The far-infrared beamline at the Canadian Light Source is a state of the art user facility, which offers significantly more far-infrared brightness than conventional globar sources. The infrared radiation is collected from a bending magnet through a 55 X 37 mrad² port to a Bruker IFS 125 HR spectrometer, which is equipped with a nine compartment scanning arm, allowing it to achieve spectral resolution better than 0.001 cm⁻¹. Currently the beamline can achieve signal to noise ratios up to 8 times that which can be achieved using a traditional thermal source. This talk will provide an overview of the the beamline, and the capabilities available to users, recent and planned improvements including the addition of a Glow Discharge cell and advances in Coherent Synchrotron Radiation. Furthermore, the process of acquiring access to the facility will be covered.

Figure We report on an all polarization-maintaining, modelocked, fiber laser system which generates coherent broadband pulses centered at 2.03 μm with a spectral FWHM bandwidth of 60 nm and 360 mW. Using this frequency comb source, we generate phase-coherent, ultra-broadband pulses centered at 6.5 μm and spanning 5.5 μm to 8 μm with DFG in CdSiP₂.

Figure A dual-comb spectrometer (DCS) has great advantages over the conventional FTIR in respect with the resolution and the measurement time. We reduce the relative linewidth of two optical frequency combs in our DCS to less than 1 Hz and extended the observable spectral bandwidth to be compatible with the FTIR. The figure shows the recorded spectrum of entire vibrational bands of ¹²C₂H₂ at 1.03 μm and 1.53 μm, CH₄ at 1.67 μm and H₂O at 1.46 μm. It takes 140 ms to record a time domain interferogram, from which the spectrum across over 1.0-1.7 μm is obtained by Fourier transformation. The interferogram is averaged more than 400,000 times successively to improve the signal to noise ratio. The horizontal axis is scaled by the absolute frequency and the transition frequencies are determined by fitting the absorption lines with the Voigt functions. The discrepancy from the previous sub-Doppler resolution measurements is typically a few MHz.

We report multiheterodyne-detected cavity-enhanced absorption spectroscopy (MH-CEAS) of CO₂ in the near-infrared using a novel frequency comb light source generated from the electro-optical modulation of a single continuous-wave laser. Using a pair of dual-drive Mach Zehnder modulators we create two frequency combs of slightly different comb mode spacing. The probe comb is coupled to a high-finesse optical cavity containing CO₂ and then combined with the local oscillator comb on a photodetector. This results in down-conversion of the optical comb to radio frequencies, therefore facilitating fast, multiplexed analysis of the CO₂ direct absorption signal. The noise-equivalent absorption coefficient for MH-CEAS as initially demonstrated here is 3×10⁻¹⁰ cm⁻¹ Hz^−1/2 per spectral element for  ∼ 50 simultaneously resolved channels of unique optical frequency.

Optical frequency combs are powerful tools for precise frequency measurements in various wavelength regions. The combs have been applied not only to metrology, but also to molecular spectroscopy. Recently, we studied high resolution spectroscopy of iodine molecule assisted by an optical frequency comb. ¹ In the study, the comb was used for frequency calibration of a scanning dye laser. In this study, we developed a frequency calibration scheme with a comb and an acousto-optic modulator to realize more precise frequency measurement in a wide frequency range. And the frequency calibration scheme was applied to Doppler-free two-photon absorption (DFTPA) spectroscopy of naphthalene. Naphthalene is one of the prototypical aromatic molecules, and its detailed structure and dynamics in excited states have been reported. Figure We measured DFTPA spectra of A¹B_1u(v₄=1) ← X¹A_g(v=0) transition around 298 nm. A part of obtained spectra is shown in the figure. The spectral lines are rotationally resolved and the resolution is about 100 kHz. The horizontal axis was calibrated by the developed frequency calibration system employing the comb. The uncertainties of the calibrated frequencies were determined by the fluctuations of the comb modes which were stabilized to a GPS-disciplined clock.
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¹ A. Nishiyama, D. Ishikawa, and M. Misono, J. Opt. Soc. Am. B 30, 2107 (2013).

