Vibrational cooling of large molecules in supersonic expansions: The case of C 60 and pyrene
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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
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Why study C60?
Figure from: J. Cami, J. Bernard-Salas, E. Peeters, and S.E. Malek. Science 329, 1180 (Sep. 2010) {2}
Symmetry Astrochemical Relevance
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How do we look for C60?
Figures from: B. Brumfield. Development of a quantum cascade laser based spectrometer for high-resolution spectroscopy of gas phase C60. UIUC, 2011. {3}
• High- temperature (~850 K) oven• Cooling via continuous supersonic expansion• CW-CRDS detection, ~1185 cm-1
C60 Vapor Pressure
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What do we expect to see?
Figure from: J. T. Stewart, B. Brumfield, B. M. Gibson, B. J. McCall. In preparation. {4}
• Branch heads possible
S / N =
• Pinhole nozzle: ~2• Slit nozzle: ~40
∆B
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What did we actually see?
Figures from: J. T. Stewart, B. Brumfield, B. M. Gibson, B. J. McCall. In preparation. {5}
Estimated:
Observed:(NEA ~0.6 ppm)
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Why didn’t we see anything?
Figure from: B. Brumfield. Development of a quantum cascade laser based spectrometer for high-resolution spectroscopy of gas phase C60. UIUC, 2011. {6}
High-Temperature Oven AlignmentGround State Population1.0
0.8
0.6
0.4
0.2
0.0
Fra
ctio
n in
Gro
und
Sta
te
300250200150100500Vibrational Temperature (K)
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Do other molecules cool efficiently?
{7}
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?
See also: M. E. Sanz, M. C. McCarthy and P. Thaddeus. J. Chem. Phys. 122, 194319 (2005) {8}
V-T Transfer
v = 0
v = 1
v = 2
kBT
kBT
Translational Energy: ↑Vibrational Energy: ↓
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How does vibrational cooling work?
See also: M. E. Sanz, M. C. McCarthy and P. Thaddeus. J. Chem. Phys. 122, 194319 (2005) {9}
V-T Transfer
v = 0
v = 1
v = 2
kBT
kBT
Translational Energy: ↑Vibrational Energy: ↓
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How does vibrational cooling work?
{10}
V-T Transfer
10-20
10
-18
10
-16
10
-14
10
-12
10
-10
10
-8
10
-6
10
-4
Fra
ctio
n in
Gro
und
Sta
te(A
fter
Coo
ling)
800700600500400300Initial Temperature (K)
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How does vibrational cooling work?
See also: M. E. Sanz, M. C. McCarthy and P. Thaddeus. J. Chem. Phys. 122, 194319 (2005) {11}
Intramode Relaxation
v = 0
v = 1
v = 2
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How does vibrational cooling work?
See also: M. E. Sanz, M. C. McCarthy and P. Thaddeus. J. Chem. Phys. 122, 194319 (2005) {12}
Intermode Relaxation
Bend Stretch
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How does vibrational cooling work?
See also: G. Ewing. Chem. Phys. 29, 253 (Apr. 1978) {13}
Cluster Predissociation
Bend IntermolecularStretch
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How can we produce colder vapor?
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)
{14}
Laser Ablation• Initial temperature >1900K
Laser Desorption• Initial temperature uncertain• Ground state detectable by R2PI
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How can we produce colder vapor?
See also: C. H. Sin, M. R. Linford, and S. R. Goates. Anal. Chem. 64, 233 (1992) {15}
Supercritical Fluid Expansion• Initial temperature solvent dependent (~450 K)• CO2 w/ toluene co-solvent – aids cooling• Low vapor flux (~1013
molecules s-1)
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Conclusions
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• 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
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• McCall Group• Brian Brumfield• Claire Gmachl