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Why water clusters?

High-resolution mid-infrared spectroscopy of deuterated water clusters using a quantum cascade laser-based cavity ringdown spectrometer. Jacob T. Stewart and Brian E. Brumfield, Department of Chemistry, University of Illinois at Urbana-Champaign

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Why water clusters?

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  1. High-resolution mid-infrared spectroscopy of deuterated water clusters using a quantum cascade laser-based cavity ringdownspectrometer Jacob T. Stewart and Brian E. Brumfield,Department of Chemistry, University of Illinois at Urbana-Champaign Benjamin J. McCall, Departments of Chemistry and Astronomy, University of Illinois at Urbana-Champaign

  2. Why water clusters? • Water is ubiquitous on Earth and essential to life • Complicated molecular structure due to hydrogen bonding • Studying small water clusters aids in understanding interactions between water molecules

  3. Measuring water clusters • One of the primary means of studying small water clusters is through spectroscopy • Lots of work in the far-infrared, much less work has been done in the infrared • No data yet on the bending mode region of small water clusters at high resolution due to limited availability of mid-IR light sources far-IR probes intermolecular vibrations mid- and near-IR probes intramolecular vibrations

  4. Quantum cascade lasers • Made from multiple stacks of quantum wells • Thickness of wells determines laser frequency • Frequency is adjusted through temperature and current Curl et al., Chem. Phys. Lett., 487, 1 (2010).

  5. Cavity ringdown spectrometer • Rhomb and polarizer act as an optical isolator • Total internal reflection causes a phase shift in the light B. E. Brumfield et al., Rev. Sci. Instrum. (2010), 81, 063102.

  6. Producing clusters • Clusters were generated in a continuous supersonic slit expansion (150 µm × 1.6 cm) • Ar was bubbled through D2O and expanded at ~250 torr • Used spectrometer to probe D2O bending region

  7. What have we observed? ArD2O • D2O and HOD monomer transitions have been removed for clarity • Almost 10 cm-1 of continuous coverage • What species are present? ArD2O (D2O)n

  8. Vibrational band of ArD2O Blue: Ar/D2O expansion Figure from Weida and Nesbitt, J. Chem. Phys., 106, 3078 (1997). Red: He/D2O expansion • How do we know this is ArD2O? Use helium! • Band structure is identical to previously observed ArH2O spectra in bending mode region observed by Weida and Nesbitt

  9. Fitting the vibrational band of ArD2O • ArD2O can be modeled as a pseudodiatomic system where the D2O subunit acts as an almost free rotor • System is described by 7 quantum numbers: • J (total angular momentum) • Asymmetric top level of D2O subunit (j, ka, and kc) • K (projection of j on intermolecular axis) • n (quanta of van der Waals stretch) • p (parity) – for e states p=(-1)J, for f states p=(-1)J+1 • For example, n=0, e(101) is a state with no van der Waals stretch; j=1, ka=0, kc=1 for D2O subunit; and K=0 • Energy level expression: + ...

  10. Fitting the vibrational band of ArD2O Coriolis coupling • Lack of P(1) and presence of R(0) indicates this is a  transition • Had to fit P- & R-branches separately from Q-branch • Upper  state has degeneracy split by Coriolis coupling with  state with same D2O quantum numbers and parity e and f states Selection rules: J = 0, only e  f allowed – Q branch J = ±1, only e  e or f  f allowed – P & R branches Figure from Weida and Nesbitt, J. Chem. Phys., 106, 3078 (1997).

  11. Constants from the fit • Fit ground and excited state constants for P- & R-branch transitions (standard deviation = 13 MHz) • Only fit excited state for Q-branch, ground state values were fixed to microwave data (standard deviation = 8 MHz) • Need to measure upper state to quantify Coriolis interaction in upper  state (101) assignment is also confirmed by combination differences Fraser et al., J. Mol. Spec., 144, 97 (1990).

  12. Another band of ArD2O • Another set of strong lines near 1199 cm-1 • These lines do not appear in He expansions – indicates Ar cluster • There are broad lines that appear in both – these are from D2O-only clusters - linewidth gives lifetime ~2 ns D2O

  13. A D2O-only cluster • This cluster of lines appears in both Ar and He expansions indicating these features are from (D2O)n • How do we determine the cluster size?

  14. Identifying cluster size • Add H2O to sample and observe how lines decrease • Assume statistical ratio of D2O, H2O, and HOD • Cluster size can be determined by a linear realtionship Cruzan et al., Science (1996), 271, 59.

  15. Next steps • Optimize expansion conditions for production of (D2O)n instead of ArD2O • Use a combination of He expansions and D2O/H2O mixtures to identify cluster composition and size • Use spectra to observe if exciting bending mode leads to predissociation Keutsch and Saykally, Proc. Natl. Acad. Sci. USA, 98, 10533 (2001).

  16. Acknowledgments • McCall Group • Claire Gmachl • Richard Saykally • Kevin Lehmann

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