Spectrometers based on Quantum Cascade (QC) lasers can be run in either continuous or pulsed operation. Although the instrumentation based upon the most recent versions of continuously operating QC lasers can have higher resolution than chirped lasers, using chirped pulse QC lasers can give an advantage when rapid changes in gas composition occur. For example, when jet engines are being tested, a variety of temperature dependent effects on the trace gas concentrations of the plume may be observed. Most pulsed QC lasers are operated in the down chirped mode, in which the chirp rate slows during the pulse. In our spectrometer the changes in frequency are recorded using two Ge etalons, one with a free spectral range of 0.0495 cm⁻¹, and the other with a fringe spacing of 0.0195 cm⁻¹.They can also be deployed in multiplex schemes in which two or more down-chirped lasers are used. In this paper we wish to show examples of the use of multiplexed chirped pulse lasers to allow overlapping spectra to be recorded. The examples of multiplex methods used are taken partly from measurements of ¹⁴NH₃ and ¹⁵NH₃ in the region from 1630 to 1622 cm⁻¹, and partly from the use of other chirped pulse lasers operating in the 8 μm region. Among the effects seen are rapid passage effects caused by the rapid down-chirp, and the use of gases such as nitrogen to cause variation in the shape of the collisional broadened absorption lines.

Simultaneous spectroscopic detection of large molecules with broad ro-vibrational spectra, and small molecules with well-resolved narrow spectral lines requires both broadband optical frequency coverage ( > 50 cm⁻¹) and high resolution ( < 0.01 cm⁻¹) to perform accurate spectral measurements. With the advent of room temperature, high power, continuous wave quantum cascade lasers (QCLs), high resolution mid-IR spectrometers for field applications became feasible. So far to address the broadband spectral coverage, external cavity (EC) QCLs with > 100 cm⁻¹tuning ranges have been spectroscopic sources of choice in the mid-IR; however EC-QCLs are rather complex opto-mechanical systems, which are vibration-sensitive, and construction of robust transportable systems is difficult. In this work we present a new method of performing broadband mid-IR spectroscopy using two free-running Fabry-Perot (FP) QCLs to perform multi-heterodyne down-conversion of optical signals to RF domain. The sample transmission spectrum probed by one multi-mode FP-QCL is down-converted to the RF domain through an optical multi-heterodyne process using a second FP-QCL as the local oscillator¹. Both a broadband multi-mode spectral measurement as well as high-resolution ( 15 MHz or  0.0005 cm⁻¹) absorption spectroscopy of NH₃ and N₂O are demonstrated and show potential for all-solid-state FP-laser-based spectrometers for chemical sensing.
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¹Y. Wang, M. G. Soskind, W. Wang, and G. Wysocki, "High-resolution multi-heterodyne spectroscopy based on Fabry-Perot quantum cascade lasers," Appl Phys Lett 104, 0311141-0311145 (2014)

External cavity quantum cascade lasers (EC-QCLs) provide significantly enhanced wavelength tunability while maintaining the high output powers of traditional QCLs throughout the mid-infrared portion of the spectrum. However, the inclusion of wavelength-selective moving parts allows vibrational and acoustic noise to be coupled into the laser as frequency and power noise. This can be particularly troublesome for cavity-enhanced spectroscopy, as attempts to increase vibrational isolation may interfere with stable cavity alignment. Here, we discuss our efforts to improve the frequency stability of our EC-QCL while maintaining tunability and consistent cavity alignment. A mid-infrared hollow silica waveguide is used to vibrationally isolate the laser from the cavity while maintaining alignment. To further increase frequency stability, the laser is side-of-fringe locked to an uncoated solid germanium etalon. Tunability is maintained by incrementing the angle of incidence upon the etalon using a piezo-driven mirror. Angle of incidence tuning and automated periodic re-locking are managed using a BeagleBone Black development board. This allows us to maintain essentially continuous frequency tuning between 1140-1220 cm⁻¹while stabilizing the laser's frequency to within 30 MHz. Other mid-infrared frequencies could easily be accessed by swapping QCL chips.

