1. High resolution rovibrational spectroscopy of large molecules using infrared frequency combs and buffer gas cooling
Bryan Changala1, Ben Spaun1, David Patterson2, Bryce Bjork1, Oliver Heckl1, John Doyle2, and Jun Ye1
1JILA, NIST and University of Colorado, Boulder, CO
2Department of Physics, Harvard University, Cambridge, MA

2. Motivations
High resolution spectroscopy reveals exquisite details of molecular structure and dynamics. Can we apply the tools we use regularly on small molecules to larger systems (dozens of atoms…)?

3. Spectroscopic challenges
Too many accessible states!
Partition room temp:
Large Doppler widths (100s of MHz)  intractably congested spectra
Small absolute absorption cross sections, weak signals

4. Outline Buffer gas cooling frequency comb spectrometer
-Cold gas phase molecule source
-Broadband, high resolution, high sensitivity spectrometer
2) Mid-IR rovibrational spectra of large molecules near μm
-Intrinsic limits to high resolution studies

5. Buffer gas cooling frequency comb spectrometer
1014 cm-3 He, cm-3 molec.
Nature, 533, 517 (2016)

6. Comb-mode resolved VIPA image

7. Fast light referencing with VIPA
Molecules..... ON OFF ON OFF … …
Time

8. Frequency scanned VIPA images

9. Starting “small”: naphthalene

10. Adamantane, C10H16
[See also lower energy studies: Pirali et al, J. Chem. Phys. 136, (2012) ]

11. Diamantane, C14H20

12. Vibrational density of states
Anharmonic-rovibrational coupling of spectroscopic bright state with dense bath of dark states (intramolecular vibrational redistribution, IVR) results in quasi-continuous absorption spectrum.

13. Conclusions & future directions
High resolution, high sensitivity, fast acquisition time, mid-infrared spectrometer using cavity-enhanced frequency comb spectrometer + buffer gas cooling IVR sets an intrinsic limit to 3 μm spectroscopy. Longer wavelength frequency comb sources (OPO & DFG) will permit progress to larger systems of fundamental interest, e.g. icosahedral systems:
Funding:
DARPA SCOUT, AFOSR, NIST, NSF-JILA PFC (JILA)
NSF, HQOC (Harvard)
NRC RAP (BS), NSF GRFP (BC), Humboldt Foundation (OH)

14. Frequency comb basics
envelope
carrier
time
Fourier
transform
freq.

15. Mid-IR OPO frequency comb
f0(i) servo
frep servo
frep
f0(i) μm
Adler, et al. Opt. Lett. 34, 1330 (2009)

16. Cavity enhanced comb spectroscopy
Circulating light
Transmission
High finesse mirrors:
FSR
Cavity resonances
frep > FSR
Comb modes
frep = FSR
frep < FSR
frequency

17. Comb mode resolved FTS Band-limited FTS Comb spectrum frequency
Foltynowicz, et al. Appl. Phys. B 110, 163 (2013)