1. 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

2. 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

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

4. 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

5. 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)

6. 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)

7. 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 ~ 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, (2010).

8. 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

9. 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

10. 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

11. 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.

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

13. 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?

14. 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.

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

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

17. 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.

18. Summary of overall spectrum
acceptor
bend
donor
bend
trimer

19. 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

20. 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

21. Acknowledgments Springborn Endowment McCall Group Claire Gmachl
Richard Saykally
Springborn Endowment