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Optical frequency comb-based measurements and the revisited assignment of high-resolution spectra of CH2Br2 in the 2960 to 3120 cm−1 region
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Leibniz Institute for Plasma Science and Technology, Greifswald, Germany.ORCID-id: 0000-0001-8082-5229
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.ORCID-id: 0000-0001-6144-4957
School of Chemistry, University of Leeds, United Kingdom.
School of Chemistry, University of Birmingham, United Kingdom.
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2023 (Engelska)Ingår i: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 25, artikel-id 8743Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

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.

Ort, förlag, år, upplaga, sidor
Royal Society of Chemistry, 2023. Vol. 25, artikel-id 8743
Nationell ämneskategori
Atom- och molekylfysik och optik
Identifikatorer
URN: urn:nbn:se:umu:diva-206029DOI: 10.1039/d2cp05881bISI: 000946509500001Scopus ID: 2-s2.0-85150414323OAI: oai:DiVA.org:umu-206029DiVA, id: diva2:1746319
Forskningsfinansiär
Knut och Alice Wallenbergs Stiftelse, 2015.0159Knut och Alice Wallenbergs Stiftelse, 2020.0303Vetenskapsrådet, 2016-03593Vetenskapsrådet, 2020-00238Vetenskapsrådet, 2018-05973Swedish National Infrastructure for Computing (SNIC)Tillgänglig från: 2023-03-28 Skapad: 2023-03-28 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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