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,