Transcript High-resolution mid-infrared spectroscopy of deuterated

High-resolution mid-infrared
spectroscopy of deuterated water
clusters using a quantum cascade laserbased 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
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Measuring water clusters
• One of the primary
means of studying small
water clusters is through
spectroscopy
• Lots of work in the farinfrared, 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
far-IR probes intermolecular
vibrations
mid- and near-IR probes
intramolecular vibrations
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Quantum cascade lasers
• Made from multiple
stacks of quantum
wells
• Thickness of wells
determines laser
frequency
• Frequency is
adjusted through
temperature and
current
Curl et al., Chem. Phys. Lett., 487, 1 (2010).
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Cavity ringdown spectrometer
•Rhomb and polarizer act as an optical
isolator
•Total internal reflection causes a phase
shift in the light
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B. E. Brumfield et al., Rev. Sci. Instrum. (2010), 81, 063102.
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
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What have we observed?
ArD2O
• D2O and HOD monomer transitions
have been removed for clarity
• Almost 10 cm-1 of continuous
coverage
• What species are present?
ArD2O
(D2O)n
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Vibrational band of ArD2O
Blue: Ar/D2O expansion
Figure from Weida and
Nesbitt, J. Chem. Phys., 106,
3078 (1997).
Red: He/D2O expansion
• 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
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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:
𝐸 𝐽, 𝐾 = 𝜈 + 𝐵 𝐽 𝐽 + 1 −
𝐾2
− 𝐷[𝐽 𝐽 + 1 −
𝐾 2 ]2
+ ...
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Fitting the vibrational band of
ArD2O
Coriolis coupling
e and f
states
• 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
Selection rules:
J = 0, only e  f allowed – Q branch
J = ±1, only e  e or f  f allowed – P & R branches
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Figure from Weida and
Nesbitt, J. Chem. Phys., 106,
3078 (1997).
Constants from the fit
• 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)
(cm-1) P&R branches (000) (Fraser et al.)
(101) (Fraser et al.)
• Need to measure upper  state to quantify Coriolis
interaction
0.09103
.09325842
.09103364
inB’’
upper 
state
D’’
(cm-1)
1.79×10-6
2.571×10-6
P&R branches
1.786×10-6
Q branch
1192.9644
1192.9620

(101) assignment is also confirmed by combination differences
B’
0.09522
0.09321
D’
2.12×10-6
Fraser et al., J. Mol. Spec., 144, 97 (1990).
2.11×10-6
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Another band of ArD2O
D2O
• 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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A D2O-only cluster
• 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?
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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
I mix
ln
 2n ln  D2O
I pure
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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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