Rotationally-Resolved Spectroscopy of the Bending Modes of Deuterated Water DimerJACOB 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?

(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

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).

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.

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

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

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

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

Overview of the spectrum

• Ar expansion• Most features also

present in He• Studies with

D2O/H2O mixtures confirm (D2O)2

Identifying (D2O)2 bands

Ka = 1 ← 0 band of donor bend

R(0) lines confirm assignment

Actually three overlapping bands

Identifying (D2O)2 bands

Ka = 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

Ka = 1 ← 0

Other component of acceptor switching splitting

0.9 cm-1

1’s

2’s

Ka = 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 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

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 1182.2 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

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

I

I

pure

mix

Cruzan et al., Science, 271 (1996), 59.

Determining cluster size• Our data from cluster of

lines near 1195.5 cm-1

• Measured each concentration 10 times

OD2ln2ln n

I

I

pure

mix

Slope = 3.9 ± 0.2Consistent with dimer