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High-resolution mid-infrared spectroscopy of deuterated water clusters using a quantum cascade laserbased cavity ringdown spectrometer 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 1 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 2 Measuring water clusters • One of the primary means of studying small water clusters is through spectroscopy • Lots of work in the farinfrared, 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 3 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). 4 Cavity ringdown spectrometer •Rhomb and polarizer act as an optical isolator •Total internal reflection causes a phase shift in the light 5 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 6 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 7 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 8 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: 𝐸 𝐽, 𝐾 = 𝜈 + 𝐵 𝐽 𝐽 + 1 − 𝐾2 − 𝐷[𝐽 𝐽 + 1 − 𝐾 2 ]2 + ... 9 Fitting the vibrational band of ArD2O Coriolis coupling e and f states • 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 Selection rules: J = 0, only e f allowed – Q branch J = ±1, only e e or f f allowed – P & R branches 10 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) (cm-1) P&R branches (000) (Fraser et al.) (101) (Fraser et al.) • Need to measure upper state to quantify Coriolis interaction 0.09103 .09325842 .09103364 inB’’ upper state D’’ (cm-1) 1.79×10-6 2.571×10-6 P&R branches 1.786×10-6 Q branch 1192.9644 1192.9620 (101) assignment is also confirmed by combination differences B’ 0.09522 0.09321 D’ 2.12×10-6 Fraser et al., J. Mol. Spec., 144, 97 (1990). 2.11×10-6 11 Another band of ArD2O D2O • 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 12 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? 13 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 I mix ln 2n ln D2O I pure 14 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 15 Keutsch and Saykally, Proc. Natl. Acad. Sci. USA, 98, 10533 (2001). Acknowledgments • • • • McCall Group Claire Gmachl Richard Saykally Kevin Lehmann 16