institute of experimental physics 1 wolfgang e. ernst xe and rb atoms on helium nanodroplets: is the...

Institute of Experimental Physics

1 Wolfgang E. Ernst

EPXe 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


Research funded by Fonds zur Förderung der wissenschaftlichen Forschung

& EU regional development fund (ERDF)

Currently at UC Berkeley

http://www.fwf.ac.at/en/index.asp




http://www.steiermark.at/


2 Wolfgang E. Ernst

EP


Experimental achievements

85RbMarkus Koch

(currently atStanford)


3 Wolfgang E. Ernst

EP


Experiment

Xe Rb ?

Van der WaalsXe

Does the doping sequence influence the reaction?Application as diagnostic tool for cold chemistry?

87Rb

ESR transition:F‘mF‘FmF2 +21 +1Free atom atB0=0.113595 T

Optically detected via5 2P1/2 5 2S1/2


4 Wolfgang E. Ernst

EP


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


5 Wolfgang E. Ernst

EP


Dalfovo et al., Phys. Rev. B 1995J. Pascale, CEN Saclay 1983Cvetko et al., J. Chem. Phys. 1994Cvetko 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 Wolfgang E. Ernst

EP


Ad 3) Confinement potential for Xe

Solvation energy for Xe in HeN (N=500) S(Xe) - 280 K

Radial probability

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)


7 Wolfgang E. Ernst

EP


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)


8 Wolfgang E. Ernst

EP


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


9 Wolfgang E. Ernst

EP


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


10 Wolfgang E. Ernst

EP


Ad 6) Double dotation of He500 with Rb and Xe, drop the constraint of a fixed Rb position.

Rb-Xe potential:free diatomicandRbXe embedded in helium

Vibrational states of free RbXe

and RbXe attached to He500



EP


Ad 6) Double dotation of He500 with Rb and Xe, drop the constraint of a fixed Rb position.

Rb-Xe potential:free diatomicandRbXe 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.



EPHelium density plots of a helium

nanodroplet (500 atoms) with two impuritiesHelium 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)


http://www.student.tugraz.at/poms/dichte2.html



EP


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 reactionb) 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



EP


Thank you for your attention !



EP


Graz, 18th - 22nd August 2014

XXIInd International Symposium on the Jahn-Teller Effect




EP


institute of experimental physics 1 wolfgang e. ernst xe and rb atoms on helium nanodroplets: is the...

Documents