1,3,5-trioxane, often used as a solid fuel or source of formaldehyde, is a symmetric top of the C_3v group. Although the microwave¹ and low-resolution vibrational spectra² have been studied extensively, only the ν₁₇ band near 1072 cm⁻¹has been observed with rotational resolution³. Here, we will present our studies of trioxane vapor from 1140-1220 cm⁻¹, covering the ν₁₆ band at a resolution of approximately 30 MHz. Solid trioxane was heated, and the resulting vapor was entrained in a continuous supersonic expansion of argon. Continuous-wave cavity ringdown spectroscopy was then performed using a frequency-stabilized external cavity quantum cascade laser (EC-QCL) as the light source. In addition to providing new ro-vibrational transition frequencies of trioxane, the present work serves to validate our newly-developed EC-QCL spectrometer and will be used to evaluate the cooling performance of the sheath-flow supercritical fluid expansion source currently under development⁴.
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¹Oka, T., Tsuchiya, K., Iwata, S., and Morino, Y. Microwave Spectrum of s-Trioxane. Bull. Chem. Soc. Jpn. 37 (1964), 4-7. ²Stair, A.T. Jr. and Nielsen, J. Rud. Vibrational Spectra of sym-Trioxane. J. Chem. Phys. 27 (1957), 402-407. ³Henninot, J-F., Bolvin, H., Demaison, J., and Lemoine, B. The Infrared Spectrum of Trioxane in a Supersonic Slit Jet. J. Mol. Spect. 152 (1992), 62-68. ⁴Gibson, B.M., Stewart, J.T., and McCall, B.J., contribution TJ14, presented at the 68th International Symposium on Molecular Spectroscopy, Columbus, OH, USA, 2013.

We demonstrate a frequency stabilized cavity ringdown spectrometer capable of measuring simultaneous isotopes of methane (¹²CH₄, ¹³CH₄, ¹²CH₃D) of enriched samples to high precision (δD\textless0.03% and δ13C\textless0.01%). The spectrometer employs coupling of two orthogonally polarized CW lasers into a ringdown cavity for simultaneous spectral measurements over the full wavelength range of 1.45-1.65μm. In addition, we discuss the necessity of modeling methane lineshapes with the Galatry profile to achieve the highest precision.

High purity and precisely mixed liquid solutions are important to a variety of industrial processes, but sensors for such solutions often have significant drawbacks such as the need for regular calibration and the inability to continuously make real-time measurements. For some specialty liquids, such as cryogenic liquids or caustic solutions used in the semiconductor industry, direct sensors for composition and contamination don’t exist at all, and indirect methods must be used instead. Fiber-loop cavity ring-down spectroscopy (FL-CRDS) can provide an ideal solution for many challenging applications. Since fibers are resistant to chemicals and extreme temperatures, a sensor based on FL-CRDS can be used in environments where other techniques and sensors can’t work. In a FL-CRDS instrument, a laser is coupled into a loop of fiber, and a small amount of light is extracted from the loop to a detector with each pass. Spliced into the loop is a sensing element that allows the evanescent field of the light otherwise confined within the fiber core to interact with the surrounding environment. Results will be presented for detection of contaminants in liquids with several types of sensing elements: fiber tapers, side-polished fibers, and core-only fibers; each with a variety of geometries. Sampling systems for both continuous flow of small samples and for monitoring of static sample baths will also be presented.

A Doppler broadening thermometry is implemented using a laser-locked cavity ring-down spectrometer^{1 2} combined with a temperature-stabilized sample cell. The temperature fluctuation of the gas sample cell is kept below 1 mK for hours. The probing laser is frequency locked at a longitudinal mode of a Fabry-Pérot interferometer made of ultra-low-expansion glass, and the spectral scan is implemented by scanning the sideband produced by an electro-optic modulator. As a result, a kHz precision has been maintained during the measurement of the spectrum of 10 GHz wide. A ro-vibrational line of C₂H₂ is measured at sample pressures of a few Pa. Using a pair of mirrors with a reflectivity of 0.99997 at 787 nm, we are able to detect absorption line profiles with a signal-to-noise ratio of 10⁵. Fitting of the recorded spectra allows us to determine the Doppler width with a statistical uncertainty of 10 ppm. Further improvements on the experimental reproducibility and investigations on the collision effects will probably lead to an optical determination of the Boltzmann constant with an uncertainty of a few ppm.
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¹ H. Pan, C.-F. Cheng, Y. R. Sun, B. Gao, A.-W. Liu, S.-M. Hu, "Laser-locked, continuously tunable high resolution cavity ring-down spectrometer," Rev. Sci. Instrum. 82, 103110 (2011) ² Y. R. Sun, H. Pan, C.-F. Cheng, A.-W. Liu, J.-T. Zhang, S.-M. Hu, "Application of cavity ring-down spectroscopy to the Boltzmann constant determination," Opt. Express, 19, 19993 (2011)

