The sticking probability of D2O-water on ice: Isotope effects and the influence of vibrational excitation

International audienceThe present study measures the sticking probability of heavy water (D2O) on H2O- and on D2O-ice and probes the influence of selective OD-stretch excitation on D2O sticking on these ices. Molecular beam techniques are combined with infrared laser excitation to allow for precise control of incident angle, translational energy, and vibrational state of the incident molecules. For a translational energy of 69 kJ/mol and large incident angles (θ ≥ 45°), the sticking probability of D2O on H2O-ice was found to be 1% lower than on D2O-ice. OD-stretch excitation by IR laser pumping of the incident D2O molecules produces no detectable change of the D2O sticking probability (<10−3). The results are compared with other gas/surface systems for which the effect of vibrational excitation on trapping has been probed experimentally

Exciton spin relaxation in single semiconductor quantum dots

We study the relaxation of the exciton spin (longitudinal relaxation time $T_{1}$) in single asymmetrical quantum dots due to an interplay of the short--range exchange interaction and acoustic phonon deformation. The calculated relaxation rates are found to depend strongly on the dot size, magnetic field and temperature. For typical quantum dots and temperatures below 100 K, the zero--magnetic field relaxation times are long compared to the exciton lifetime, yet they are strongly reduced in high magnetic fields. We discuss explicitly quantum dots based on (In,Ga)As and (Cd,Zn)Se semiconductor compounds.Comment: accepted for Phys. Rev.

Alignment dependent chemisorption of vibrationally excited CH4(ν3) on Ni(100), Ni(110), and Ni(111)

International audienceWe present a stereodynamics study of the dissociative chemisorption of vibrationally excited methane on the (100), (110), and (111) planes of a nickel single crystal surface. Using linearly polarized infrared excitation of the antisymmetric C-H stretch normal mode vibration (ν3), we aligned the angular momentum and C-H stretch amplitude of CH4(ν3) in the laboratory frame and measured the alignment dependence of state-resolved reactivity of CH4 for the ν3 = 1, J = 0-3 quantum states over a range of incident translational energies. For all three surfaces studied, in-plane alignment of the C-H stretch results in the highest dissociation probability and alignment along the surface normal in the lowest reactivity. The largest alignment contrast between the maximum and minimum reactivity is observed for Ni(110), which has its surface atoms arranged in close-packed rows separated by one layer deep troughs. For Ni(110), we also probed for alignment effects relative to the direction of the Ni rows. In-plane C-H stretch alignment perpendicular to the surface rows results in higher reactivity than parallel to the surface rows. The alignment effects on Ni(110) and Ni(100) are independent of incident translational energy between 10 and 50 kJ/mol. Quantum state-resolved reaction probabilities are reported for CH4(ν3) on Ni(110) for translational energies between 10 and 50 kJ/mol

Transformation Pathways of Silica under High Pressure

Concurrent molecular dynamics simulations and ab initio calculations show that densification of silica under pressure follows a ubiquitous two-stage mechanism. First, anions form a close-packed sub-lattice, governed by the strong repulsion between them. Next, cations redistribute onto the interstices. In cristobalite silica, the first stage is manifest by the formation of a metastable phase, which was observed experimentally a decade ago, but never indexed due to ambiguous diffraction patterns. Our simulations conclusively reveal its structure and its role in the densification of silica.Comment: 14 pages, 4 figure

The Moral of the Tale: Stories, Trust, and Public Engagement with Clinical Ethics via Radio and Theatre.

Trust is frequently discussed with reference to the professional-patient relationship. However, trust is less explored in relation to the ways in which understanding of, and responses to, questions of ethics are discussed by both the "public" and "experts." Public engagement activity in healthcare ethics may invoke "trust" in analysing a moral question or problem but less frequently conceives of trust as integral to "public engagement" itself. This paper explores the relationship between trust and the ways in which questions of healthcare ethics are identified and negotiated by both "experts" and the public. Drawing on two examples from the author's "public engagement" work-a radio programme for the British Broadcasting Corporation and work with a playwright and theatre-the paper interrogates the ways in which "public engagement" is often characterized. The author argues that the common approach to public engagement in questions of ethics is unhelpfully constrained by a systemic disposition which continues to privilege the professional or expert voice at the expense of meaningful exchange and dialogue. By creating space for novel interactions between the "expert" and the "public," authentic engagement is achieved that enables not only the participants to flourish but also contributes to trust itself

Bad influence? – an investigation into the purported negative influence of foreign domestic helpers on children's second language English acquisition

This paper explores the purported negative influence of foreign domestic helpers (FDHs) on child second language acquisition (SLA) by studying Hong Kong Cantonese children's listening ability in second language (L2) English. 31 kindergarten third graders aged 4;6 to 6, and 29 first year secondary students aged 11-14 who have had a Filipino domestic helper at home took part in the study. In addition, 34 youngsters (20 in kindergarten, 14 in secondary) who did not have a Filipino helper participated as controls. Results from two listening tasks (picture choosing task, sound discrimination task) suggest that informants do not differ from the control in their abilities to listen to American-, British-, and Hong Kong English, and that they are better at listening to Filipino-accented English than the control. These findings cast doubts on the anecdotal belief of the harmful effect FDHs have on children's language acquisition including an L2. Moreover, the additional effect of being familiar with another variety of English is arguably a desirable outcome given that English is used as a lingua franca among non-native speakers on a daily basis in this highly globalised world

Data from: Phylogenomic Species Delimitation of Studfishes (Fundulidae: Fundulus): Evidence for Cryptic Species in Agreement with the Central Highlands Vicariance Hypothesis

The Central Highlands ecoregion of the eastern United States represents a hotspot of freshwater biodiversity, with replicated patterns of vicariant speciation east and west of the Mississippi River. Previous phylogeographic investigation of the studfishes (Fundulus subgenus Fundulus) revealed evidence for vicariant speciation in the Central Highlands, but data were limited to a small number of gene sequences generated with Sanger sequencing. We used double digest restriction-site associated DNA sequencing (ddRADseq) to improve resolution of phylogeographic patterns and better characterize population genetic variation. Our sample design included individuals from the Fundulus catenatus species group (F. catenatus, F. bifax, and F. stellifer) and two outgroup taxa (F. julisia and F. rathbuni). Phylogenetic analyses support a monophyletic F. catenatus complex and a sister relationship with Mobile Basin studfishes (F. bifax and F. stellifer). Population genomics and species delimitation tests provide evidence for three species-level subdivisions of F. catenatus. We describe F. catenatus as limited to the Tennessee River and its drainages, F. caddo in the Ouachita Highlands, and F. cryptocatenatus occupying the remainder of the range. Modally, F. catenatus was characterized by fewer left pectoral rays (16 vs. 17). Fundulus caddo had modally fewer anal rays (15 vs. 16) and lateral scale rows (13 vs. 14). Fundulus cryptocatenatus was characterized by modally higher dorsal rays (15 vs. 14) and fewer caudal rays (16 vs. 17). The geographic distribution is likely the result of multiple pre-Pleistocene vicariance events congruent with the Central Highlands Vicariance Hypothesis as well as separate, possibly subsequent, dispersal events. Overall, results of this study corroborate previous evidence for a complex biogeographic history of taxa endemic to rivers of the Central Highlands ecoregion. The improved resolution of genomic variation among studfish populations will guide future studies of morphological variation and will improve conservation plans for rare and endemic taxa in a freshwater biodiversity hotspot