Precision molecular spectroscopy in the near- and mid-infrared using frequency comb-based Fourier transform spectrometers

Abstract: Absorption spectroscopy is a powerful scientific tool for non-invasive and remote sensing applications ranging from atmospheric monitoring to astrophysics. In spectroscopic detection schemes it is necessary to have spectral models for any molecular species to be detected or quantified. Such models are often based on spectroscopic measurements or at the very least require experimental validation. The experimental data need to be accurate in terms of absorption line positions and intensities, but should also cover as many absorption lines as possible, i.e. broadband measurements are highly desirable.Fourier transform spectroscopy (FTS) based on optical frequency combs (OFCs) can supply laboratory data that meet these requirements. OFCs provide a broad optical bandwidth and high spectral brightness, and also revolutionized our ability to measure optical frequencies, which had a profound impact on the frequency accuracy of spectroscopic measurements. The combination of OFCs and FTS, using the recently developed sub-nominal resolution technique, allows for measuring broadband absorption spectra with very high resolution, and a frequency accuracy provided by the OFCs. The aim of the work in this thesis was to expand the application of sub-nominal OFC-FTS to provide the much needed high-accuracy data for validation and development of spectroscopic databases of molecules relevant for a wide range of sensing application.We developed a spectrometer to target the strong molecular absorption bands in the mid-infrared using two OFC sources based on difference frequency generation (DFG) emitting in the 3 μm and 8 μm wavelength ranges. We measured the spectra of iodomethane, CH3I, and dibromomethane, CH2Br2, around 3 μm, fitted Hamiltonian models to several bands using the PGOPHER software, and reported molecular constants. For CH3I we improved on previous models, while for CH2Br2 we presented a new interpretation of the spectrum. We also reported the first assessments of line intensities of CH3I performed using multispectral fitting. At 8 μm, we implemented OFC-FTS based on a fiber-based compact DFG OFC source and measured low pressure spectra of nitrous oxide, N2O, methane, CH4, and formaldehyde, H2CO. After the frequency accuracy was confirmed by excellent agreement with an earlier accurate study of N2O, we compiled extensive line lists for CH4 and H2CO containing hundreds of transition frequencies with a precision improved by one order of magnitude compared to previously available data, and also reported line intensities for most transitions. For CH4 the new data were used to improve a global Hamiltonian model, while the H2CO data were incorporated into an algorithm based on spectroscopic networks to yield better precision in predicted energy levels and transition frequencies.We also further developed a recent implementation of double resonance (DR) spectroscopy where optical pumping by a continuous-wave laser was used to populate selected vibrational energy levels of CH4 not populated at room temperature, and a near-infrared OFC probed sub-Doppler transitions from the pumped states. Such measurements are necessary to validate theoretical predictions of transitions between excited vibrational levels that are relevant for high-temperature environments such as the atmospheres of hot celestial objects. We reported an improved measurement setup using a new pump laser, new enhancement cavity with an updated OFC-cavity locking scheme, and measured transitions between more highly excited rotational levels than was previously reported. The higher rotational excitations lead to a larger number of DR transitions, which could be readily detected in the broadband high-resolution OFC-FTS spectra. We retrieved parameters of 88 lines of which we could assign 79 to theoretically predicted transitions. We found systematic frequency discrepancies with the predictions, that had not been observed earlier for lower rotational levels.These implementations of sub-nominal OFC-FTS thus provided highly accurate line lists and improved spectral models of absorption bands of several molecules in the universally important mid-infrared region, as well as the first detection of 88 transitions between excited vibrational states of CH4 relevant for high-temperature environments. We demonstrated the high potential of these techniques for collecting large amounts of accurate spectroscopic data, that further the scope of applicability of molecular spectroscopy.