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

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…)?

Spectroscopic challenges Too many accessible states! Partition functions @ room temp: 105-106 ++ Large Doppler widths (100s of MHz)  intractably congested spectra Small absolute absorption cross sections, weak signals

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 3-3.3 μm -Intrinsic limits to high resolution studies

Buffer gas cooling frequency comb spectrometer 1014 cm-3 He, 1012-13 cm-3 molec. Nature, 533, 517 (2016)

Comb-mode resolved VIPA image

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

Frequency scanned VIPA images

Starting “small”: naphthalene

Adamantane, C10H16 [See also lower energy studies: Pirali et al, J. Chem. Phys. 136, 024310 (2012) ]

Diamantane, C14H20

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.

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)

Frequency comb basics envelope carrier time Fourier transform freq.

Mid-IR OPO frequency comb f0(i) servo frep servo frep f0(i) 2.8-4.8 μm Adler, et al. Opt. Lett. 34, 1330 (2009)

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

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