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

Magic Numbers in Protonated Water Chang et al. J. Chem. Phys. 122, 074315 (2005) Johnson et al. Science 304, 1137 2004.

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

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.

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-2 + 2H2O M+(H20)n + hv

Mass Spectrum of Li+(H2O)n Recall Protonated Water MS. No Prominent Magic Numbers!! Figure: Chang et al. J. Chem. Phys. 122, 074315 (2005)

Li+(H2O)n Free-OH IR Spectra DA DAA

Li+(H2O)n Peak Centers DA DAA

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

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

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

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 3696-3701 cm-1 DA 3714-3719 cm-1 Na+ DAA 3700-3704 cm-1 DA 3713-3719 cm-1

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).

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).

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

Poissonian Behavior in Na+(H2O)6

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

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

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