1. Multiple Photon Absorption in Hydrated Cesium Ion Clusters Jordan Beck, Jim Lisy June 17,2008 OSU International Symposium on Molecular Spectroscopy

2. Initial Motivation  Project started with desire to find a unique IR signature of magic number clusters similar to Johnson, Duncan, Jordan, et. al. with H + (H 2 O) n Science 304 (5674), pp. 1137-1140, 2004

3. Initial Motivation  There exists in the literature clear evidence of magic numbers in hydrated alkali metal clusters  Example mass spectra from Castleman  We wanted to see if there was an IR signature to these magic number clusters J. Phys. Chem., Vol 99, No. 19, 1995

4. Experimental Overview Ion Gun Conversion Dynode/ Electron Multiplier Electrostatic Lenses Conical Nozzle Ion Deflector Ion Selecting Quadrupole Mass Filter Ion Guiding Quadrupole Ion Guide Ion Analyzing Quadrupole Mass Filter Source Chamber Detection Chamber Skimmer Ion Guiding Octapole Ion Guide Ion Guiding Chamber Continuum Surelite II-10 Hz Nd 3+ :YAG (1064 nm) Tunable LaserVision OPO/A

5. Ion Selecting Quadrupole Mass Filter Ion Guiding Quadrupole Ion Guide Ion Analyzing Quadrupole Mass Filter Detection Chamber Continuum Surelite II-10 Hz Nd 3+ :YAG (1064 nm) Tunable LaserVision OPO/A Experimental Overview Typical Experiment 1) Select cluster ion of interest in first quadrupole (Q1), e.g. Cs + (H 2 O) 20 2) Laser interaction in second quadrupole (Q2) 3) Third quadrupole (Q3) set to monitor loss of most labile ligand, e.g. Cs + (H 2 O) 19 The Solution 3) Third quadrupole (Q3) set to monitor multiple ligand loss channels, e.g. Cs + (H 2 O) 17,18 First two steps are the same The Problem The loss of one water in the typical experiments leads to large background in large clusters due to spontaneous evaporation

6. An Example of a ‘Typical’ Experiment An Example of Monitoring Multiple Loss Channels First Surprise: We are able to see the loss of three water molecules Second Surprise: The spectra seem to change

7. Observations 1) Peaks get narrower as more waters lost. 2) Peak centers shift as more waters lost. 3) Some peaks get suppressed as more waters lost. Multiple Loss Channels Show Different Features We noticed that the cross sections of the loss channels decrease with the number of waters lost. This begs the question: Do the intensities decrease uniformly over the whole spectral range? One way to check is to scale the spectra so that they have the same free OH peak intensity.

8. Looking at Free OH More Closely Here we see loss of up to 6 waters! Also, a narrowing of the free OH peak with successive water loss. Why and how do we see multiple ligand loss?

9. Initial Interpretation Water Binding Energy ~ 30-55 kJ/mol Argon Binding Energy ~ 5 kJ/mol Photon Energy = 37-45 kJ/mol Multiple Photon Absorption Necessary to Lose Multiple Waters What are the mechanisms for Multiple Photon Absorption? xxxxxxx quasi- continuum hvhv xxxxxx hvhv x hvhv x hvhv x Coherent absorption: Probability of absorption of n photons proportional to Can be ruled out due to anharmonicity of OH stretch and power dependence data. xxxxxxx Heat Bath Vibrational Mode Levels hvhv xxxxxx x Energy leaks to bath xxxxxxx Repeat Incoherent absorption: Probability of absorption of n photons proportional to I Cannot be definitively ruled out.

10. Theor Chem Acc 114, 357-379 (2005) Cs + (H 2 O) 18 Cs + (H 2 O) 8 J. Chem. Phys. 126, 074302 (2007) Implications of Incoherent Absorption We would expect the intensity of the hydrogen bonded features to grow relative to the free OH peaks with multiple water loss. 1.Intrinsic absorption cross sections of H-bonded modes larger than free OH modes 2.H-bonds are more effectively coupled into the framework of the molecule than the free OH modes

11. Spectra Broken Down Into General Regions Free Cyclic HB Bent HB Sym & Asym Linear HB Free Sym Bent HB Cyclic HB Linear HB

12. J. Chem. Phys. 126, 074302 (2007) A closer look at Cs + (H 2 O) 8 Eight identical free OH oscillators with identical absorption cross sections

13. If the photon absorptions are random, uncorrelated events, then they can be modeled with a Poisson Distribution: Small non-argonated clusters dissociate one water molecule per photon (red box). Multi-Photon Absorption

14. RRKM/EE formalism can be used to model these experiments  Rice-Ramsperger-Kassel-Marcus (RRKM) technique used to obtain rate data  Evaporative Ensemble (EE) technique used to model internal energy distributions.

15. Spontaneous Fragmentation: 5.1% Experimental: 7.8%

16. Cluster Relative Intensity Cs + (H 2 O) 10 Cs + (H 2 O) 9 Cs + (H 2 O) 10 1.0 at Mid Q2 0.65 at Detector 0.312 at Detector Cs + (H 2 O) 8 0.038 at Detector

18. Conclusions  Clear, unambiguous evidence of multiple photon absorption  Different OH stretches exhibit different multiple photon behavior  Modeling experiments with RRKM/EE techniques and Poisson statistics is pointing us in the right direction

19. Acknowledgements  Professor Jim Lisy  Lisy Lab Jason Rodriguez Amy Nicely Oscar Rodriguez  Dr. Timothy Vaden  Dr. P. Tarakeshwar  Funding: NSF CHE 0415859

23. Similar Features Observed in Larger Clusters

24. One Photon Causes Multiple Water Loss Experimental fragmentation of one water loss less than calculated value. Experimental fragmentation of two water loss greater than calculated value. Notes: 1. This indicates that the absorption of one photon can cause the loss of more than one water. 2. There is still significant population in the loss of 3 (and 4) waters channels. This cannot be accounted for by one photon absorption.

25. Cs + (H 2 O) 20 as an Example

26. Model of Experiment – Free OH

27. Spontaneous Fragmentation Modeled

28. Laser Induced Fragmentation Modeled

29. One Photon Absorption Causes Two Water Loss

30. Two Photon Absorption

31. Another Example of Multiple Water Loss Different fragment channels of H + (H 2 O) 21 Courtesy of Professor A. Fujii, Tohoku University

34. Summary of Small Hydrated Cs + Spectra

35. Multiple Ligand Loss Also Observed in Argonated Clusters

36. xxxxxxx quasi- continuum hvhv xxxxxx hvhv x hvhv x hvhv x xxxxxxx Heat Bath Vibrational Mode Levels hvhv xxxxxx x Energy leaks to bath xxxxxxx Repeat Incoherent absorption: Probability of absorption of n photons proportional to I Cannot be definitively ruled out.

37. Model Experiment Provides Much Information