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Continuous probing of cold complex molecules with infrared frequency comb spectroscopy

Nature volume 533, pages 517–520 (2016)Cite this article

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Abstract

For more than half a century, high-resolution infrared spectroscopy has played a crucial role in probing molecular structure and dynamics. Such studies have so far been largely restricted to relatively small and simple systems, because at room temperature even molecules of modest size already occupy many millions of rotational/vibrational states, yielding highly congested spectra that are difficult to assign. Targeting more complex molecules requires methods that can record broadband infrared spectra (that is, spanning multiple vibrational bands) with both high resolution and high sensitivity. However, infrared spectroscopic techniques have hitherto been limited either by narrow bandwidth and long acquisition time1, or by low sensitivity and resolution2. Cavity-enhanced direct frequency comb spectroscopy (CE-DFCS) combines the inherent broad bandwidth and high resolution of an optical frequency comb with the high detection sensitivity provided by a high-finesse enhancement cavity3,4, but it still suffers from spectral congestion5. Here we show that this problem can be overcome by using buffer gas cooling6 to produce continuous, cold samples of molecules that are then subjected to CE-DFCS. This integration allows us to acquire a rotationally resolved direct absorption spectrum in the C–H stretching region of nitromethane, a model system that challenges our understanding of large-amplitude vibrational motion7,8,9. We have also used this technique on several large organic molecules that are of fundamental spectroscopic and astrochemical relevance, including naphthalene10, adamantane11 and hexamethylenetetramine12. These findings establish the value of our approach for studying much larger and more complex molecules than have been probed so far, enabling complex molecules and their kinetics to be studied with orders-of-magnitude improvements in efficiency, spectral resolution and specificity.

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Figure 1: Buffer gas cooling of nitromethane.
Figure 2: A schematic of the combined CE-DFCS and buffer gas cooling apparatus.
Figure 3: Survey absorption spectrum of nitromethane.
Figure 4: Survey absorption spectrum of several large molecules.

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Acknowledgements

We acknowledge funding from DARPA SCOUT, AFOSR, NIST and NSF-JILA PFC for this research. J.M.D. and D.P. acknowledge funding from the NSF and HQOC. B.S. is supported through an NRC Postdoctoral Fellowship. O.H.H. is partially supported through a Humboldt Fellowship. P.B.C. is supported by the NSF GRFP (award no. DGE1144083). We thank J. Baraban for input and discussion. We thank D. Perry for providing us with G. O. Sørensen’s original nitromethane ground state data.

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Authors and Affiliations

  1. Department of Physics, JILA, National Institute of Standards and Technology and University of Colorado, University of Colorado, Boulder, 80309, Colorado, USA

    Ben Spaun, P. Bryan Changala, Bryce J. Bjork, Oliver H. Heckl & Jun Ye

  2. Department of Physics, Harvard University, Cambridge, 02138, Massachusetts, USA

    David Patterson & John M. Doyle

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Contributions

P.B.C., D.P., J.M.D. and J.Y. originally designed this experiment. B.S., P.B.C. and J.Y. discussed and implemented the experimental technique, and B.S. and P.B.C. analysed all data. B.S., P.B.C., B.J.B. and O.H.H. operated laboratory equipment. All authors wrote the paper and contributed to technical discussions regarding this work.

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Correspondence to Ben Spaun or Jun Ye.

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Extended data figures and tables

Extended Data Figure 1 Reduced term values of the rotational sub-levels of ν3 + ν6 (m = 0).

These are plotted against the total angular momentum, J (scaled as J(J + 1)). The reduced energies are equal to the absolute energy E, offset by 2,950 cm−1, minus J(J + 1) times the average of the B and C rotational constants. The solid lines connect sets of levels with respect to Ka (the projection of J onto the molecular inertial a axis) and their parity (e/f) symmetry label. For clarity, e and f states are shown in triangles and circles, respectively. States of different Ka values are shown in different colours. Inset, magnified view of the boxed area of the main plot, showing pairs of perturbed eigenstates, split symmetrically about the zeroth-order bright state position, are indicated in bold markers (see Methods for additional details).

Extended Data Figure 2 Evidence of cluster-free cooling.

The plot compares our measured buffer gas cooled C2H2 spectrum (bottom trace) with that of the Ne–C2H2 complex (upper trace; reprinted with permission from figure 1 of ref. 45, copyright 1998, AIP Publishing LLC). Three acetylene monomer transitions in the buffer gas cooled spectrum, including two hot band transitions and a 13C feature as described in the text, have been labelled. The buffer gas cooled spectrum has been rebinned with a bin size of 5 frequency elements (~40 MHz total).

Extended Data Figure 3 The vibrational density of states for several large hydrocarbons.

In increasing order, the total density of states (that is, not symmetry selected) versus vibrational energy is shown for adamantane (C10H16), naphthalene (C10H8), dodecahedrane (C20H20), diamantane (C14H20), anthracene (C14H10), and pyrene (C16H10). These curves were calculated using a direct state count algorithm and a combination of previously observed and calculated vibrational frequencies (see Methods for details). The horizontal line at 100 states per cm−1 marks the empirical threshold symmetry selected state density for IVR60,61. The vertical line at 3,000 cm−1 indicates the approximate energy for CH stretch fundamental vibrations.

Extended Data Table 1 Rotational Hamiltonian fit results
Full size table
Extended Data Table 2 Line list for the 2,953 cm−1 band of nitromethane
Full size table
Extended Data Table 3 Continued from Extended Data Table 2
Full size table
Extended Data Table 4 Naphthalene ν29 band line list
Full size table

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Spaun, B., Changala, P., Patterson, D. et al. Continuous probing of cold complex molecules with infrared frequency comb spectroscopy. Nature 533, 517–520 (2016). https://doi.org/10.1038/nature17440

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