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Vibrational cooling of large molecules
in supersonic expansions: The case of
C60 and pyrene
Bradley M. Gibson and Jacob T. Stewart, Department
of Chemistry, University of Illinois at Urbana-Champaign
Benjamin J. McCall, Departments of Chemistry and Astronomy,
University of Illinois at Urbana-Champaign
Why study C60?
Symmetry
Figure from:
Astrochemical Relevance
J. Cami, J. Bernard-Salas, E. Peeters, and S.E. Malek. Science 329, 1180 (Sep. 2010)
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How do we look for C60?
• High- temperatureC(~850
K) Pressure
oven
Vapor
60
• Cooling via continuous
supersonic expansion
• CW-CRDS detection, ~1185 cm-1
Figures from: B. Brumfield. Development of a quantum cascade laser based spectrometer for high-resolution spectroscopy of
gas phase C60. UIUC, 2011.
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What do we expect to see?
• Branch heads possible
S/N=
𝑛𝑧 𝑓𝑆 ′ 𝐿𝑗𝑒𝑡
∆𝑣(𝜎𝑁𝐸𝐴 )
∆B
• Pinhole nozzle: ~2
• Slit nozzle: ~40
Figure from:
J. T. Stewart, B. Brumfield, B. M. Gibson, B. J. McCall. In preparation.
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What did we actually see?
Estimated:
Observed:
(NEA ~0.6 ppm)
Figures from: J. T. Stewart, B. Brumfield, B. M. Gibson, B. J. McCall. In preparation.
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Why didn’t we see anything?
High-Temperature
Ground State Oven
Population
Alignment
1.0
Fraction in Ground State
0.8
0.6
0.4
0.2
0.0
0
Figure from:
50
100
200
150
Vibrational Temperature (K)
250
300
B. Brumfield. Development of a quantum cascade laser based spectrometer for high-resolution spectroscopy of
gas phase C60. UIUC, 2011.
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Do other molecules cool efficiently?
Pyrene
• Oven temp ~430 K
• Estimated from absorption depth
• Tvib 60-90 K
D2O
• Oven temp ~800 K
• Estimated from hot band
• Tvib > 1000 K
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How does vibrational cooling work?
V-T Transfer
v=2
v=1
kBT
kBT
v=0
Translational Energy: ↑
Vibrational Energy: ↓
See also:
M. E. Sanz, M. C. McCarthy and P. Thaddeus. J. Chem. Phys. 122, 194319 (2005)
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How does vibrational cooling work?
V-T Transfer
v=2
kBT
v=1
kBT
v=0
Translational Energy: ↑
Vibrational Energy: ↓
See also:
M. E. Sanz, M. C. McCarthy and P. Thaddeus. J. Chem. Phys. 122, 194319 (2005)
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How does vibrational cooling work?
V-T Transfer
-4
10
-6
Fraction in Ground State
(After Cooling)
10
-8
10
-10
10
-12
10
-14
10
-16
10
-18
10
-20
10
300
400
500
600
Initial Temperature (K)
700
800
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How does vibrational cooling work?
Intramode Relaxation
v=2
v=1
v=0
See also:
M. E. Sanz, M. C. McCarthy and P. Thaddeus. J. Chem. Phys. 122, 194319 (2005)
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How does vibrational cooling work?
Intermode Relaxation
Bend
See also:
Stretch
M. E. Sanz, M. C. McCarthy and P. Thaddeus. J. Chem. Phys. 122, 194319 (2005)
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How does vibrational cooling work?
Cluster Predissociation
Bend
See also:
Intermolecular
Stretch
G. Ewing. Chem. Phys. 29, 253 (Apr. 1978)
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How can we produce colder vapor?
Laser Ablation
• Initial temperature >1900K
Laser Desorption
• Initial temperature uncertain
• Ground state detectable by R2PI
See also:
E. E. B. Campbell, I. V. Hertel, Ch. Kusch, R. Mitzner, and B. Winter. Synth. Mat. 77, 173 (1996)
R. E. Haufler, L-S. Wang, L. P. F. Chibante, C. Jin, J. Conceicao, Y. Chai, and R. E. Smalley. Chem. Phys. Lett. 179, 449
(1991)
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How can we produce colder vapor?
Supercritical Fluid Expansion
• Initial temperature solvent dependent (~450 K)
• CO2 w/ toluene co-solvent – aids cooling
• Low vapor flux (~1013 molecules s-1)
See also:
C. H. Sin, M. R. Linford, and S. R. Goates. Anal. Chem. 64, 233 (1992)
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Conclusions
•
•
•
•
C60 signal not yet observed
Lack of signal likely due to poor vibrational cooling
Efficient cooling highly dependent upon initial temperature
New vaporization technique required
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Acknowledgements
• McCall Group
• Brian Brumfield
• Claire Gmachl
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