High-resolution mid-infrared spectroscopy of deuterated water clusters using a quantum cascade laser-based cavity ringdown spectrometer

Jacob T. Stewart and Brian E. Brumfield, Department of Chemistry, University of Illinois at Urbana-Champaign

Benjamin J. McCall, Departments of Chemistry and Astronomy, University of Illinois at Urbana-Champaign

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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 2

Measuring water clusters• One of the primary means

of studying small water clusters is through spectroscopy• Lots of work in the far-

infrared, much less work has been done in the infrared• No data yet on the bending

mode region of small water clusters at high resolution due to limited availability of mid-IR light sources

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far-IR probes intermolecular vibrations

mid- and near-IR probes intramolecular vibrations

Quantum cascade lasers• Made from multiple

stacks of quantum wells• Thickness of wells

determines laser frequency• Frequency is adjusted

through temperature and current

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Curl et al., Chem. Phys. Lett., 487, 1 (2010).

Cavity ringdown spectrometer

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B. E. Brumfield et al., Rev. Sci. Instrum. (2010), 81, 063102.

•Rhomb and polarizer act as an optical isolator•Total internal reflection causes a phase shift in the light

Producing clusters• Clusters were generated

in a continuous supersonic slit expansion (150 µm × 1.6 cm)• Ar was bubbled through

D2O and expanded at ~250 torr• Used spectrometer to

probe D2O bending region 6

What have we observed?

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• D2O and HOD monomer transitions have been removed for clarity

• Almost 10 cm-1 of continuous coverage

• What species are present?

ArD2O

(D2O)n

ArD2O

Vibrational band of ArD2O

8• How do we know this is ArD2O? Use helium!• Band structure is identical to previously observed ArH2O spectra in bending

mode region observed by Weida and Nesbitt

Blue: Ar/D2O expansion

Red: He/D2O expansion

Figure from Weida and Nesbitt, J. Chem. Phys., 106, 3078 (1997).

Fitting the vibrational band of ArD2O

• ArD2O can be modeled as a pseudodiatomic system where the D2O subunit acts as an almost free rotor

• System is described by 7 quantum numbers:• J (total angular momentum)• Asymmetric top level of D2O subunit (j, ka, and kc)• K (projection of j on intermolecular axis)• n (quanta of van der Waals stretch)• p (parity) – for e states p=(-1)J, for f states p=(-1)J+1

• For example, n=0, e(101) is a state with no van der Waals stretch; j=1, ka=0, kc=1 for D2O subunit; and K=0

• Energy level expression:9

+ ...

Fitting the vibrational band of ArD2O

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• Lack of P(1) and presence of R(0) indicates this is a transition

• Had to fit P- & R-branches separately from Q-branch

• Upper state has degeneracy split by Coriolis coupling with state with same D2O quantum numbers and parity

Figure from Weida and Nesbitt, J. Chem. Phys., 106, 3078 (1997).

Selection rules:J = 0, only e f allowed – Q branchJ = ±1, only e e or f f allowed – P & R branches

e and f

states

Coriolis coupling

Constants from the fit

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(cm-1) P&R branches (000) (Fraser et al.) (101) (Fraser et al.)

B’’ 0.09103 .09325842 .09103364

D’’ 1.79×10-6 2.571×10-6 1.786×10-6

Fraser et al., J. Mol. Spec., 144, 97 (1990).

(cm-1) P&R branches Q branch 1192.9644 1192.9620

B’ 0.09522 0.09321

D’ 2.12×10-6 2.11×10-6

(101) assignment is also confirmed by combination differences

• Fit ground and excited state constants for P- & R-branch transitions (standard deviation = 13 MHz)

• Only fit excited state for Q-branch, ground state values were fixed to microwave data (standard deviation = 8 MHz)

• Need to measure upper state to quantify Coriolis interaction in upper state

Another band of ArD2O

• Another set of strong lines near 1199 cm-1

• These lines do not appear in He expansions – indicates Ar cluster• There are broad lines that appear in both – these are from D2O-only

clusters - linewidth gives lifetime ~2 ns

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D2O

A D2O-only cluster

13• This cluster of lines appears in both Ar and He expansions indicating these features are from (D2O)n

• How do we determine the cluster size?

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 realtionship

14OD2

ln2ln nII

pure

mix

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

Next steps

• Optimize expansion conditions for production of (D2O)n instead of ArD2O• Use a combination of He expansions and D2O/H2O

mixtures to identify cluster composition and size• Use spectra to observe if exciting bending mode

leads to predissociation

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Keutsch and Saykally, Proc. Natl. Acad. Sci. USA, 98, 10533 (2001).

Acknowledgments• McCall Group• Claire Gmachl• Richard Saykally• Kevin Lehmann

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