Universal rate constants for reactive collisions of ultracold molecules

A simple quantum defect model gives analytic expressions for the complex scattering length and threshold collision rates of ultracold molecules. If the probability of reaction in the short-range part of the collision is high, the model gives universal rate constants for s- and p-wave collisions that are independent of short-range dynamics. This model explains the magnitudes of the recently measured rate constants for collisions of two ultracold 40K87Rb molecules, or an ultracold 40K atom with the 40K87Rb molecule [Ospelkaus et al., Science 327, 853 (2010)].Comment: 4 pages, 2 figures; v2: final version, accepted for publication in Physical Review Letter

Polar molecule reactive collisions in quasi-1D systems

We study polar molecule scattering in quasi-one-dimensional geometries. Elastic and reactive collision rates are computed as a function of collision energy and electric dipole moment for different confinement strengths. The numerical results are interpreted in terms of first order scattering and of adiabatic models. Universal dipolar scattering is also discussed. Our results are relevant to experiments where control of the collision dynamics through one dimensional confinement and an applied electric field is envisioned.Comment: 25 pages, 13 figure

Universal rates for reactive ultracold polar molecules in reduced dimensions

Analytic expressions describe universal elastic and reactive rates of quasi-two-dimensional and quasi-one-dimensional collisions of highly reactive ultracold molecules interacting by a van der Waals potential. Exact and approximate calculations for the example species of KRb show that stability and evaporative cooling can be realized for spin-polarized fermions at moderate dipole and trapping strength, whereas bosons or unlike fermions require significantly higher dipole or trapping strengths.Comment: 4 pages, 3 figure

A Simple Quantum Model of Ultracold Polar Molecule Collisions

We present a unified formalism for describing chemical reaction rates of trapped, ultracold molecules. This formalism reduces the scattering to its essential features, namely, a propagation of the reactant molecules through a gauntlet of long-range forces before they ultimately encounter one another, followed by a probability for the reaction to occur once they do. In this way, the electric-field dependence should be readily parametrized in terms of a pair of fitting parameters (along with a $C_6$ coefficient) for each asymptotic value of partial wave quantum numbers $|L,M \rangle$. From this, the electric field dependence of the collision rates follows automatically. We present examples for reactive species such as KRb, and non-reactive species, such as RbCs

Risk assessment of gas emission at the surface of French abandoned coal mines and prevention techniques

En France, les procédures réglementaires d'abandon des concessions minières, de charbon en particulier, nécessitent, de la part de l'exploitant, la réalisation d'une analyse des risques consécutifs à l'existence de vieux travaux souterrains et la mise en place de mesures compensatoires destinées à réduire ces risques. Parmi ceux-ci, le risque de remontée de gaz en surface est peut-être le moins facile à apprécier. Le gaz de mine, constitué du grisou originellement contenu dans le charbon, c'est-à-dire de méthane mais aussi d'autres gaz comme le dioxyde de carbone, s'accumule dans le réservoir souterrain constitué par les vides laissés par l'exploitation. Il peut migrer vers la surface du sol sous l'effet de plusieurs mécanismes - remontée du niveau d'eau, variations de la pression barométrique, etc. - et s'accumuler au sein de volumes confinés dans les constructions, présentant ainsi des risques importants pour les personnes et les biens (explosions, asphyxie...). Cette communication décrit en détail la méthode générale utilisée en France pour analyser ce risque, à partir du contexte géologique et des caractéristiques propres à l'exploitation du bassin concerné. Elle présente également les moyens de prévention mis en place pour gérer ce risque

Incidence of rough and irregular atmospheric ice particles from Small Ice Detector 3 measurements

NERC, NE/E011225/1 © Author(s) 2013. This work is distributed under the Creative Commons Attribution 3.0 LicenseThe knowledge of properties of ice crystals such as size, shape, concavity and roughness is critical in the context of radiative properties of ice and mixed phase clouds. Limitations of current cloud probes to measure these properties can be circumvented by acquiring two-dimensional light scattering patterns instead of particle images. Such patterns were obtained in situ for the first time using the Small Ice Detector 3 (SID-3) probe during several flights in a variety of mid-latitude mixed phase and cirrus clouds. The patterns are analyzed using several measures of pattern texture, selected to reveal the magnitude of particle roughness or complexity. The retrieved roughness is compared to values obtained from a range of well-characterized test particles in the laboratory. It is found that typical in situ roughness corresponds to that found in the rougher subset of the test particles, and sometimes even extends beyond the most extreme values found in the laboratory. In this study we do not differentiate between small-scale, fine surface roughness and large-scale crystal complexity. Instead, we argue that both can have similar manifestations in terms of light scattering properties and also similar causes. Overall, the in situ data is consistent with ice particles with highly irregular or rough surfaces being dominant. Similar magnitudes of roughness were found in growth and sublimation zones of cirrus. The roughness was found to be negatively correlated with the halo ratio, but not with other thermodynamic or microphysical properties found in situ. Slightly higher roughness was observed in cirrus forming in clean oceanic airmasses than in a continental, polluted one. Overall, the roughness and complexity is expected to lead to increased shortwave cloud reflectivity, in comparison with cirrus composed of more regular, smooth ice crystal shapes. These findings put into question suggestions that climate could be modified through aerosol seeding to reduce cirrus cover and optical depth, as the seeding may result in decreased shortwave reflectivity.Peer reviewe