Line positions and intensities of the ν1{\nu}_1 band of 12^{12}CH3_3I using mid-infrared optical frequency comb Fourier transform spectroscopy

We present a new spectral analysis of the ${\nu}_1$ and ${\nu}_3$+${\nu}_1$-${\nu}_3$ bands of $^{12}$CH$_3$I around 2971 cm$^{-1}$ based on a high-resolution spectrum spanning from 2800 cm$^{-1}$ to 3160 cm$^{-1}$, measured using an optical frequency comb Fourier transform spectrometer. From this spectrum, we previously assigned the ${\nu}_4$ and ${\nu}_3$+${\nu}_4$-${\nu}_3$ bands around 3060 cm$^{-1}$ using PGOPHER, and the line list was incorporated in the HITRAN database. Here, we treat the two fundamental bands, ${\nu}_1$ and ${\nu}_4$, together with the perturbing states, 2${\nu}_2$+${\nu}_3$ and ${\nu}_2$+2${\nu}_6$$^{\pm2}$, as a four-level system connected via Coriolis and Fermi interactions. A similar four-level system is assumed to connect the ${\nu}_3$+${\nu}_1$-${\nu}_3$ and ${\nu}_3$+${\nu}_4$-${\nu}_3$ hot bands, which appear due to the population of the low-lying ${\nu}_3$ state at room temperature, with the 2${\nu}_2$+2${\nu}_3$ and ${\nu}_2$+${\nu}_3$-${\nu}_6$$^{\pm2}$ perturbing states. This treatment provides a good global agreement of the simulated spectra with experiment, and hence accurate line lists and band parameters of the four connected vibrational states in each system. Overall, we assign 4665 transitions in the fundamental band system, with an average error of 0.00071 cm$^{-1}$, a factor of two better than earlier work on the ${\nu}_1$ band using conventional Fourier transform infrared spectroscopy. The ${\nu}_1$ band shows hyperfine splitting, resolvable for transitions with J $\le$ 2 x K. Finally, the spectral intensities of 65 lines of the ${\nu}_1$ band and 7 lines of the ${\nu}_3$+${\nu}_1$-${\nu}_3$ band are reported for the first time using the Voigt line shape as a model in multispectral fitting

OPTICAL FREQUENCY COMB FOURIER TRANSFORM SPECTROSCOPY

Fourier transform spectroscopy (FTS) based on optical frequency combs offers a number of advantages over conventional Fourier transform infrared (FTIR) spectroscopy based on incoherent sources\footnote{J. Mandon, G. Guelachvili, and N. Picque, Nat. Photonics 3, 99 (2009).}. The high spectral brightness of the comb sources allows measuring spectra with high signal-to-noise ratios in acquisition times of the order of seconds. What is more, the resolution of comb-based FTS is given by the linewidth of the comb modes rather than the optical path difference (OPD) in the spectrometer, provided that the OPD is matched to the inverse of the comb mode spacing\footnote{P. Maslowski, et al., Phys. Rev. A 93, 021802 (2016); L. Rutkowski, et al., J. Quant. Spectrosc. Radiat. Transf. 204, 63 (2018).}. This implies that spectra with kHz resolution can be measured using OPD of the order of a few tens of cm\footnote{L. Rutkowski, et al., Opt. Express 25, 21711 (2017).}, which is impossible in conventional FTIR spectrometers. To increase the sensitivity of direct absorption measurements, frequency combs can be efficiently coupled into enhancement cavities that increase the interaction length with the sample\footnote{M. J. Thorpe, and J. Ye, Appl. Phys. B 91, 397 (2008); A. Foltynowicz, et al., Phys. Rev. Lett. 107, 233002 (2011).}. In another cavity-enhanced approach, the profiles of the cavity modes are measured directly, and complex refractive index spectra of entire molecular bands are determined from the broadening and shift of the cavity modes caused by the molecular sample\footnote{A. C. Johansson, et al., Opt. Express 26, 20633 (2018).}. Comb-based FTS can also be combined with other detection methods, such as Faraday rotation spectroscopy to detect broadband interference-free spectra of paramagnetic molecules\footnote{A. C. Johansson, J. Westberg, G. Wysocki, and A. Foltynowicz, Appl. Phys. B 124, 79 (2018).}, or photoacoustic spectroscopy that allows detection in a very small sample volume\footnote{I. Sadiek, et al., Phys. Chem. Chem. Phys. 20, 27849 (2018).}. I will present the various implementations of comb-based FTS and show examples of high-resolution measurements of entire absorption bands in the near- and mid-infrared wavelength range

Cavity-enhanced optical frequency comb spectroscopy in the mid-infrared - application to trace detection of H2O2

We demonstrate the first cavity-enhanced optical frequency comb spectroscopy in the mid-infrared wavelength region and report the sensitive real-time trace detection of hydrogen peroxide in the presence of a large amount of water. The experimental apparatus is based on a mid-infrared optical parametric oscillator synchronously pumped by a high power Yb:fiber laser, a high finesse broadband cavity, and a fast-scanning Fourier transform spectrometer with autobalancing detection. The comb spectrum with a bandwidth of 200 nm centered around 3.75 {\mu}m is simultaneously coupled to the cavity and both degrees of freedom of the comb, i.e., the repetition rate and carrier envelope offset frequency, are locked to the cavity to ensure stable transmission. The autobalancing detection scheme reduces the intensity noise by a factor of 300, and a sensitivity of 5.4 {\times} 10^-9 cm^-1 Hz^-1/2 with a resolution of 800 MHz is achieved (corresponding to 6.9 {\times} 10^-11 cm^-1 Hz^-1/2 per spectral element for 6000 resolved elements). This yields a noise equivalent detection limit for hydrogen peroxide of 8 parts-per-billion (ppb); in the presence of 2.8% of water the detection limit is 130 ppb. Spectra of acetylene, methane and nitrous oxide at atmospheric pressure are also presented, and a line shape model is developed to simulate the experimental data.Comment: submitted to special FLAIR 2011 issue of Appl. Phys.

