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1. Roaming pathways and survival probability in real-time collisional dynamics of cold and controlled bialkali molecules

   https://doi.org/10.1038/s41598-021-90004-0  

   Kłos, Jacek ; Guan, Qingze ; Li, Hui ; Li, Ming ; Tiesinga, Eite ; Kotochigova, Svetlana ( May 2021 , Scientific Reports)

   Perfectly controlled molecules are at the forefront of the quest to explore chemical reactivity at ultra low temperatures. Here, we investigate for the first time the formation of the long-lived intermediates in the time-dependent scattering of cold bialkali$$^{23}\hbox {Na}^{87}$$${}^{23}{\text{Na}}^{87}$Rb molecules with and without the presence of infrared trapping light. During the nearly 50 nanoseconds mean collision time of the intermediate complex, we observe unconventional roaming when for a few tens of picoseconds either NaRb or$$\hbox {Na}_2$$${\text{Na}}_2$and$$\hbox {Rb}_2$$${\text{Rb}}_2$molecules with large relative separation are formed before returning to the four-atom complex. We also determine the likelihood of molecular loss when themore »trapping laser is present during the collision. We find that at a wavelength of 1064 nm the$$\hbox {Na}_2\hbox {Rb}_2$$${\text{Na}}_2{\text{Rb}}_2$complex is quickly destroyed and thus that the$$^{23}\hbox {Na}^{87}$$${}^{23}{\text{Na}}^{87}$Rb molecules are rapidly lost.

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2. Laser cooling with adiabatic transfer on a Raman transition

   https://doi.org/10.1088/1367-2630/ab2f3c  

   Greve, G. P. ; Wu, B. ; Thompson, J. K. ( July 2019 , New Journal of Physics)

   Sawtooth Wave Adiabatic Passage (SWAP) laser cooling was recently demonstrated using a narrow-linewidth single-photon optical transition in atomic strontium and may prove useful for cooling other atoms and molecules. However, many atoms and molecules lack the appropriate narrow optical transition. Here we use such an atom,⁸⁷Rb, to demonstrate that two-photon Raman transitions with arbitrarily-tunable linewidths can be used to achieve 1D SWAP cooling without significantly populating the intermediate excited state. Unlike SWAP cooling on a narrow transition, Raman SWAP cooling allows for a final 1D temperature well below the Doppler cooling limit (here, 25 times lower); and the effectivemore »excited state decay rate can be modified in time, presenting another degree of freedom during the cooling process. We also develop a generic model for Raman Landau–Zener transitions in the presence of small residual free-space scattering for future applications of SWAP cooling in other atoms or molecules.

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3. Directing the reactivity of metal hydrides for selective CO ₂ reduction

   https://doi.org/10.1073/pnas.1811396115  

   Ceballos, Bianca M. ; Yang, Jenny Y. ( November 2018 , Proceedings of the National Academy of Sciences)

   A critical challenge in electrocatalytic CO₂reduction to renewable fuels is product selectivity. Desirable products of CO₂reduction require proton equivalents, but key catalytic intermediates can also be competent for direct proton reduction to H₂. Understanding how to manage divergent reaction pathways at these shared intermediates is essential to achieving high selectivity. Both proton reduction to hydrogen and CO₂reduction to formate generally proceed through a metal hydride intermediate. We apply thermodynamic relationships that describe the reactivity of metal hydrides with H⁺and CO₂to generate a thermodynamic product diagram, which outlines the free energy of product formation as a function of proton activity andmore »hydricity (∆G_H−), or hydride donor strength. The diagram outlines a region of metal hydricity and proton activity in which CO₂reduction is favorable and H⁺reduction is suppressed. We apply our diagram to inform our selection of [Pt(dmpe)₂](PF₆)₂as a potential catalyst, because the corresponding hydride [HPt(dmpe)₂]⁺has the correct hydricity to access the region where selective CO₂reduction is possible. We validate our choice experimentally; [Pt(dmpe)₂](PF₆)₂is a highly selective electrocatalyst for CO₂reduction to formate (>90% Faradaic efficiency) at an overpotential of less than 100 mV in acetonitrile with no evidence of catalyst degradation after electrolysis. Our report of a selective catalyst for CO₂reduction illustrates how our thermodynamic diagrams can guide selective and efficient catalyst discovery.

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4. Critical computational analysis illuminates the reductive-elimination mechanism that activates nitrogenase for N ₂ reduction

   https://doi.org/10.1073/pnas.1810211115  

   Raugei, Simone ; Seefeldt, Lance C. ; Hoffman, Brian M. ( October 2018 , Proceedings of the National Academy of Sciences)

   Recent spectroscopic, kinetic, photophysical, and thermodynamic measurements show activation of nitrogenase for N₂→ 2NH₃reduction involves the reductive elimination (re) of H₂from two [Fe–H–Fe] bridging hydrides bound to the catalytic [7Fe–9S–Mo–C–homocitrate] FeMo-cofactor (FeMo-co). These studies rationalize the Lowe–Thorneley kinetic scheme’s proposal of mechanistically obligatory formation of one H₂for each N₂reduced. They also provide an overall framework for understanding the mechanism of nitrogen fixation by nitrogenase. However, they directly pose fundamental questions addressed computationally here. We here report an extensive computational investigation of the structure and energetics of possible nitrogenase intermediates using structural models for the active site with a broad rangemore »in complexity, while evaluating a diverse set of density functional theory flavors. (i) This shows that to prevent spurious disruption of FeMo-co having accumulated 4[e⁻/H⁺] it is necessary to include: all residues (and water molecules) interacting directly with FeMo-co via specific H-bond interactions; nonspecific local electrostatic interactions; and steric confinement. (ii) These calculations indicate an important role of sulfide hemilability in the overall conversion ofE₀to a diazene-level intermediate. (iii) Perhaps most importantly, they explain (iiia) how the enzyme mechanistically couples exothermic H₂formation to endothermic cleavage of the N≡N triple bond in a nearly thermoneutralre/oxidative-addition equilibrium, (iiib) while preventing the “futile” generation of two H₂without N₂reduction: hydrideregenerates an H₂complex, but H₂is only lost when displaced by N₂, to form an end-on N₂complex that proceeds to a diazene-level intermediate.

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5. Millimetre-long transport of photogenerated carriers in topological insulators

   https://doi.org/10.1038/s41467-019-13711-3  

   Hou, Yasen ; Wang, Rui ; Xiao, Rui ; McClintock, Luke ; Clark Travaglini, Henry ; Paulus Francia, John ; Fetsch, Harry ; Erten, Onur ; Savrasov, Sergey Y. ; Wang, Baigeng ; et al ( December 2019 , Nature Communications)

   Excitons are spin integer particles that are predicted to condense into a coherent quantum state at sufficiently low temperature. Here by using photocurrent imaging we report experimental evidence of formation and efficient transport of non-equilibrium excitons in Bi_2-xSb_xSe₃nanoribbons. The photocurrent distributions are independent of electric field, indicating that photoexcited electrons and holes form excitons. Remarkably, these excitons can transport over hundreds of micrometers along the topological insulator (TI) nanoribbons before recombination at up to 40 K. The macroscopic transport distance, combined with short carrier lifetime obtained from transient photocurrent measurements, indicates an exciton diffusion coefficient at least 36 m² s⁻¹, which correspondsmore »to a mobility of 6 × 10⁴ m² V⁻¹ s⁻¹at 7 K and is four order of magnitude higher than the value reported for free carriers in TIs. The observation of highly dissipationless exciton transport implies the formation of superfluid-like exciton condensate at the surface of TIs.

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