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EPInstitute of Experimental Physics Xe and Rb Atoms on Helium Nanodroplets: is the van der Waals Attraction Strong Enough to Form a Molecule? Johannes Poms, Andreas W. Hauser, and Wolfgang E. Ernst Currently at UC Berkeley Institute of Experimental Physics Research funded by Fonds zur Förderung der wissenschaftlichen Forschung & EU regional development fund (ERDF) Wolfgang E. Ernst 68th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 17-21, 2013 1 EPInstitute of Experimental Physics Experimental achievements 85Rb Markus Koch (currently at Stanford) Wolfgang E. Ernst 68th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 17-21, 2013 2 EPInstitute of Experimental Physics Experiment Xe Rb ? Van der Waals Xe 87Rb Does the doping sequence influence the reaction? Application as diagnostic tool for cold chemistry? ESR transition: F‘mF‘FmF 2 +21 +1 Free atom at B0=0.113595 T Optically detected via 5 2P1/2 5 2S1/2 Wolfgang E. Ernst 68th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 17-21, 2013 3 EPInstitute of Experimental Physics Theoretical approach 1) Select pair potentials from previous work 2) Calculate energy and density profile of undoped HeN (DFT) 3) Add single Xe, DFT simulation find confinement potential 4) Add only Rb to HeN, find binding potential Rb-HeN and requ 5) Place Rb at requ, Xe in center & obtain changed conf. pot. for Xe 6) Use Rb-He potential to correct step 5, drop fixed Rb constraint Wolfgang E. Ernst 68th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 17-21, 2013 4 EPInstitute of Experimental Physics Ad 1) Pair potentials Dalfovo et al., Phys. Rev. B 1995 J. Pascale, CEN Saclay 1983 Cvetko et al., J. Chem. Phys. 1994 Cvetko et al., J. Chem. Phys. 1994 Ad 2 & 3) DFT simulation (Trento-Orsay density functional) Minimize the free energy F[] with helium density: Dopant-helium interaction 14 to 16 He atoms in 1st shell J. Poms, A. W. Hauser, and W. E. Ernst, PhysChemChemPhys 14, 15158 (2012) Wolfgang E. Ernst 68th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 17-21, 2013 5 EPInstitute of Experimental Physics Ad 3) Confinement potential for Xe Solvation energy for Xe in HeN (N=500) S(Xe) - 280 K Potential analytically described by: (K. K. Lehmann, Mol. Phys. 1999) With a= 588502 K Å3 and b = 29.22 Å in the Lehmann model. (a related to C6 of He-Xe potential And b to droplet radius) Radial probability Wolfgang E. Ernst 68th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 17-21, 2013 6 EPInstitute of Experimental Physics Ad 4) Rb on He500 Helium density distribution, requ = 20 Å and binding (or solvation) energy - 14 K (sensitive to diatomic potential, Barranco: - 9 K) Wolfgang E. Ernst 68th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 17-21, 2013 7 EPInstitute of Experimental Physics Ad 5) Double dotation of He500 with Rb and Xe, neglect direct Rb-Xe interaction Helium density distribution: 3-dim. and radial distribution (Xe at z = 0) Note: negligible difference between He,Rb & He,RbXe Wolfgang E. Ernst 68th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 17-21, 2013 8 EPInstitute of Experimental Physics Ad 6) Double dotation of He500 with Rb and Xe, add direct Rb-Xe interaction (Rb fixed). In a further step, drop the constraint of a fixed Rb position (by scanning over the solvation energy of Xe in He500 and simultaneously optimizing the position of the Rb atom for each point). Blue: Rb moves deeper into droplet, potential becomes slightly deeper and steeper, but barrier reduces only from 25 to 23.4 K Wolfgang E. Ernst 68th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 17-21, 2013 9 EPInstitute of Experimental Physics Ad 6) Double dotation of He500 with Rb and Xe, drop the constraint of a fixed Rb position. Rb-Xe potential: free diatomic and RbXe embedded in helium Vibrational states of free RbXe and RbXe attached to He500 Wolfgang E. Ernst 68th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 17-21, 2013 10 EPInstitute of Experimental Physics Ad 6) Double dotation of He500 with Rb and Xe, drop the constraint of a fixed Rb position. Rb-Xe potential: free diatomic and RbXe embedded in helium Tunneling probability for a single Xe atom or a Xe atom surrounded by a 15 He atom shell through a one-dimensional barrier of 23.4 K height: Significant only for energies between 22 and 25 K, not at all for 0.4 K. Wolfgang E. Ernst 68th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 17-21, 2013 11 EPInstitute of Experimental Physics Helium density plots of a helium nanodroplet (500 atoms) with two impurities Helium density plot of a helium nanodroplet with two impurities: one xenon atom is moving through the droplet one rubidium atom is allowed to move too. Animation by Johannes Poms (http://www.student.tugraz.at/poms/dichte2.html) Johannes Poms, Andreas W. Hauser, and Wolfgang E. Ernst, Phys. Chem. Chem. Phys. 14, 15158 (2012) Wolfgang E. Ernst 68th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 17-21, 2013 12 EPInstitute of Experimental Physics Conclusions Analyze reaction on helium nanodroplet by DFT using Orsay-Trento functional Xe + Rb XeRb Simulation of double-doped HeN with N = 500 shows: 1. Alkali metal atom stays on the surface, Xe resides in the droplet center, 2. Separation by a potential barrier of 23.4 K prevents reaction, 3. Consequences for the doping sequence: a) Xe first, Rb second no reaction b) Rb first, Xe second possibly formation of RbXe on surface vibrational excitation of RbXe may induce sinking of Xe into the droplet and controlled fragmentation. 4. Planned experiments: laser-induced fluorescence spectroscopy around alkali D-lines for each scenario, ESR spectroscopy Wolfgang E. Ernst 68th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 17-21, 2013 13 EPInstitute of Experimental Physics Thank you for your attention ! Wolfgang E. Ernst 68th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 17-21, 2013 14 EPInstitute of Experimental Physics XXIInd International Symposium on the Jahn-Teller Effect Graz, 18th - 22nd August 2014 Institute of Experimental Physics Wolfgang E. Ernst 68th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 17-21, 2013 15 EPInstitute of Experimental Physics Wolfgang E. Ernst 68th Int. 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