Institute of Experimental Physics

1 Wolfgang E. Ernst

EPPhotoionization 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

EP

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 = 20-100 Å)

DFT code courtesy of Franco Dalfovo, U. Trento (Italy), Roman Schmied, Princeton

z

Rb atomon droplet surface

He1000Rb iondives into the droplet,surrounds itself withpolarized He

snowballformation

Institute of Experimental Physics

3 Wolfgang E. Ernst

EP

67th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 18-22, 2012

Theoretical predictions for alkali ions in HeN

PHYSICAL REVIEW B, VOLUME 64, 094512 (2001)

Alkali ions in superfluid 4He 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 4He Clusters Doped with Alkali and Alkali-Earth IonsD. E. Galli,*,† D. M. Ceperley,‡ and L. Reatto†

J. Phys. Chem. A 2011, 115, 7300–7309

Radial 4He density, (r), around ions for the largest

4He droplets

studied in this work: number of atoms N = 128.

Prediction: Closure of a rigid HeN shell around Rb+ for N14 and around Cs+ for N18. Second and higher shells may exhibit more mobility (like liquid).

Institute of Experimental Physics

4 Wolfgang E. Ernst

EP

67th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 18-22, 2012

Alkali atoms on HeN – two-step ionization through excited states

Pseudodiatomic potentials HeN-RbFrom: M. Theisen, F. Lackner, F. Ancilotto, C. Callegari, and W.E. Ernst, Eur. Phys. J. D 61, 403–408 (2011)

HeN-Rb 521/2 and HeN-Cs 621/2 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 HeN-Rb and HeN-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

EP

67th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 18-22, 2012

Experiment

Source conditions:

T = 10 - 20 K, p = 40 to 60 bar, 5 μm nozzle

droplet size:

N ~ 103 to 105 He atoms

Pick-up conditions:

Pick-up cell temparature is optimized for single atom pick-up

Institute of Experimental Physics

6 Wolfgang E. Ernst

EP

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: 33691 cm-1

Pulsed Ti:S laser, 12600 – 13000 cm-1

5²S1/2

5²P3/2

5²P1/2

pulsed dye laser

Rb

+e-

TOF

6²S1/2

5²D3/2,5/2

+

6²S1/2

6²P3/2

6²P1/2

7²S1/2

6²D3/2,5/2

Cs

Pulsed Ti:S laser11200 – 11900 cm-1

pulsed dye laser

IE: 31406 cm-1

e- TOF

ionization products monitored:

Ak, Ak-Hen (n < 20),

Ak-HeN (N > 500)

Institute of Experimental Physics

7 Wolfgang E. Ernst

EP

67th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 18-22, 2012

Snowballs observed

Rb+-Hen (PhD thesis Moritz Theisen) Cs+-Hen

Moritz Theisen, Florian Lackner, and Wolfgang E. Ernst,THE JOURNAL OF CHEMICAL PHYSICS 135, 074306 (2011)

187 HeRb

185 HeRb

1285,87 HeRb

1685,87 HeRb

Favored structures n=16 1st shell n=12 stable structure (predicted by Gianturco and coworkers)

50HeCs

17.5HeCs

12HeCs

Favored structures Cs+-Hen

– n18 1st shell “frozen” n~50 2nd shell (“liquid”?)

– n=12 stable structure Cs2

+-Hen

– n=8 stable structure

Institute of Experimental Physics

8 Wolfgang E. Ernst

EP

67th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 18-22, 2012

Appearance potential (IP approached from „above“)

TOF ion detection for Rb-Hen and Cs-Hen 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, 083401-1–083401-4

Institute of Experimental Physics

9 Wolfgang E. Ernst

EP

67th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 18-22, 2012

IP approached from „below“ (through Rydberg series)

Cs-HeN 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

EP

67th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 18-22, 2012

IP approached from „below“ (through Rydberg series)

Cs-HeN Rydberg series:F. Lackner, G. Krois, M. Theisen, M. Koch and W. E. Ernst,Phys. Chem. Chem. Phys., 2011, 13, 18781–18788

HeN droplet sizes varied via different nozzle temperatures and pressures

Cs-He7000

Cs-He4000

Rb-HeN Rydberg series:Florian Lackner, Günter Krois, Markus Koch, and Wolfgang E. Ernst,J. Phys. Chem. Lett. 2012, 3, 1404−1408

Rb-He7500

Rb-He3200

Institute of Experimental Physics

11 Wolfgang E. Ernst

EP

67th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 18-22, 2012

IP approached from „below“ (through Rydberg series)

e. g. free Cs atom:

EI,at = 31406.5 cm-1

dat for S, P and D levels (for high n):

dS= 4.1, dP= 3.6, dD= 2.5

atom on the droplet, fit parameter:

EI,dr and ddr for Σ, Π, and Δ states

d…Quantum defect, depends on n and l

EI…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-HeN Rydberg series

with N 104

Ionization-threshold red-shift for all Rb-HeN Rydberg series

with N 2 to 8 x 103

Institute of Experimental Physics

12 Wolfgang E. Ernst

EP

67th Int. Symp. on Molecular Spectroscopy, Columbus, Ohio June 18-22, 2012

Summary and conclusion

Approximate energy potentials of CsHeN and Cs+HeN

for N = 2000 and R ≈ 27 Å. Ψ2 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-HeN lowered compared to free Ak due to strong Ak+-He binding

· IP redshift increases with increasing HeN size

Outlook:· Ionization of other metal atoms

with location inside droplet· More details on Rydberg states

See next talk: Andreas Kautsch WJ06 – Cr in HeN

See Friday: Florian Lackner FD08 11:04 – Rb-HeN Rydberg