1. The University of Illinois-Urbana Champaign
MAGIC NUMBERS IN ALKALI CATION WATER CLUSTERS: INFARED SPECTROSCOPY OF Li+(H2O)n and Na+(H2O)n
Jason D. Rodriguez, Matthew L. Ackerman, Dorothy J. Miller, James M. Lisy
The University of Illinois-Urbana Champaign

2. Magic Numbers in Protonated Water
Chang et al. J. Chem. Phys. 122, (2005)
Johnson et al. Science 304,

3. Magic Numbers in Alkali Metal Ions
Hartke and Schulz used MD to probe the structure of alkali cation micro hydration structures for M+(H2O)n (M=Na,K,Cs) to attempt to study magic numbers
Possible explanation: Magic numbers due to low-energy cage structures that contain only three-coordinated DDA or DAA water molecules DA
DAA

4. Cluster Generation
Lithium and sodium ions collide with fully expanded neutral clusters
Generates an ensemble of unstable clusters with high initial internal energies
Clusters Then Stabilize by Evaporative Cooling.

5. Apparatus M+(H2O)n-2 + 2H2O M+(H20)n + hv Nd3+:YAG Laser (1064 nm)
Tunable LiNbO3 OPO
Nd3+:YAG Laser (1064 nm)
20 Hz, 20 ns pulse width
M+(H2O)n H2O
M+(H20)n + hv

6. Mass Spectrum of Li+(H2O)n
Recall Protonated Water MS.
No Prominent Magic Numbers!!
Figure: Chang et al. J. Chem. Phys. 122, (2005)

7. Li+(H2O)n Free-OH IR SpectraDA
DAA

8. Li+(H2O)n Peak Centers DA
DAA

9. Mass Spectrum of Na+(H2O)n
No Prominent Magic Numbers!!

10. Na+(H2O)n Free-OH IR Spectra
Most DA peaks occur as shoulders.
DAA DA

11. Na+(H2O)n Peak Centers DA
n=22 has no DA peak
DAA

12. Big Water Summary
The mass spectra for both Li+ and Na+ do not contain prominent magic numbers and we do not see any significant changes in Free-OH IR spectra
DAA and DA type Free-OH stretches are present in both Li+ and Na+
Li+
DAA cm-1
DA cm-1
Na+
DAA cm-1
DA cm-1

13. Multi-Photon Absorption
In these clusters, MPA is not “Traditional”
RA05 Prof. Neumark: system underwent change in cross-section upon absorption of 1 photon
In our system: we have multiple uncoupled identical oscillators that upon absorption leave the cross-section unchanged
Na+(H2O)18
Number of Photons Absorbed
Loss Channel 1
Loss of both 1 and 2 waters may occur 2
Loss of 2 waters 3
Loss of 3 waters
F. Schulz, B. Hartke, Theor. Chem. Acc. 114, 357 (2005).

14. Looking at the Smaller Clusters
Loss of multiple waters not expected. DA
DAA
n=8
DA more intense than DAA DA
DAA
F. Schulz, B. Hartke, Theor. Chem. Acc. 114, 357 (2005).

15. A Closer Look at n=8 DA DAA
F. Schulz, B. Hartke, Theor. Chem. Acc. 114, 357 (2005).

16. Poissonian Behavior in Na+(H2O)6

17. Poissonian Behavior in Na+(H2O)8
DAA DA
Good agreement at low power.

18. Conclusions
There is no discernable pattern for peaks in free-OH region for big hydrated Li and Na clusters
In small hydrated clusters loss of multiple waters is due to MPA and there is a good correlation between number of waters lost and number of photons absorbed
MPA was unexpected in our studies, but we are eager to investigate the impact MPA plays on the IR signatures of the large hydrated alkali metal cations

19. Acknowledgements Lisy Research Group Prof. James M. Lisy
Ms. Dorothy J. Miller
Mr. Matthew L. Ackerman
Mr. Jordan P. Beck
Ms. Amy Willmarth