Continuous Probing of Cold Complex Molecules with Infrared Frequency Comb Spectroscopy Ben Spaun1, P
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LETTER doi:10.1038/nature17440 Continuous probing of cold complex molecules with infrared frequency comb spectroscopy Ben Spaun1, P. Bryan Changala1, David Patterson2, Bryce J. Bjork1, Oliver H. Heckl1, John M. Doyle2 & Jun Ye1 For more than half a century, high-resolution infrared spectroscopy Fourier transform microwave spectroscopy has advanced the field of has played a crucial role in probing molecular structure and rotational spectroscopy16,17, but spectral congestion has limited appli- dynamics. Such studies have so far been largely restricted to cation to relatively simple molecules with less than 10 atoms4,5. This relatively small and simple systems, because at room temperature limitation is addressed by combining CE-DFCS with a buffer gas cooling even molecules of modest size already occupy many millions of method that can rotationally and translationally cool large molecules to 18,19 rotational/vibrational states, yielding highly congested spectra ~10 K and below . The simulated nitromethane (CH3NO2) absorp- that are difficult to assign. Targeting more complex molecules tion spectra in Fig. 1 illustrate the significant gains in resolving power requires methods that can record broadband infrared spectra (that and sensitivity associated with cooling from 300 K to 10 K: molecules is, spanning multiple vibrational bands) with both high resolution have a five-times narrower Doppler-broadened linewidth and occupy and high sensitivity. However, infrared spectroscopic techniques many fewer and lower rovibrational energy levels, giving a drastically have hitherto been limited either by narrow bandwidth and long simplified absorption spectrum (compared to the unresolvable room acquisition time1, or by low sensitivity and resolution2. Cavity- temperature spectrum20) with clearly distinguishable absorption lines enhanced direct frequency comb spectroscopy (CE-DFCS) combines with enhanced peak amplitudes. Supersonic expansion jets21,22 pro- the inherent broad bandwidth and high resolution of an optical vide another means for cooling molecules, but buffer gas cells do not frequency comb with the high detection sensitivity provided by a require high pumping speeds and the associated elaborate pumping high-finesse enhancement cavity3,4, but it still suffers from spectral congestion5. Here we show that this problem can be overcome by 6 a 1 b using buffer gas cooling to produce continuous, cold samples of T = 300 K 0.8 molecules that are then subjected to CE-DFCS. This integration 0.6 0.5 allows us to acquire a rotationally resolved direct absorption FWHM = 33.3(10) MHz 0.4 T = 16(1) K spectrum in the C–H stretching region of nitromethane, a model 0.4 trans 0.2 system that challenges our understanding of large-amplitude 0.3 vibrational motion7–9. We have also used this technique on several 1 T = 150 K large organic molecules that are of fundamental spectroscopic and 0.8 0.2 astrochemical relevance, including naphthalene10, adamantane11 0.6 −ln(Transmission) 0.1 and hexamethylenetetramine12. These findings establish the value of 0.4 0 our approach for studying much larger and more complex molecules 0.2 −100 0 100 than have been probed so far, enabling complex molecules and their 10 T = 50 K Offset frequency (MHz) kinetics to be studied with orders-of-magnitude improvements in 8 efficiency, spectral resolution and specificity. 6 c 4 Absorption (a.u.) The massively parallel CE-DFCS technique is virtually equivalent to 4 3.5 Trot = 10.7(12) K thousands of simultaneous, highly sensitive, absorption measurements 2 ) with thousands of narrow linewidth lasers. The broadband (hundreds 3 of nanometres) spectrum of a frequency comb consists of tens of thou- 500 T = 8 K 2.5 sands of discrete, narrow frequency modes equally separated by the 400 300 comb repetition rate, frep, with a common carrier envelope frequency 2 200 ln(Population offset, fceo. Both frep and fceo can be precisely stabilized, allowing for 1.5 complete control of each of the tens of thousands of separate frequency 100 13,14 1 modes in the comb . By matching evenly spaced comb lines with 2,954.4 2,954.45 2,954.5 2,954.55 0 5 10 15 resonant frequency modes of a high-finesse optical cavity filled with a Frequency (cm−1) E (cm−1) molecular species, the absorption path length and overall absorption i sensitivity of the comb can be enhanced by four orders of magnitude, Figure 1 | Buffer gas cooling of nitromethane. a, A simulated portion of the to kilometre length scales3. A Fourier-transform spectrometer (FTS) nitromethane spectrum as a function of temperature. Individual absorption is used to measure the absorption spectrum, and only needs a resolu- lines are much more resolvable at low temperatures, as Doppler broadening tion better than the cavity free spectral range (FSR: 100–1,000 MHz) in decreases and the molecular population moves to the lowest available energy levels. b, Observed Doppler-broadened absorption profile of nitromethane, order to resolve a single comb line. Thus, a standard FTS can achieve showing a translational temperature (Ttrans) of ~16 K. c, Measured an instrument linewidth limited only by the stability of the comb nitromethane rotational population as a function of energy, Ei, revealing a ~ itself, in our case, 50 kHz (ref. 15). The highly multiplexed nature rotational temperature of Trot ≈ 11 K (see Methods for more details). The of CE-DFCS has the potential to advance the field of infrared rovibra- units cm−1 correspond to wavenumber, and are used to represent frequency tional spectroscopy just as the recent development of chirped-pulsed (a) and energy (c). 