Institute of Experimental Physics 1 Wolfgang E. Ernst Photoionization of Alkali-Doped Helium Nanodroplets Moritz Theisen, Florian Lackner, Günter Krois, Markus Koch, and Wolfgang E. Ernst 67th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 18-22, 2012 Institute of Experimental Physics Research funded by Fonds zur Förderung der wissenschaftlichen Forschung & EU regional development fund (ERDF)

Institute of Experimental Physics 2 Wolfgang E. Ernst 67th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 18-22, 2012 Helium Droplets cooling clusters evaporative (adiabatic expansion) grow cooling ■ Excellent cryostat, at T~0.4K ■ Weak interactions ■ Liquid (in fact superfluid) ■ Confinement (r = Å) DFT code courtesy of Franco Dalfovo, U. Trento (Italy), Roman Schmied, Princeton z Rb atom on droplet surface He 1000 Rb ion dives into the droplet, surrounds itself with polarized He  snowball formation

Institute of Experimental Physics 3 Wolfgang E. Ernst 67th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 18-22, 2012 Theoretical predictions for alkali ions in He N PHYSICAL REVIEW B, VOLUME 64, (2001) Alkali ions in superfluid 4 He and structure of the snowball M. Buzzacchi, D. E. Galli, and L. Reatto Ak + -He pair potentials from: R. Ahlrichs et al., J. Chem. Phys. 88, 6290 (1988) A. D. Koutselos, E. A. Mason, and L. A. Viehland, J. Chem. Phys. 93, 7125 (1990) Path Integral Monte Carlo Study of 4 He Clusters Doped with Alkali and Alkali-Earth Ions D. E. Galli,*,† D. M. Ceperley,‡ and L. Reatto† J. Phys. Chem. A 2011, 115, 7300–7309 Radial 4 He density,  (r), around ions for the largest 4 He droplets studied in this work: number of atoms N = 128. Prediction: Closure of a rigid He N shell around Rb + for N  14 and around Cs + for N  18. Second and higher shells may exhibit more mobility (like liquid).

Institute of Experimental Physics 4 Wolfgang E. Ernst 67th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 18-22, 2012 Alkali atoms on He N – two-step ionization through excited states Pseudodiatomic potentials He N -Rb From: M. Theisen, F. Lackner, F. Ancilotto, C. Callegari, and W.E. Ernst, Eur. Phys. J. D 61, 403–408 (2011) He N -Rb 5 2  1/2 and He N -Cs 6 2  1/2 are stable intermediate states for further excitation.  Measure vertical ionization potential, distinguish from free atom IP.  Experimental method: two-step laser ionization through a stable intermediate state,  Approach the IP‘s of He N -Rb and He N -Cs from „above“ (appearance potential) and „below“ (through Rydberg states). Qualitative scheme: from neutral ground state to the ion. Potential well depth  5 cm -1

Institute of Experimental Physics 5 Wolfgang E. Ernst 67th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 18-22, 2012 Experiment Source conditions: T = K, p = 40 to 60 bar, 5 μm nozzle droplet size: N ~ 10 3 to 10 5 He atoms Pick-up conditions: Pick-up cell temparature is optimized for single atom pick-up

Institute of Experimental Physics 6 Wolfgang E. Ernst 67th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 18-22, 2012 Time-of-flight (TOF) mass spectroscopic detection MCP detector pulsed lasers ions IE: cm -1 Pulsed Ti:S laser, – cm -1 5²S 1/2 5²P 3/2 5²P 1/2 pulsed dye laser Rb + e-e- TOF 6²S 1/2 5²D 3/2,5/2 + 6²S 1/2 6²P 3/2 6²P 1/2 7²S 1/2 6²D 3/2,5/2 Cs Pulsed Ti:S laser – cm -1 pulsed dye laser IE: cm -1 e-e- TOF ionization products monitored: Ak, Ak-He n (n < 20), Ak-He N (N > 500)

Institute of Experimental Physics 7 Wolfgang E. Ernst 67th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 18-22, 2012 Snowballs observed Rb + -He n (PhD thesis Moritz Theisen) Cs + -He n Moritz Theisen, Florian Lackner, and Wolfgang E. Ernst, THE JOURNAL OF CHEMICAL PHYSICS 135, (2011) Favored structures  n=16 1 st shell  n=12 stable structure (predicted by Gianturco and coworkers) Favored structures  Cs + -He n – n  18 1st shell “frozen” n~50 2 nd shell (“liquid”?) – n=12 stable structure  Cs 2 + -He n – n=8 stable structure  

Institute of Experimental Physics 8 Wolfgang E. Ernst 67th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 18-22, 2012 Appearance potential (IP approached from „above“) TOF ion detection for Rb-He n and Cs-He n with n  500 Moritz Theisen, Florian Lackner, Günter Krois, and Wolfgang E. Ernst, J. Phys. Chem. Lett. 2011, 2, 2778–2782 Ref. 16: E. Loginov, M. Drabbels, Phys. Rev. Lett. 2011, 106, –

Institute of Experimental Physics 9 Wolfgang E. Ernst 67th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 18-22, 2012 IP approached from „below“ (through Rydberg series) Cs-He N Rydberg series: F. Lackner, G. Krois, M. Theisen, M. Koch and W. E. Ernst, Phys. Chem. Chem. Phys., 2011, 13, 18781– 18788

Institute of Experimental Physics 10 Wolfgang E. Ernst 67th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 18-22, 2012 IP approached from „below“ (through Rydberg series) Cs-He N Rydberg series: F. Lackner, G. Krois, M. Theisen, M. Koch and W. E. Ernst, Phys. Chem. Chem. Phys., 2011, 13, 18781– He N droplet sizes varied via different nozzle temperatures and pressures Cs-He 7000 Cs-He 4000 Rb-He N Rydberg series: Florian Lackner, Gu ̈ nter Krois, Markus Koch, and Wolfgang E. Ernst, J. Phys. Chem. Lett. 2012, 3, 1404−1408 Rb-He 7500 Rb-He 3200

Institute of Experimental Physics 11 Wolfgang E. Ernst 67th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 18-22, 2012 IP approached from „below“ (through Rydberg series) e. g. free Cs atom: E I,at = cm -1 d at for S, P and D levels (for high n): d S = 4.1, d P = 3.6, d D = 2.5 atom on the droplet, fit parameter: E I,dr and d dr for Σ, Π, and Δ states d…Quantum defect, depends on n and l E I …Ionization energy R ∞ …Rydberg constant Compare the “on-droplet” peak position to “free-atom” transitions using a model based on the Rydberg-formula: Δν(n) Ionization-threshold red-shift: ~52 cm -1 for all Cs-He N Rydberg series with N  10 4 Ionization-threshold red- shift for all Rb-He N Rydberg series with N  2 to 8 x 10 3

Institute of Experimental Physics 12 Wolfgang E. Ernst 67th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 18-22, 2012 Summary and conclusion Approximate energy potentials of CsHe N and Cs + He N for N = 2000 and R ≈ 27 Å.  Ψ  2 and Franck-Condon vertical excitation region relevant to the experimental excitation scheme are indicated. Energy values are only approximate. The dotted line near R ≈ 27 Å shows the boundary of the helium droplet.  Upon Ak ionization, He shell forms around ion – snowball formation  IP of Ak-He N lowered compared to free Ak due to strong Ak + -He binding  IP redshift increases with increasing He N size Outlook:  Ionization of other metal atoms with location inside droplet  More details on Rydberg states See next talk: Andreas Kautsch WJ06 – Cr in He N See Friday: Florian Lackner FD08 11:04 – Rb-He N Rydberg