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Line positions and intensities of the ν1 band of 12CH3I using mid-infrared optical frequency comb Fourier transform spectroscopy
Umeå University, Faculty of Science and Technology, Department of Physics.ORCID iD: 0000-0001-6144-4957
Umeå University, Faculty of Science and Technology, Department of Physics.ORCID iD: 0000-0002-6191-7926
Umeå University, Faculty of Science and Technology, Department of Physics. Leibniz Institute for Plasma Science and Technology (INP), Greifswald, Germany.ORCID iD: 0000-0001-8082-5229
2023 (English)In: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 306, article id 108646Article in journal (Refereed) Published
Abstract [en]

We present a new spectral analysis of the ν1 and ν3+ν1−ν3 bands of 12CH3I 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 ν4 and ν3+ν4−ν3 bands around 3060 cm–1 using PGOPHER, and the line list was incorporated in the HITRAN database. Here, we treat the two fundamental bands, ν1 and ν4, together with the perturbing states, 2ν2+ν3 and ν2+2ν6±2, as a four-level system connected via Coriolis and Fermi interactions. A similar four-level system is assumed to connect the two ν3+ν1−ν3 and ν3+ν4−ν3 hot bands, which appear due to the population of the low-lying ν3 state at room temperature, with the 2ν2+2ν3 and ν2+ν3+2ν6±2 perturbing states. This spectroscopic 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. It also allows revisiting the analysis of the ν4 and ν3+ν4−ν3 bands, which were previously treated as separate bands, not connected to their ν1 and ν3+ν1−ν3 counterparts. 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 ν1 band using conventional Fourier transform infrared spectroscopy. The ν1 band shows hyperfine splitting, resolvable for transitions with J ≤ 2 × K. Finally, the spectral intensities of 65 lines of the ν1 band and 7 lines of the ν3+ν1−ν3 band are reported for the first time using the Voigt line shape as a model in multispectral fitting. The reported line lists and intensities will serve as a reference for high-resolution molecular spectroscopic databases, and as a basis for line selection in future monitoring applications of CH3I.

Place, publisher, year, edition, pages
Elsevier, 2023. Vol. 306, article id 108646
National Category
Atom and Molecular Physics and Optics Condensed Matter Physics
Identifiers
URN: urn:nbn:se:umu:diva-209110DOI: 10.1016/j.jqsrt.2023.108646ISI: 001007953500001Scopus ID: 2-s2.0-85159161690OAI: oai:DiVA.org:umu-209110DiVA, id: diva2:1763941
Funder
Knut and Alice Wallenberg Foundation, 2015.0159Knut and Alice Wallenberg Foundation, 2020.0303Swedish Research Council, 2016-03593Swedish Research Council, 2020-00238Available from: 2023-06-08 Created: 2023-06-08 Last updated: 2023-11-13Bibliographically approved
In thesis
1. Precision molecular spectroscopy in the near- and mid-infrared using frequency comb-based Fourier transform spectrometers
Open this publication in new window or tab >>Precision molecular spectroscopy in the near- and mid-infrared using frequency comb-based Fourier transform spectrometers
2023 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Molekylär precisionsspektroskopi i det när- och mellaninfraröda med frekvenskamsbaserade Fouriertransformspektrometrar
Abstract [en]

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.

Place, publisher, year, edition, pages
Umeå: Umeå University, 2023. p. 94
Keywords
optical frequency comb, molecular spectroscopy, Fourier transform spectrometer, high resolution, mid-infrared, line list, double-resonance spectroscopy, difference frequency generation
National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
urn:nbn:se:umu:diva-216403 (URN)978-91-8070-131-0 (ISBN)978-91-8070-130-3 (ISBN)
Public defence
2023-12-07, NAT.D.480, Naturvetarhuset, Umeå, 09:00 (English)
Opponent
Supervisors
Available from: 2023-11-16 Created: 2023-11-10 Last updated: 2023-11-13Bibliographically approved

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Hjältén, AdrianFoltynowicz, AleksandraSadiek, Ibrahim

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