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A methane line list with sub-MHz accuracy in the 1250 to 1380 cm−1 range from optical frequency comb Fourier transform spectroscopy
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Laboratoire Interdisciplinaire Carnot de Bourgogne, UMR 6303 CNRS/Université Bourgogne Franche-Comté, 9 Av. A. Savary, BP 47870, Dijon Cedex, France.
Laboratoire Interdisciplinaire Carnot de Bourgogne, UMR 6303 CNRS/Université Bourgogne Franche-Comté, 9 Av. A. Savary, BP 47870, Dijon Cedex, France.
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2022 (Engelska)Ingår i: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 288, artikel-id 108252Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

We use a Fourier transform spectrometer based on a difference frequency generation optical frequency comb to measure high-resolution, low-pressure, room-temperature spectra of methane in the 1250 – 1380-cm−1 range. From these spectra, we retrieve line positions and intensities of 678 lines of two isotopologues: 157 lines from the 12CH4 ν4 fundamental band, 131 lines from the 13CH4 ν4 fundamental band, as well as 390 lines from two 12CH4 hot bands, ν2 + ν4 ν2 and 2ν4ν4. For another 165 lines from the 12CH4 ν4 fundamental band we retrieve line positions only. The uncertainties of the line positions range from 0.19 to 2.3 MHz, and their median value is reduced by a factor of 18 and 59 compared to the previously available data for the 12CH4 fundamental and hot bands, respectively, obtained from conventional FTIR absorption measurements. The new line positions are included in the global models of the spectrum of both methane isotopologues, and the fit residuals are reduced by a factor of 8 compared to previous absorption data, and 20 compared to emission data. The experimental line intensities have relative uncertainties in the range of 1.5 – 7.7%, similar to those in the previously available data; 235 new 12CH4 line intensities are included in the global model.

Ort, förlag, år, upplaga, sidor
2022. Vol. 288, artikel-id 108252
Nyckelord [en]
(Exo-)Planetary atmospheres, Atmospheric window, Fourier transform spectroscopy, High-resolution spectroscopy, Methane, Optical frequency comb
Nationell ämneskategori
Atom- och molekylfysik och optik
Identifikatorer
URN: urn:nbn:se:umu:diva-196126DOI: 10.1016/j.jqsrt.2022.108252ISI: 000811064700004Scopus ID: 2-s2.0-85130577614OAI: oai:DiVA.org:umu-196126DiVA, id: diva2:1670262
Tillgänglig från: 2022-06-15 Skapad: 2022-06-15 Senast uppdaterad: 2023-11-10Bibliografiskt granskad
Ingår i avhandling
1. Precision molecular spectroscopy in the near- and mid-infrared using frequency comb-based Fourier transform spectrometers
Öppna denna publikation i ny flik eller fönster >>Precision molecular spectroscopy in the near- and mid-infrared using frequency comb-based Fourier transform spectrometers
2023 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
Alternativ titel[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.

Ort, förlag, år, upplaga, sidor
Umeå: Umeå University, 2023. s. 94
Nyckelord
optical frequency comb, molecular spectroscopy, Fourier transform spectrometer, high resolution, mid-infrared, line list, double-resonance spectroscopy, difference frequency generation
Nationell ämneskategori
Atom- och molekylfysik och optik
Forskningsämne
fysik
Identifikatorer
urn:nbn:se:umu:diva-216403 (URN)978-91-8070-131-0 (ISBN)978-91-8070-130-3 (ISBN)
Disputation
2023-12-07, NAT.D.480, Naturvetarhuset, Umeå, 09:00 (Engelska)
Opponent
Handledare
Tillgänglig från: 2023-11-16 Skapad: 2023-11-10 Senast uppdaterad: 2023-11-13Bibliografiskt granskad

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Germann, MatthiasHjältén, AdrianFoltynowicz, Aleksandra

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