rotationally-resolved spectroscopy of the bending modes of deuterated water dimer
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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 - PowerPoint PPT PresentationTRANSCRIPT
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Rotationally-Resolved Spectroscopy of the Bending Modes of Deuterated Water DimerJACOB T. STEWART AND BENJAMIN J. MCCALLDEPARTMENT OF CHEMISTRY, UNIVERSITY OF ILLINOIS
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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
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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
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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).
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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.
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Tunneling in water dimer
Keutsch, F. N., & Saykally, R. J. PNAS, 98 (2001) 10533.
• Three large amplitude motions lead to tunneling between 8 equivalent minima• Splittings caused by tunneling can be observed
experimentally
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rigiddimer
acceptorswitching interchange bifurcation
Experimentally determined splittings are a measure of barriers on the potential energy surface
Keutsch, F. N., & Saykally, R. J. PNAS, 98 (2001) 10533.
Bottom half are “1’s”
Top half are “2’s”
Tunneling in water dimer
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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
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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
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Overview of the spectrum
• Ar expansion• Most features also
present in He• Studies with
D2O/H2O mixtures confirm (D2O)2
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Identifying (D2O)2 bands
Ka = 1 ← 0 band of donor bend
R(0) lines confirm assignment
Actually three overlapping bands
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Identifying (D2O)2 bands
Ka = 2 ← 1 band of donor bend
Lack of R(0) lines confirm assignment
Actually three overlapping bands
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Other component of acceptor switching splitting
2.4 cm-1
1’s
2’s
Ka = 1 ← 0
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Other component of acceptor switching splitting
0.9 cm-1
1’s
2’s
Ka = 2 ← 1
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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
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Trying to assign interchange tunneling levels
Exciting donor bend perturbs interchange tunneling
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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
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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
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Acknowledgments• McCall Group• Claire Gmachl• Richard Saykally
Springborn Endowment
http://bjm.scs.illinois.edu
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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
OD2ln2ln n
II
pure
mix
Cruzan et al., Science, 271 (1996), 59.
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Determining cluster size• Our data from cluster of
lines near 1195.5 cm-1
• Measured each concentration 10 times
OD2ln2ln n
II
pure
mix
Slope = 3.9 ± 0.2Consistent with dimer