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?

Figure from: J. Cami, J. Bernard-Salas, E. Peeters, and S.E. Malek. Science 329, 1180 (Sep. 2010) {2}

Symmetry Astrochemical Relevance

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

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

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)

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)

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

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: ↓

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: ↓

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)

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

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

How does vibrational cooling work?

See also: G. Ewing. Chem. Phys. 29, 253 (Apr. 1978) {13}

Cluster Predissociation

Bend IntermolecularStretch

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

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)

Conclusions

{16}

• 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

Acknowledgements

{17}

• McCall Group• Brian Brumfield• Claire Gmachl