1JILA, National Institute of Standards and Technology and University of Colorado, Department of Physics, University of Colorado, Boulder, Colorado 80309, USA. 2Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA. 26 MAY 2016 | VOL 533 | NATURE | 517 © 2016 Macmillan Publishers Limited. All rights reserved RESEARCH LETTER in the buffer gas cell. To read out the fractional absorption of each comb mode, we use a custom-built (doubled-passed) fast-scanning HR mirror + – – FTS with a scanning arm (0.7 m) sufficiently long for single comb mode + + – – 4,5 + To FTS Comb PBS resolution . ∆ν light – The molecular absorption linewidth ( ), dominated by 15–30 MHz + + – + – + – Doppler broadening (Fig. 1b), is significantly smaller than the To PD H frequency spacing between comb modes transmitted through the lock Grating enhancement cavity (272 MHz). Thus, for given values of frep and fceo, the frequency comb is resonant with only a fraction of the molecular Molecule inlet He inlet absorption features that lie within the comb bandwidth. To ensure that Figure 2 | A schematic of the combined CE-DFCS and buffer gas cooling absorption lines are not missed by the discrete frequency comb modes, apparatus. Light from a mid-infrared frequency comb is coupled into a we step the comb repetition rate by ∆frep after averaging four FTS data high-finesse enhancement cavity formed by two high reflectivity (HR) acquisitions (~30 s total). This shifts the frequency of each comb mode 6 mirrors surrounding a 5–10 K buffer gas cell filled with cold molecules. by n∆frep, where n ≈ 10 is the comb mode number. We choose ∆frep Warm molecules enter through the side of the cell and are quickly cooled such that n∆frep ≤ ∆ν/5, allowing us to measure the Doppler width, to ~10 K through multiple collisions with the helium buffer gas, which and therefore the translational temperature, of molecules in the buffer enters the cell from a separate inlet. Comb light reflected from the cavity gas cell in real time. The complete spectrum containing all absorption is routed by a polarizing beam splitter (PBS) and dispersed by a grating, lines is then generated by interleaving multiple FTS spectra, each and an ~10 nm segment of the comb spectrum is used to generate a Pound–Drever–Hall (PDH) error signal to lock the comb to the cavity. corresponding to a different value of frep. The PDH lock allows for continuous transmission of thousands of We demonstrate the simultaneous advantages in resolution, sensi- frequency comb modes spanning ~100 nm (refs 4, 30). Transmitted comb tivity and bandwidth in this cold molecule–comb spectroscopy system light is coupled into a Fourier-transform spectrometer (FTS), which by gathering rotationally resolved spectra in the C–H stretching region measures the fractional absorption of each transmitted comb line. (~3.3 μm) of nitromethane (CH3NO2, a model system for understanding intramolecular vibrational coupling and large amplitude internal 7–9 infrastructure while allowing us to acquire higher-resolution molecular motion ), adamantane (C10H16) and hexamethylenetetramine infrared spectra spanning multiple vibrational bands in the mid-infra- (C6N4H12, HMT) for the first time. As shown in Fig. 3a, we clearly resolve red with comparable absorption sensitivity. over 1,000 nitromethane absorption lines spanning multiple vibrational Figure 2 shows the important components of the combined bands, including the entire fundamental C–H stretching region, with CE-DFCS and buffer gas cooling apparatus. A mid-infrared frequency an excellent signal-to-noise ratio for less than three hours of data acqui- comb, tunable from 2.8 μm to 4.8 μm, is produced in an optical para- sition. The comb bandwidth is sufficiently large to simultaneously 4,15 metric oscillator (OPO) pumped by a 1 μm ytterbium fibre comb . gather spectra containing the ν3 + ν6 (symmetric + antisymmetric) Both frep ≈ 136 MHz and fceo of the mid-infrared comb are referenced to N–O stretching combination band and the ν1 symmetric C–H stretching a microwave caesium clock. The OPO comb output is then coupled into band (a small section of the ν1 band is shown magnified in Fig. 3b). The a high finesse (F ≈ 6,000) optical cavity surrounding a 5–10 K buffer comb was also tuned to a higher centre frequency to acquire the portion gas cell.