1. 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 Benjamin J. McCall, Departments of Chemistry and Astronomy, University of Illinois at Urbana-Champaign 1

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 2

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 3 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 4 Curl et al., Chem. Phys. Lett., 487, 1 (2010).

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

6. Producing clusters Clusters were generated in a continuous supersonic slit expansion (150 µm × 1.6 cm) Ar was bubbled through D 2 O and expanded at ~ 250 torr Used spectrometer to probe D 2 O bending region 6

7. What have we observed? 7 D 2 O and HOD monomer transitions have been removed for clarity Almost 10 cm -1 of continuous coverage What species are present? ArD 2 O (D 2 O) n ArD 2 O

8. Vibrational band of ArD 2 O 8 How do we know this is ArD 2 O? Use helium! Band structure is identical to previously observed ArH 2 O spectra in bending mode region observed by Weida and Nesbitt Blue: Ar/D 2 O expansion Red: He/D 2 O expansion Figure from Weida and Nesbitt, J. Chem. Phys., 106, 3078 (1997).

9. Fitting the vibrational band of ArD 2 O ArD 2 O can be modeled as a pseudodiatomic system where the D 2 O subunit acts as an almost free rotor System is described by 7 quantum numbers: J (total angular momentum) Asymmetric top level of D 2 O subunit (j, k a, and k c ) 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 (1 01 ) is a state with no van der Waals stretch; j=1, k a =0, k c =1 for D 2 O subunit; and K=0 Energy level expression: 9

10. Fitting the vibrational band of ArD 2 O 10 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 D 2 O quantum numbers and parity Figure from Weida and Nesbitt, J. Chem. Phys., 106, 3078 (1997). Selection rules:  J = 0, only e  f allowed – Q branch  J = ±1, only e  e or f  f allowed – P & R branches  e and  f states Coriolis coupling

11. Constants from the fit 11 (cm -1 )P&R branches  (0 00 ) (Fraser et al.)  (1 01 ) (Fraser et al.) B’’0.09103.09325842.09103364 D’’1.79×10 -6 2.571×10 -6 1.786×10 -6 Fraser et al., J. Mol. Spec., 144, 97 (1990). (cm -1 )P&R branchesQ branch 1192.96441192.9620 B’0.095220.09321 D’2.12×10 -6 2.11×10 -6  (1 01 ) assignment is also confirmed by combination differences 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

12. Another band of ArD 2 O 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 D 2 O-only clusters - linewidth gives lifetime ~ 2 ns 12 D2OD2O

13. A D 2 O-only cluster 13 This cluster of lines appears in both Ar and He expansions indicating these features are from (D 2 O) n How do we determine the cluster size?

14. Identifying cluster size Add H 2 O to sample and observe how lines decrease Assume statistical ratio of D 2 O, H 2 O, and HOD Cluster size can be determined by a linear realtionship 14 Cruzan et al., Science (1996), 271, 59.

15. Next steps Optimize expansion conditions for production of (D 2 O) n instead of ArD 2 O Use a combination of He expansions and D 2 O/H 2 O 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).

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