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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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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
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
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
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).
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.
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
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
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
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: + ...
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).
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).
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
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?
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.
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).
Acknowledgments • McCall Group • Claire Gmachl • Richard Saykally • Kevin Lehmann