Sensitive and broadband measurement of dispersion in a cavity using a Fourier transform spectrometer with kHz resolution

Optical cavities provide high sensitivity to dispersion since their resonance frequencies depend on the index of refraction. We present a direct, broadband, and accurate measurement of the modes of a high finesse cavity using an optical frequency comb and a mechanical Fourier transform spectrometer with a kHz-level resolution. We characterize 16000 cavity modes spanning 16 THz of bandwidth in terms of center frequency, linewidth, and amplitude. We retrieve the group delay dispersion of the cavity mirror coatings and pure N${_2}$ with 0.1 fs${^2}$ precision and 1 fs${^2}$ accuracy, as well as the refractivity of the 3{\nu}1+{\nu}3 absorption band of CO${_2}$ with 5 x 10${^{-12}}$ precision. This opens up for broadband refractive index metrology and calibration-free spectroscopy of entire molecular bands

Quantum-noise-limited optical frequency comb spectroscopy

We achieve a quantum-noise-limited absorption sensitivity of 1.7/times10$^{-12}$ cm$^{-1}$ per spectral element at 400 s of acquisition time with cavity-enhanced frequency comb spectroscopy, the highest demonstrated for a comb-based technique. The system comprises a frequency comb locked to a high-finesse cavity and a fast-scanning Fourier transform spectrometer with an ultra-low-noise autobalancing detector. Spectra with a signal-to-noise ratio above 1000 and a resolution of 380 MHz are acquired within a few seconds. The measured absorption lineshapes are in excellent agreement with theoretical predictions.Comment: 18 pages, 4 figures; http://prl.aps.org/pdf/PRL/v107/i23/e23300

Line Positions and Intensities of the {\nu}4_4 Band of Methyl Iodide Using Mid-Infrared Optical Frequency Comb Fourier Transform Spectroscopy

We use optical frequency comb Fourier transform spectroscopy to measure high-resolution spectra of iodomethane, CH$_3$I in the C-H stretch region from 2800 to 3160 cm$^{-1}$. The fast-scanning Fourier transform spectrometer with auto-balanced detection is based on a difference frequency generation comb with repetition rate, f$_{rep}$, of 125 MHz. A series of spectra with sample point spacing equal to f$_{rep}$ are measured at different f$_{rep}$ settings and interleaved to yield sampling point spacing of 11 MHz. Iodomethane is introduced into a 76 m long multipass absorption cell by its vapor pressure at room temperature. The measured spectrum contains three main ro-vibrational features: the parallel vibrational overtone and combination bands centered around 2850 cm$^{-1}$, the symmetric stretch ${\nu}_1$ band centered at 2971 cm$^{-1}$, and the asymmetric stretch ${\nu}_4$ band centered at 3060 cm$^{-1}$. The spectra of the ${\nu}_4$ band and the nearby ${\nu}_3$+${\nu}_4$-${\nu}_3$ hot band are simulated using PGOPHER and a new assignment of these bands is presented. The resolved ro-vibrational structures are used in a least square fit together with the microwave data to provide the upper state parameters. We assign 2603 transitions to the ${\nu}_4$ band with standard deviation (observed - calculated) of 0.00034 cm$^{-1}$, and 831 transitions to the ${\nu}_3$+${\nu}_4$-${\nu}_3$ hot band with standard deviation of 0.00084 cm$^{-1}$. The hyperfine splittings due to the ${^{127}}$I nuclear quadrupole moment are observed for transitions with J$\leq$2xK. Finally, intensities of 157 isolated transitions in the ${\nu}_4$ band are reported for the first time using the Voigt line shape as a model in multispectral fitting

Optical Frequency Comb Fourier Transform Spectroscopy of 14^{14}N2_216^{16}O at 7.8 {\mu}m

We use a Fourier transform spectrometer based on a compact mid-infrared difference frequency generation comb source to perform broadband high-resolution measurements of nitrous oxide, $^{14}$N$_2$$^{16}$O, and retrieve line center frequencies of the $\nu$$_1$ fundamental band and the $\nu$$_1$ + $\nu$$_2$ - $\nu$$_2$ hot band. The spectrum spans 90 cm$^{-1}$ around 1285 cm$^{-1}$ with a sample point spacing of 3 ${\times}$ 10$^{-4}$ cm$^{-1}$ (9 MHz). We report line positions of 67 lines in the $\nu$$_1$ fundamental band between P(37) and R(39), and 78 lines in the $\nu$$_1$ + $\nu$$_2$ - $\nu$$_2$ hot band (split into two components with e/f rotationless parity) between P(34) and R(33), with uncertainties in the range of 90-600 kHz. We derive upper state constants of both bands from a fit of the effective ro-vibrational Hamiltonian to the line center positions. For the fundamental band, we observe excellent agreement in the retrieved line positions and upper state constants with those reported in a recent study by AlSaif et al. using a comb-referenced quantum cascade laser [J Quant Spectrosc Radiat Transf, 2018;211:172-178]. We determine the origin of the hot band with precision one order of magnitude better than previous work based on FTIR measurements by Toth [http://mark4sun.jpl.nasa.gov/n2o.html], which is the source of the HITRAN2016 data for these bands