Atmospheric xenon is strongly mass fractionated, the result of a process that apparently continued through the Archean and perhaps beyond. Previous models that explain Xe fractionation by hydrodynamic hydrogen escape cannot gracefully explain how Xe escaped when Ar and Kr did not, nor allow Xe to escape in the Archean. Here we show that Xe is the only noble gas that can escape as an ion in a photo-ionized hydrogen wind, possible in the absence of a geomagnetic field or along polar magnetic field lines that open into interplanetary space. To quantify the hypothesis we construct new 1-D models of hydrodynamic diffusion-limited hydrogen escape from highly-irradiated CO 2 -H 2 -H atmospheres. The models reveal three minimum requirements for Xe escape: solar EUV irradiation needs to exceed 10× that of the modern Sun; the total hydrogen mixing ratio in the atmosphere needs to exceed 1% (equiv. to 0.5% CH 4 ); and transport amongst the ions in the lower ionosphere needs to lift the Xe ions to the base of the outflowing hydrogen corona. The long duration of Xe escape implies that, if a constant process, Earth lost the hydrogen from at least one ocean of water, roughly evenly split between the Hadean and the Archean. However, to account for both Xe's fractionation and also its depletion with respect to Kr and primordial 244 Pu, Xe escape must have been limited to small apertures or short episodes, which suggests that Xe escape was restricted to polar windows by a geomagnetic field, or dominated by outbursts of high solar activity, or limited to transient episodes of abundant hydrogen, or a combination of these. Xenon escape stopped when the hydrogen (or methane) mixing ratio became too small, or EUV radiation from the aging Sun became too weak, or charge exchange between Xe + and O 2 rendered Xe neutral.1

Ultracold molecules offer a broad variety of applications, ranging from metrology to quantum computing. However, forming "real" ultracold molecules, i.e., in deeply bound levels, is a very difficult proposition. Here, we show how photoassociation in the vicinity of a Feshbach resonance enhances molecular formation rates by several orders of magnitude. We illustrate this effect in heteronuclear systems, and find giant rate coefficients even in deeply bound levels. We also give a simple analytical expression for the photoassociation rate and discuss future applications of the Feshbach-optimized photoassociation technique.

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We investigate a hybrid system composed of ultracold Rydberg atoms immersed in an atomic Bose-Einstein condensate (BEC). The coupling between the Rydberg electrons and BEC atoms leads to the excitation of phonons, the exchange of which induces Yukawa interaction between Rydberg atoms. Due to the small electron mass, the effective charge associated with this quasiparticle-mediated interaction can be large, while its range is equal to the healing length of the BEC, which can be tuned by adjusting the scattering length of the BEC atoms. We find that for small healing lengths, the distortion of the BEC can "image" the wave function density of the Rydberg electron, while large healing lengths induce an attractive Yukawa potential between the two Rydberg atoms that can form a new type of ultra-long-range molecule. We discuss both cases for a realistic system.Impurities in a Bose-Einstein condensate (BEC) have attracted much attention and motivated the investigation of a wide range of phenomena. For example, the motion of a single impurity in a BEC can probe the superfluid dynamics [1][2][3], while an ionic impurity in a BEC can form a mesoscopic molecular ion [4]. Due to the selfenergy induced by phonons (excitations of the BEC), a neutral impurity can self-localize in both a homogeneous and a harmonically trapped BEC [5][6][7], which sheds light on polaron physics [8,9]. Exchanging phonons between multiple impurities induces an attractive Yukawa potential between each pair of impurities [10,11], which leads to the so called "co-self-localization" [12] and is related to forming bipolarons and multipolarons [13]. Recent experiments, where an atom of a BEC is excited into a Rydberg state [14] to study phonon excitations and collective oscillations, open the door to exploration of the electron-phonon coupling in ultracold degenerate gases, a phenomenon responsible for the formation of Cooper pairs of two repelling electrons in BCS superconductivity [15].In this Letter, we study Rydberg atoms immersed in a homogeneous BEC, as sketched in Fig. 1(a). Rydberg atoms consist of an ion core and a highly excited electron with its oscillatory wave function Ψ e extending to large distances of the order of ∼n 2 a 0 (n: principle quantum number, a 0 : Bohr radius). As pointed out by Fermi [16], the interaction between the quasi-free electron at x and a ground state atom at r can be approximated at low scattering energies by a contact interaction parametrized by an energy-dependent s-wave scattering lengthWhile the s-wave approximation is valid for qualitative analysis, we include higher-partial wave contributions for quantitative results [17]. A s (k) depends on the scattering energy via the local wave number k(r) given by where R y is the Rydberg constant, 0 the vacuum permittivity, e and m e the charge and mass, respectively, of the electron with angular momentum e and quantum defect δ e . For low-e state, Eq. (1) gives an effective interaction between Rydberg and ground state atoms aswhich leads to an attraction and formation of ...

