Rotationally-Resolved Spectroscopy of the Bending Modes of Deuterated Water Dimer JACOB T. STEWART AND BENJAMIN J. MCCALL DEPARTMENT OF CHEMISTRY, UNIVERSITY OF ILLINOIS

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

What do we know about water dimer? (H 2 O) 2 and (D 2 O) 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-IR probes intramolecular vibrations

Previous work on bending modes of water dimer Gas phase spectra of (H 2 O) 2 observed by cavity ringdown spectroscopy No rotational resolution, difficult to determine band centers Could not observe tunneling patterns Paul et al., J. Phys. Chem. A, 103, 2972 (1999).

Previous work on bending modes of water dimer Spectra taken in the Saykally group of a He/D 2 O expansion Possible hints of (D 2 O) 2 features Laser stopped working (damaged mirrors) Huneycutt, PhD thesis, University of California, Berkeley, 2003.

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

rigid dimer acceptor switching interchangebifurcation Experimentally determined splittings are a measure of barriers on the potential energy surface Keutsch, F. N., & Saykally, R. J. PNAS, 98 (2001) Bottom half are “1’s” Top half are “2’s” Tunneling in water dimer

Expected band structure Either perpendicular (ΔK a = ±1) or parallel bands (ΔK a = 0) Selection rules only allow 1s ↔ 1s or 2s ↔ 2s Two sets of bands separated by acceptor switching tunneling Each set composed of three bands

Producing and measuring clusters Clusters were generated in a continuous supersonic slit expansion (150 µm × 1.6 cm) Gas was bubbled through D 2 O at room temperature Ar at ~ 250 torr He at ~ 900 torr Used cavity ringdown spectroscopy to obtain spectrum

Overview of the spectrum Ar expansion Most features also present in He Studies with D 2 O/H 2 O mixtures confirm (D 2 O) 2

Identifying (D 2 O) 2 bands K a = 1 ← 0 band of donor bend R(0) lines confirm assignment Actually three overlapping bands

Identifying (D 2 O) 2 bands K a = 2 ← 1 band of donor bend Lack of R(0) lines confirm assignment Actually three overlapping bands

Other component of acceptor switching splitting 2.4 cm -1 1’s 2’s K a = 1 ← 0

Other component of acceptor switching splitting 0.9 cm -1 1’s 2’s K a = 2 ← 1

Acceptor switching splitting in the excited state Using previous estimates of Paul et al. for the ground state, we can calculate excited state splitting For K a = 1 in excited state, acceptor switching splitting is 19 GHz (17 GHz in ground state) For K a = 2 in excited state, acceptor switching splitting is 44 GHz (42 GHz in ground state) Exciting donor bend has little to no effect on acceptor switching

Trying to assign interchange tunneling levels Exciting donor bend perturbs interchange tunneling

Band center Band center can be calculated from assignment After taking tunneling into account, band center is cm -1 About 10 cm -1 lower than matrix studies Close agreement with calculations on ab initio surface

Conclusions Observed first rotationally resolved spectrum of donor bend of water dimer Found excitation of donor bend has basically no effect on acceptor switching tunneling Excitation of donor bend appears to perturb the interchange tunneling, making detailed fit difficult Additional bands should be accessible with more widely tunable laser TJ12, 2015 McPherson, 4:40

Acknowledgments McCall Group Claire Gmachl Richard Saykally Springborn Endowment

Determining 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 relationship 21 Cruzan et al., Science, 271 (1996), 59.

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