We have introduced a wide-beam-coupled enhanced-cavity absorption cell (ECAC) into a 3-μm difference-frequency generation spectrometer in order to reduce transit-time broadening of Lamb dips. It contains concave and convex mirrors with a curvature radius of ±7 m separated by 37.5 cm, has a finesse of 770, and is coupled with a Gaussian beam having a 1/e² radius of 1.9 mm at beam waist. The spectrometer is applied to record sub-Doppler resolution spectra of the ν₃ band of CH₄ and the ν₁ and ν₄ bands of CH₃D, and the transit-time broadening is estimated 30 kHz for these molecules. The observed Lamb dips are about 80 kHz (HWHM) wide, which is one third of those recorded using another ECAC coupled with a 1/e² radius of 0.7 mm at the beam waist. Some A₁-A₂ splittings of the low J levels for CH₃D are first resolved, and the absolute transition frequencies are determined with a relative uncertainty of 10⁻⁹.

Quantitative measurements of the absorption cross-sections and reaction rates constants of free radicals by spectroscopic means requires the knowledge of the absolute concentration of the target species. We have demonstrated earlier¹ that such information can be retrieved from absorption measurements of the well-known "reporter" molecule, co-produced in radical synthesis. This method is limited to photochemical protocols allowing for production of "reporters" stochiometrically with the target species. This limitation can be overcome by use of the optical calorimetry (OC) which measures heat signatures of a photochemical protocol. These heat signatures are directly related to the amount of species produced and the thermochemical data of the reactants and stable products whose accuracy is usually substantially higher than that of the absorption data for prospective "reporters". The implementation of the OC method presented in this talk is based on the measurements of the frequency shift of the resonances due to the change in the optical density of the reactiove sample within a Fabry-Perot cavity caused by deposition of heat from the absorbed photolysis beam and subsequent chemical reactions. Preliminary results will be presented and future development of this experimental technique will be discussed.
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¹D. Melnik, R. Chhantyal-Pun and T. A. Miller, J. Phys. Chem. A, 114, 11583, (2010)

Hyperfine-resolved saturation spectra were measured for a selection of low and medium J rotational lines in the A²Π–X²Σ⁺ system of CN using two copropagating laser beams tuned to transitions in the (2–0) and (1–0) bands. A bleach laser was amplitude modulated and fixed in frequency near the center of a rotational line of the (2–0) vibrational band, while a probe laser was frequency-modulated and scanned across selected lines of the (1–0) vibrational band, sharing a common lower state with the bleach laser. Locking the probe laser with a tunable radio frequency offset to a cavity that tracks the slowly drifting bleach laser greatly improved the quality of the double-resonance saturation signals, by stabilizing the relative frequency of the two beams. The sub-Doppler resonances were fit with Lorentzian line shapes having a typical full-width at half maximum of 2–3 MHz. The hyperfine spectra observed depend on the hyperfine structure within both rovibronic transitions excited, permitting the determination of hyperfine molecular constants in the ν = 2 state and the refinement of previously published values in the ν = 1 state. Four nuclear magnetic dipole and two electric quadrupole hyperfine constants were determined for each of the upper states from a fit with a weighted root mean squared error of 0.5 MHz. The vibrational dependence of these constants is weak or negligible. Acknowledgements: Work at Brookhaven National Laboratory was carried out under contract No. DE-AC02-98CH10886 with the U.S. Department of Energy and supported by its Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences.