Analysis of the high-resolution mid-infrared spectrum of deuterated water clusters Jacob T. Stewart and Bradley M. Gibson, Department of Chemistry, University of Illinois at Urbana-Champaign Benjamin J. McCall, Departments of Chemistry and Astronomy, University of Illinois at Urbana-Champaign

Why study small water clusters? Liquid water plays a key role in many important systems (biological, solutions, etc.) Complex molecular structure due to hydrogen bonding Gas phase clusters are model systems to study intermolecular interactions Once I bring up the picture from Science, highlight recent rotational spectroscopy on water hexamer Note that (B) shows comparison of spectroscopic data and ab initio calculations We are interested in water dimer in this study

Why study small water clusters? Saykally and Wales, Science, 336 (2012) 814. Pérez et al., Science, 336 (2012) 897.

What do we know about water dimers? (H2O)2 and (D2O)2 extensively studied in microwave and far-IR (rotations and intermolecular modes) Data used to develop potential energy surfaces Intramolecular stretches have been measured at high resolution No rotationally-resolved spectra of bending modes far-IR probes intermolecular vibrations mid- and near-IR probes intramolecular vibrations

Tunneling in water dimer Three large amplitude motions lead to tunneling between 8 equivalent minima Splittings caused by tunneling can be observed experimentally Keutsch, F. N., & Saykally, R. J. PNAS, 98 (2001) 10533.

Tunneling in water dimer Experimentally determined splittings are a measure of barriers on the potential energy surface rigid dimer acceptor switching interchange bifurcation Keutsch, F. N., & Saykally, R. J. PNAS, 98 (2001) 10533.

How do we measure (D2O)2? Fabry-Perot quantum cascade lasers provided by Claire Gmachl at Princeton Housed in a liquid nitrogen cryostat Lasers can scan from ~1180-1200 cm-1 (but not continuously) Items to highlight: - QCL that can cover D2O bending region Rhomb acts as optical isolator Use cavity ringdown as sensitive absorption technique Frequency calibration using wavemeter and SO2 B. E. Brumfield et al., Rev. Sci. Instrum., 81, 063102 (2010).

Producing water clusters Clusters were generated in a continuous supersonic slit expansion (150 µm × 1.6 cm) Gas was bubbled through D2O at room temperature Ar at ~250 torr He at ~900 torr

Vibrational bands of ArD2O What have we seen? Ar expansion at 250 torr D2O monomer lines removed Vibrational bands of ArD2O Gap in laser coverage

Combination differences indicate same lower state Closer look at ArD2O (110)(101) band of ArD2O P-branch Q-branch R-branch For full analysis, see Li et al., JMS, 272 (2012) 27. Two more Q-branches Combination differences indicate same lower state

Closer look at ArD2O Q-branch (111)(000) band R-branch P-branch R-branch P-branch For full analysis, see Li et al., JMS, 272 (2012) 27.

Finding D2O clusters Clusters of lines that appears in Ar and He expansions

We know this is from a D2O only cluster – but what size? Finding D2O clusters We know this is from a D2O only cluster – but what 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 relationship Cruzan et al., Science, 271 (1996), 59.

Identifying cluster size Our data from cluster of lines near 1195.5 cm-1 Measured each concentration 10 times Slope = 3.9 ± 0.2 Consistent with dimer

What else have we found? He expansion instead of Ar Baseline drifts occur as D2O level in bubbler decreases Likely trimer transitions

Previous measurements of (H2O)2 bending region Hydrogen bond acceptor bend at lower frequency (close to monomer band center) Hydrogen bond donor bend at higher frequency acceptor bend donor bend Paul et al., JPCA, 103 (1999) 2972.

Summary of overall spectrum acceptor bend donor bend trimer

Trying to sort out the mess We have observed over 500 lines using He expansions No obvious patterns yet found Analysis complicated by tunneling splitting of rotational levels

Conclusions and future directions We have measured (D2O)2 intramolecular bending modes with rotational resolution Also observed bending mode of ArD2O We will be building an external cavity QCL system which can expand our coverage and fill in gaps Analysis of the observed features is ongoing

Acknowledgments Springborn Endowment McCall Group Claire Gmachl Richard Saykally http://bjm.scs.illinois.edu Springborn Endowment