1. Institute of Experimental Physics 1 Wolfgang E. Ernst 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 68th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 17-21, 2013 Institute of Experimental Physics Research funded by Fonds zur Förderung der wissenschaftlichen Forschung & EU regional development fund (ERDF) Currently at UC Berkeley

2. Institute of Experimental Physics 2 Wolfgang E. Ernst 68th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 17-21, 2013 Experimental achievements 85 Rb Markus Koch (currently at Stanford)

3. Institute of Experimental Physics 3 Wolfgang E. Ernst 68th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 17-21, 2013 Experiment Xe Rb  ? Van der Waals Xe Does the doping sequence influence the reaction? Application as diagnostic tool for cold chemistry? 87 Rb ESR transition:  F‘m F ‘  Fm F   2 +2  1 +1  Free atom at B 0 =0.113595 T Optically detected via 5 2 P 1/2  5 2 S 1/2

4. Institute of Experimental Physics 4 Wolfgang E. Ernst 68th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 17-21, 2013 Theoretical approach 1) Select pair potentials from previous work 2) Calculate energy and density profile of undoped He N (DFT) 3) Add single Xe, DFT simulation  find confinement potential 4) Add only Rb to He N, find binding potential Rb-He N and r equ 5) Place Rb at r equ, Xe in center & obtain changed conf. pot. for Xe 6) Use Rb-He potential to correct step 5, drop fixed Rb constraint

5. Institute of Experimental Physics 5 Wolfgang E. Ernst 68th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 17-21, 2013 Dalfovo et al., Phys. Rev. B 1995 J. Pascale, CEN Saclay 1983 Cvetko et al., J. Chem. Phys. 1994 Ad 1) Pair potentials J. Poms, A. W. Hauser, and W. E. Ernst, PhysChemChemPhys 14, 15158 (2012) 14 to 16 He atoms in 1st shell Ad 2 & 3) DFT simulation (Trento-Orsay density functional) Minimize the free energy F[  ] with  helium density: Dopant-helium interaction

6. Institute of Experimental Physics 6 Wolfgang E. Ernst 68th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 17-21, 2013 Ad 3) Confinement potential for Xe Solvation energy for Xe in He N (N=500) S(Xe)  - 280 K Radial probability Potential analytically described by: (K. K. Lehmann, Mol. Phys. 1999) With a= 588502 K Å 3 and b = 29.22 Å in the Lehmann model. (a related to C 6 of He-Xe potential And b to droplet radius)

7. Institute of Experimental Physics 7 Wolfgang E. Ernst 68th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 17-21, 2013 Ad 4) Rb on He 500 Helium density distribution, r equ = 20 Å and binding (or solvation) energy  - 14 K (sensitive to diatomic potential, Barranco: - 9 K)

8. Institute of Experimental Physics 8 Wolfgang E. Ernst 68th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 17-21, 2013 Ad 5) Double dotation of He 500 with Rb and Xe, neglect direct Rb-Xe interaction Helium density distribution: 3-dim.andradial distribution (Xe at z = 0) Note: negligible difference between  He,Rb &  He,RbXe

9. Institute of Experimental Physics 9 Wolfgang E. Ernst 68th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 17-21, 2013 Ad 6) Double dotation of He 500 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 He 500 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

10. Institute of Experimental Physics 10 Wolfgang E. Ernst 68th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 17-21, 2013 Ad 6) Double dotation of He 500 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 He 500

11. Institute of Experimental Physics 11 Wolfgang E. Ernst 68th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 17-21, 2013 Ad 6) Double dotation of He 500 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.

12. Institute of Experimental Physics 12 Wolfgang E. Ernst 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)http://www.student.tugraz.at/poms/dichte2.html Johannes Poms, Andreas W. Hauser, and Wolfgang E. Ernst, Phys. Chem. Chem. Phys. 14, 15158 (2012) 68th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 17-21, 2013

13. Institute of Experimental Physics 13 Wolfgang E. Ernst 68th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 17-21, 2013 Conclusions Analyze reaction on helium nanodroplet by DFT using Orsay-Trento functional Xe + Rb  XeRb Simulation of double-doped He N 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

14. Institute of Experimental Physics 14 Wolfgang E. Ernst 68th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 17-21, 2013 Thank you for your attention !

15. Institute of Experimental Physics 15 Wolfgang E. Ernst 68th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 17-21, 2013 Graz, 18 th - 22 nd August 2014 XXIInd International Symposium on the Jahn-Teller Effect Institute of Experimental Physics

16. 16 Wolfgang E. Ernst 68th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 17-21, 2013