Post on 05-Jan-2016
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