gravel, with almost no recruitment to muddy sand. In a laboratory experiment, evenly dispersed mussels rapidly self-organized into power-law clusters similar to those observed in nature. Collectively, the results indicate that facilitation of recruitment to existing mussels plays a major role in soft-bottom spatial pattern development. The interaction between large-scale resource availability (hard substrate) and local-scale recruitment facilitation may be responsible for creating complex power-law spatial structure in soft-bottom mussel beds

CUI@CSCW: Collaborating through Conversational User Interfaces

This virtual workshop seeks to bring together the burgeoning communities centred on the design, development, application and study of so-called Conversational User Interfaces (CUIs). CUIs are used in myriad contexts, from online support chatbots through to entertainment devices in the home. In this workshop, we will examine the challenges involved in transforming CUIs into everyday computing devices capable of supporting collaborative activities across space and time. Additionally, this workshop seeks to establish a cohesive CUI community and research agenda within CSCW. We will examine the roles in which CSCW research can contribute insights into understanding how CUIs are or can be used in a variety of settings, from public to private, and how they can be brought into a potentially unlimited number of tasks. This proposed workshop will bring together researchers from academia and practitioners from industry to survey the state-of-the-art in terms of CUI design, use, and understanding, and will map new areas for work including addressing the technical, social, and ethical challenges that lay ahead. By bringing together existing researchers and new ideas in this space, we intend to foster a strong community and enable potential future collaborations

Nonequilibrium clumped isotope signals in microbial methane

Methane is a key component in the global carbon cycle with a wide range of anthropogenic and natural sources. Although isotopic compositions of methane have traditionally aided source identification, the abundance of its multiply-substituted “clumped” isotopologues, e.g., 13CH3D, has recently emerged as a proxy for determining methane-formation temperatures; however, the impact of biological processes on methane’s clumped isotopologue signature is poorly constrained. We show that methanogenesis proceeding at relatively high rates in cattle, surface environments, and laboratory cultures exerts kinetic control on 13CH3D abundances and results in anomalously elevated formation temperature estimates. We demonstrate quantitatively that H2 availability accounts for this effect. Clumped methane thermometry can therefore provide constraints on the generation of methane in diverse settings, including continental serpentinization sites and ancient, deep groundwaters.National Science Foundation (U.S.) (EAR-1250394)National Science Foundation (U.S.) (EAR-1322805)Deep Carbon Observatory (Program)Natural Sciences and Engineering Research Council of CanadaDeutsche Forschungsgemeinschaft (Gottfried Wilhelm Leibniz Program)United States. Dept. of Defense (National Defense Science and Engineering Graduate Fellowship)Neil & Anna Rasmussen FoundationGrayce B. Kerr Fund, Inc. (Fellowship)MIT Energy Initiative (Shell-MITEI Graduate Fellowship)Shell International Exploration and Production B.V. (N. Braunsdorf and D. Smit of Shell PTI/EG grant