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Rotationally-Resolved Spectroscopy of the Bending Modes of Deuterated Water Dimer

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

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Rotationally-Resolved Spectroscopy of the Bending Modes of Deuterated Water Dimer

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  1. Rotationally-Resolved Spectroscopy of the Bending Modes of Deuterated Water Dimer Jacob T. Stewart and Benjamin J. McCall Department of chemistry, University of Illinois

  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

  3. What do we know about water dimer? • (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-IR probes intramolecular vibrations

  4. Previous work on bending modes of water dimer • Gas phase spectra of (H2O)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).

  5. Previous work on bending modes of water dimer • Spectra taken in the Saykally group of a He/D2O expansion • Possible hints of (D2O)2 features • Laser stopped working (damaged mirrors) Huneycutt, PhD thesis, University of California, Berkeley, 2003.

  6. 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.

  7. Tunneling in water dimer Top half are “2’s” Experimentally determined splittings are a measure of barriers on the potential energy surface Bottom half are “1’s” Keutsch, F. N., & Saykally, R. J. PNAS, 98 (2001) 10533. rigid dimer acceptor switching interchange bifurcation

  8. Expected band structure • Either perpendicular (ΔKa = ±1) or parallel bands (ΔKa = 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

  9. Producing and measuring 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 • Used cavity ringdown spectroscopy to obtain spectrum

  10. Overview of the spectrum • Arexpansion • Most features also present in He • Studies with D2O/H2O mixtures confirm (D2O)2

  11. Identifying (D2O)2 bands Ka = 1 ← 0 band of donor bend R(0) lines confirm assignment Actually three overlapping bands

  12. Identifying (D2O)2 bands Ka = 2 ← 1 band of donor bend Lack of R(0) lines confirm assignment Actually three overlapping bands

  13. Other component of acceptor switching splitting Ka = 1 ← 0 1’s 2’s 2.4 cm-1

  14. Other component of acceptor switching splitting Ka = 2← 1 1’s 2’s 0.9 cm-1

  15. 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 Ka= 1 in excited state, acceptor switching splitting is 19 GHz (17 GHz in ground state) • For Ka= 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

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

  17. Band center • Band center can be calculated from assignment • After taking tunneling into account, band center is 1182.2 cm-1 • About 10 cm-1 lower than matrix studies • Close agreement with calculations on ab initio surface

  18. 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

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

  20. Determining 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.

  21. Determining 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

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