[1] We present a detailed theoretical analysis of nonthermal escape of molecular hydrogen from Mars induced by collisions with hot atomic oxygen from the Martian corona. To accurately describe the energy transfer in O + H 2 (v, j) collisions, we performed extensive quantum-mechanical calculations of state-to-state elastic, inelastic, and reactive cross sections. The escape flux of H 2 molecules was evaluated using a simplified 1D column model of the Martian atmosphere with realistic densities of atmospheric gases and hot oxygen production rates for low solar activity conditions. An average intensity of the non-thermal escape flux of H 2 of 1.9 Â 10 5 cm À2 s À1 was obtained considering energetic O atoms produced in dissociative recombinations of O 2 + ions. Predicted ro-vibrational distribution of the escaping H 2 was found to contain a significant fraction of higher rotational states. While the non-thermal escape rate was found to be lower than Jeans rate for H 2 molecules, the non-thermal escape rates of HD and D 2 are significantly higher than their respective Jeans rates. The accurate evaluation of the collisional escape flux of H 2 and its isotopes is important for understanding non-thermal escape of molecules from Mars, as well as for the formation of hot H 2 Martian corona. The described molecular ejection mechanism is general and expected to contribute to atmospheric escape of H 2 and other light molecules from planets, satellites, and exoplanetary bodies. Citation: Gacesa, M., P. Zhang, and V. Kharchenko (2012), Non-thermal escape of molecular hydrogen from Mars,

We present a theoretical analysis of optical pathways for formation of cold ground state (NaCa) + molecular ions via an intermediate state. The formation schemes are based on ab initio potential energy curves and transition dipole moments calculated using effective-core-potential methods of quantum chemistry. In the proposed approach, starting from a mixture of cold trapped Ca + ions immersed into an ultracold gas of Na atoms, (NaCa) + molecular ions are photoassociated in the excited E 1 Σ + electronic state and allowed to spontaneously decay either to the ground electronic state or an intermediate state from which the population is transferred to the ground state via an additional optical excitation. By analyzing all possible pathways, we find that the efficiency of a two-photon scheme, via either B 1 Σ + or C 1 Σ + potential, is sufficient to produce significant quantities of ground state (NaCa) + molecular ions. A single-step process results in lower formation rates that would require either a high density sample or a very intense photoassociation laser to be viable.

We examine here for the first time the effects of both global and regional dust storms on the formation of the Martian hot O corona and associated photochemical loss of O. Our study is conducted by utilizing our integrated model framework, which couples our Martian hot O corona model with a multifluid magnetohydrodynamic model for Mars for the dusty and clear atmospheric condition cases. We present our results with the most up-to-date cross sections for the O( 3 P)-CO 2 collisions. The main effect of dust storms on the ionosphere is the upward shift of the ionosphere on the dayside, which results in an increase in production of hot O at all altitudes above the ionospheric peak. However, the dust-induced inflation of the neutral upper atmosphere results in an enhancement in collisional loss of hot O and thus effectively suppresses the hot O density, reducing the global photochemical loss rate by~28% for the global dust storm scenario. The relative density structure of the hot O corona does not show any significant changes, while its magnitude decreases at all altitudes.

We report new elastic and inelastic cross sections for O( 3 P)+CO 2 scattering at collision energies from 0.03 to 5 eV, of major importance to O escape from Mars, Venus, and CO 2 -rich atmospheres. The cross sections were calculated from first principles using three newly constructed ab-initio potential energy surfaces correlating to the lowest energy asymptote of the complex. The surfaces were restricted to a planar geometry with the CO 2 molecule assumed to be in linear configuration fixed at equilibrium. Quantum-mechanical coupled-channel formalism with a large basis set was used to compute state-to-state integral and differential cross sections for elastic and inelastic O( 3 P)+CO 2 scattering between all pairs of rotational states of CO 2 molecule. The elastic cross sections are 35% lower at 0.5 eV and more than 50% lower at 4+ eV than values commonly used in studies of processes in upper and middle planetary atmospheres of Mars, Earth, Venus, and CO 2 -rich planets. Momentum transfer cross sections, of interest for energy transport, were found to be proportionally lower than predicted by mass-scaling.