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

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

Modeling the cavity dispersion in cavity-enhanced optical frequency comb Fourier transform spectroscopy

Cavity enhanced optical frequency comb spectroscopy is a technique that allows for quick and sensitive measurements of molecular absorption spectra. Locking the comb lines of an optical frequency comb to the cavity modes of an enhancement cavity and then extracting the spectral information with a Fourier transform spectrometer grants easy access to wide segments of absorption spectra. One of the main obstacles complicating the analysis of the measurements is the inevitable dispersion occurring inside the cavity. In this project, absorption measurements of CO2 were performed using an existing and well established setup consisting of a near-infrared optical frequency comb locked to a Fabry- Pérot enhancement cavity using the Pound-Drever-Hall technique, and a Fourier transform spectrometer. The purpose was to improve theoretical models of the measured absorption spectra by creating and verifying a model for the cavity dispersion, stemming mostly from the cavity mirrors but also from the normal dispersion of the intracavity medium. Until now, the cavity dispersion has been treated as an unknown and was included as a fitting parameter together with the CO2 concentration when applying fits to the absorption measurements. The dispersion model was based on previously performed precise measurements of the positions of the cavity modes. The model was found to agree well with measurements. In addition, pre-calculating the dispersion drastically reduced computation time and seemed to improve the overall robustness of the fitting routine. A complicating factor was found to be small discrepancies between the locking frequencies as determined prior to the measurements and the values yielding optimum agreement with the model. These apparent shifts of the locking points were found to have a systematic dependence on the distance between the locking points. The exact cause of this was not determined but the results indicate that with the locking points separated by more than about 10nm the shifts are negligible

Modeling the cavity dispersion in cavity-enhanced optical frequency comb Fourier transform spectroscopy

Cavity enhanced optical frequency comb spectroscopy is a technique that allows for quick and sensitive measurements of molecular absorption spectra. Locking the comb lines of an optical frequency comb to the cavity modes of an enhancement cavity and then extracting the spectral information with a Fourier transform spectrometer grants easy access to wide segments of absorption spectra. One of the main obstacles complicating the analysis of the measurements is the inevitable dispersion occurring inside the cavity. In this project, absorption measurements of CO2 were performed using an existing and well established setup consisting of a near-infrared optical frequency comb locked to a Fabry- Pérot enhancement cavity using the Pound-Drever-Hall technique, and a Fourier transform spectrometer. The purpose was to improve theoretical models of the measured absorption spectra by creating and verifying a model for the cavity dispersion, stemming mostly from the cavity mirrors but also from the normal dispersion of the intracavity medium. Until now, the cavity dispersion has been treated as an unknown and was included as a fitting parameter together with the CO2 concentration when applying fits to the absorption measurements. The dispersion model was based on previously performed precise measurements of the positions of the cavity modes. The model was found to agree well with measurements. In addition, pre-calculating the dispersion drastically reduced computation time and seemed to improve the overall robustness of the fitting routine. A complicating factor was found to be small discrepancies between the locking frequencies as determined prior to the measurements and the values yielding optimum agreement with the model. These apparent shifts of the locking points were found to have a systematic dependence on the distance between the locking points. The exact cause of this was not determined but the results indicate that with the locking points separated by more than about 10nm the shifts are negligible

Molekylär precisionsspektroskopi i det när- och mellaninfraröda med frekvenskamsbaserade Fouriertransformspektrometrar

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

Molekylär precisionsspektroskopi i det när- och mellaninfraröda med frekvenskamsbaserade Fouriertransformspektrometrar

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

Optical frequency comb-based measurements and the revisited assignment of high-resolution spectra of CH2Br2 in the 2960 to 3120 cm−1 region

Brominated organic compounds are toxic ocean-derived trace gases that affect the oxidation capacity of the atmosphere and contribute to its bromine burden. Quantitative spectroscopic detection of these gases is limited by the lack of accurate absorption cross-section data as well as rigorous spectroscopic models. This work presents measurements of high-resolution spectra of dibromomethane, CH2Br2, from 2960 cm−1 to 3120 cm−1 by two optical frequency comb-based methods, Fourier transform spectroscopy and a spatially dispersive method based on a virtually imaged phased array. The integrated absorption cross-sections measured using the two spectrometers are in excellent agreement with each other within 4%. A revisited rovibrational assignment of the measured spectra is introduced, in which the progressions of features are attributed to hot bands rather than different isotopologues as was previously done. Overall, twelve vibrational transitions, four for each of the three isotopologues CH281Br2, CH279Br81Br, and CH279Br2, are assigned. These four vibrational transitions are attributed to the fundamental ν6 band and the nearby nν4 + ν6 − nν4 hot bands (with n = 1–3) due to the population of the low-lying ν4 mode of the Br–C–Br bending vibration at room temperature. The new simulations show very good agreement in intensities with the experiment as predicted by the Boltzmann distribution factor. The spectra of the fundamental and the hot bands show progressions of strong QKa(J) rovibrational sub-clusters. The band heads of these sub-clusters are assigned and fitted to the measured spectra, providing accurate band origins and the rotational constants for the twelve states with an average error of 0.0084 cm−1. A detailed fit of the ν6 band of the CH279Br81Br isotopologue is commenced after assigning 1808 partially resolved rovibrational lines, with the band origin, rotational, and centrifugal constants as fit parameters, resulting in an average error of 0.0011 cm−1

Optical frequency comb Fourier transform spectroscopy of 14N216O at 7.8 µm

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, 14N216O, and retrieve line center frequencies of the ν1 fundamental band and the ν1 + ν2 – ν2 hot band. The spectrum spans 90 cm−1 around 1285 cm−1 with a sample point spacing of 3 × 10−4 cm−1 (9 MHz). We report line positions of 72 lines in the ν1 fundamental band between P(37) and R(38), and 112 lines in the ν1 + ν2 – ν2 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