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1. Quantum scattering of icosahedron fullerene C60 with noble-gas atoms

   https://doi.org/10.1038/s41598-024-59481-x  

   Kłos, Jacek; Tiesinga, Eite; Kotochigova, Svetlana (April 2024, Scientific Reports)

   Abstract There exist multiple ways to cool neutral molecules. A front runner is the technique of buffer gas cooling, where momentum-changing collisions with abundant cold noble-gas atoms cool the molecules. This approach can, in principle, produce the most diverse samples of cold molecules. We present quantum mechanical and semiclassical calculations of the elastic scattering differential cross sections and rate coefficients of the C₆₀fullerene with He and Ar noble-gas atoms in order to quantify the effectiveness of buffer gas cooling for this molecule. We also develop new three-dimensional potential energy surfaces for this purpose using dispersion-corrected density functional theory (DFT) with counterpoise correction. The icosahedral anisotropy of the molecular system is reproduced by expanding the potential in terms of symmetry-allowed spherical harmonics. Long-range dispersion coefficients have been computed from frequency dependent polarizabilities of C₆₀and the noble-gas atoms. We find that the potential of the fullerene with He is about five times shallower than that with Ar. Anisotropic corrections are very weak for both systems and omitted in the quantum scattering calculations giving us a nearly quantitative estimate of elastic scattering observables. Finally, we have computed differential cross sections at the collision energies used in experiments by Han et al. (Chem Phys Lett 235:211, 1995), corrected for the sensitivity of their apparatus, and we find satisfactory agreement for C₆₀scattering with Ar. 

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2. Direct observation of bimolecular reactions of ultracold KRb molecules

   https://doi.org/10.1126/science.aay9531  

   Hu, M. -G.; Liu, Y.; Grimes, D. D.; Lin, Y. -W.; Gheorghe, A. H.; Vexiau, R.; Bouloufa-Maafa, N.; Dulieu, O.; Rosenband, T.; Ni, K. -K. (November 2019, Science)

   Femtochemistry techniques have been instrumental in accessing the short time scales necessary to probe transient intermediates in chemical reactions. In this study, we took the contrasting approach of prolonging the lifetime of an intermediate by preparing reactant molecules in their lowest rovibronic quantum state at ultralow temperatures, thereby markedly reducing the number of exit channels accessible upon their mutual collision. Using ionization spectroscopy and velocity-map imaging of a trapped gas of potassium-rubidium (KRb) molecules at a temperature of 500 nanokelvin, we directly observed reactants, intermediates, and products of the reaction⁴⁰K⁸⁷Rb +⁴⁰K⁸⁷Rb → K₂Rb₂* → K₂+ Rb₂. Beyond observation of a long-lived, energy-rich intermediate complex, this technique opens the door to further studies of quantum-state–resolved reaction dynamics in the ultracold regime. 

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3. Rovibrational quantum state resolution of the C ₆₀ fullerene

   https://doi.org/10.1126/science.aav2616  

   Changala, P. Bryan; Weichman, Marissa L.; Lee, Kevin F.; Fermann, Martin E.; Ye, Jun (January 2019, Science)

   The unique physical properties of buckminsterfullerene, C₆₀, have attracted intense research activity since its original discovery. Total quantum state–resolved spectroscopy of isolated C₆₀molecules has been of particularly long-standing interest. Such observations have, to date, been unsuccessful owing to the difficulty in preparing cold, gas-phase C₆₀in sufficiently high densities. Here we report high-resolution infrared absorption spectroscopy of C₆₀in the 8.5-micron spectral region (1180 to 1190 wave number). A combination of cryogenic buffer-gas cooling and cavity-enhanced direct frequency comb spectroscopy has enabled the observation of quantum state–resolved rovibrational transitions. Characteristic nuclear spin statistical intensity patterns confirm the indistinguishability of the 60 carbon-12 atoms, while rovibrational fine structure encodes further details of the molecule’s rare icosahedral symmetry. 

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4. An improved source of spin-polarized electrons based on spin exchange in optically pumped rubidium vapor

   https://doi.org/10.1063/5.0149691  

   Ahrendsen, K. J.; Trantham, K. W.; Tupa, D.; Gay, T. J. (August 2023, Review of Scientific Instruments)

   We have improved a polarized electron source in which unpolarized electrons undergo collisions with a mixture of buffer gas molecules and optically spin-polarized Rb atoms. With a nitrogen buffer gas, the source reliably provides spin polarization between 15% and 25% with beam currents >4 μA. Vacuum pump upgrades mitigate problems caused by denatured diffusion pump oil, leading to longer run times. A new differential pumping scheme allows the use of higher buffer gas pressures up to 800 mTorr. With a new optics layout, the Rb polarization is continuously monitored by a probe laser and improved pump laser power provides more constant high polarization. We have implemented an einzel lens to better control the energy of the electrons delivered to the target chamber and to preferentially select electron populations of higher polarization. The source is designed for studies of biologically relevant chiral molecule samples, which can poison photoemission-based GaAs polarized electron sources at very low partial pressures. It operates adjacent to a target chamber that rises to pressures as high as 10−4 Torr and has been implemented in a first experiment with chiral cysteine targets. 

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5. Imaging rotational energy transfer: comparative stereodynamics in CO + N ₂ and CO + CO inelastic scattering

   https://doi.org/10.1039/D3CP02229C  

   Sun, Zhong-Fa; Scheidsbach, Roy J.; van Hemert, Marc C.; van der Avoird, Ad; Suits, Arthur G.; Parker, David H. (July 2023, Physical Chemistry Chemical Physics)

   State-to-state rotational energy transfer in collisions of ground ro-vibrational state 13 CO molecules with N 2 molecules has been studied using the crossed molecular beam method under kinematically equivalent conditions used for 13 CO + CO rotationally inelastic scattering described in a previously published report (Sun et al. , Science , 2020, 369 , 307–309). The collisionally excited 13 CO molecule products are detected by the same (1 + 1′ + 1′′) VUV (Vacuum Ultra-Violet) resonance enhanced multiphoton ionization scheme coupled with velocity map ion imaging. We present differential cross sections and scattering angle resolved rotational angular momentum alignment moments extracted from experimentally measured 13 CO + N 2 scattering images and compare them with theoretical predictions from quasi-classical trajectories (QCT) on a newly calculated 13 CO–N 2 potential energy surface (PES). Good agreement between experiment and theory is found, which confirms the accuracy of the 13 CO–N 2 potential energy surface for the 1460 cm −1 collision energy studied by experiment. Experimental results for 13 CO + N 2 are compared with those for 13 CO + CO collisions. The angle-resolved product rotational angular momentum alignment moments for the two scattering systems are very similar, which indicates that the collision induced alignment dynamics observed for both systems are dominated by a hard-shell nature. However, compared to the 13 CO + CO measurements, the primary rainbow maximum in the DCSs for 13 CO + N 2 is peaked consistently at more backward scattering angles and the secondary maximum becomes much less obvious, implying that the 13 CO–N 2 PES is less anisotropic. In addition, a forward scattering component with high rotational excitation seen for 13 CO + CO does not appear for 13 CO–N 2 in the experiment and is not predicted by QCT theory. Some of these differences in collision dynamics behaviour can be predicted by a comparison between the properties of the PESs for the two systems. More specific behaviour is also predicted from analysis of the dependence on the relative collision geometry of 13 CO + N 2 trajectories compared to 13 CO + CO trajectories, which shows the special ‘do-si-do’ pathway invoked for 13 CO + CO is not effective for 13 CO + N 2 collisions. 

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