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Precision molecular spectroscopy in the near- and mid-infrared using frequency comb-based Fourier transform spectrometers
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.ORCID-id: 0000-0001-6144-4957
2023 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)Alternativ titel
Molekylär precisionsspektroskopi i det när- och mellaninfraröda med frekvenskamsbaserade Fouriertransformspektrometrar (Svenska)
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 [en]
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: urn:nbn:se:umu:diva-216403ISBN: 978-91-8070-131-0 (digital)ISBN: 978-91-8070-130-3 (tryckt)OAI: oai:DiVA.org:umu-216403DiVA, id: diva2:1811176
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
Delarbeten
1. Line positions and intensities of the ν4 band of methyl iodide using mid-infrared optical frequency comb Fourier transform spectroscopy
Öppna denna publikation i ny flik eller fönster >>Line positions and intensities of the ν4 band of methyl iodide using mid-infrared optical frequency comb Fourier transform spectroscopy
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2020 (Engelska)Ingår i: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 255, artikel-id 107263Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

We use optical frequency comb Fourier transform spectroscopy to measure high-resolution spectra of iodomethane, CH3I, 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 a standard deviation (observed - calculated) of 0.00034 cm(-1), and 831 transitions to the nu(3)+nu(4)-nu(3) hot band with a standard deviation of 0.00084 cm(-1). For comparison, in the earlier work using standard FT-IR with 162 MHz resolution [Anttila, et al., J. Mol. Spectrosc. 1986; 119:190-200] 1830 transition were assigned to the nu(4) band, and 380 transitions to the nu(3)+nu(4)-nu(3) hot band, with standard deviations of 0.00083 cm(-1) and 0.0013 cm(-1), respectively. The hyperfine splittings due to the 127 I nuclear quadrupole moment are observed for transitions with J <= 2 x K. 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.

Ort, förlag, år, upplaga, sidor
Elsevier, 2020
Nyckelord
Methyl iodide, High-resolution spectroscopy, Optical frequency comb, Fourier transform spectroscopy
Nationell ämneskategori
Atom- och molekylfysik och optik
Identifikatorer
urn:nbn:se:umu:diva-176794 (URN)10.1016/j.jqsrt.2020.107263 (DOI)000581971300031 ()2-s2.0-85090048420 (Scopus ID)
Tillgänglig från: 2020-11-25 Skapad: 2020-11-25 Senast uppdaterad: 2023-11-10Bibliografiskt granskad
2. Optical frequency comb Fourier transform spectroscopy of 14N216O at 7.8 µm
Öppna denna publikation i ny flik eller fönster >>Optical frequency comb Fourier transform spectroscopy of 14N216O at 7.8 µm
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2021 (Engelska)Ingår i: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 271, artikel-id 107734Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

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.

Ort, förlag, år, upplaga, sidor
Elsevier, 2021
Nyckelord
Fourier transform spectroscopy, High-resolution spectroscopy, Nitrous oxide, Optical frequency comb
Nationell ämneskategori
Atom- och molekylfysik och optik
Identifikatorer
urn:nbn:se:umu:diva-184121 (URN)10.1016/j.jqsrt.2021.107734 (DOI)000677683500004 ()2-s2.0-85106642195 (Scopus ID)
Forskningsfinansiär
Knut och Alice Wallenbergs Stiftelse, 2015.0159Vetenskapsrådet, 2016-03593
Tillgänglig från: 2021-06-10 Skapad: 2021-06-10 Senast uppdaterad: 2023-11-13Bibliografiskt granskad
3. A methane line list with sub-MHz accuracy in the 1250 to 1380 cm−1 range from optical frequency comb Fourier transform spectroscopy
Öppna denna publikation i ny flik eller fönster >>A methane line list with sub-MHz accuracy in the 1250 to 1380 cm−1 range from optical frequency comb Fourier transform spectroscopy
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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.

Nyckelord
(Exo-)Planetary atmospheres, Atmospheric window, Fourier transform spectroscopy, High-resolution spectroscopy, Methane, Optical frequency comb
Nationell ämneskategori
Atom- och molekylfysik och optik
Identifikatorer
urn:nbn:se:umu:diva-196126 (URN)10.1016/j.jqsrt.2022.108252 (DOI)000811064700004 ()2-s2.0-85130577614 (Scopus ID)
Tillgänglig från: 2022-06-15 Skapad: 2022-06-15 Senast uppdaterad: 2023-11-10Bibliografiskt granskad
4. Optical frequency comb-based measurements and the revisited assignment of high-resolution spectra of CH2Br2 in the 2960 to 3120 cm−1 region
Öppna denna publikation i ny flik eller fönster >>Optical frequency comb-based measurements and the revisited assignment of high-resolution spectra of CH2Br2 in the 2960 to 3120 cm−1 region
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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
Nationell ämneskategori
Atom- och molekylfysik och optik
Identifikatorer
urn:nbn:se:umu:diva-206029 (URN)10.1039/d2cp05881b (DOI)000946509500001 ()2-s2.0-85150414323 (Scopus ID)
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
5. Line positions and intensities of the ν1 band of 12CH3I using mid-infrared optical frequency comb Fourier transform spectroscopy
Öppna denna publikation i ny flik eller fönster >>Line positions and intensities of the ν1 band of 12CH3I using mid-infrared optical frequency comb Fourier transform spectroscopy
2023 (Engelska)Ingår i: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 306, artikel-id 108646Artikel i tidskrift (Refereegranskat) 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.

Ort, förlag, år, upplaga, sidor
Elsevier, 2023
Nationell ämneskategori
Atom- och molekylfysik och optik Den kondenserade materiens fysik
Identifikatorer
urn:nbn:se:umu:diva-209110 (URN)10.1016/j.jqsrt.2023.108646 (DOI)001007953500001 ()2-s2.0-85159161690 (Scopus ID)
Forskningsfinansiär
Knut och Alice Wallenbergs Stiftelse, 2015.0159Knut och Alice Wallenbergs Stiftelse, 2020.0303Vetenskapsrådet, 2016-03593Vetenskapsrådet, 2020-00238
Tillgänglig från: 2023-06-08 Skapad: 2023-06-08 Senast uppdaterad: 2023-11-13Bibliografiskt granskad
6. Optical frequency comb Fourier transform spectroscopy of formaldehyde in the 1250 to 1390 cm−1 range: experimental line list and improved MARVEL analysis
Öppna denna publikation i ny flik eller fönster >>Optical frequency comb Fourier transform spectroscopy of formaldehyde in the 1250 to 1390 cm−1 range: experimental line list and improved MARVEL analysis
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2024 (Engelska)Ingår i: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 312, artikel-id 108782Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

We use optical frequency comb Fourier transform spectroscopy to record high-resolution, low-pressure, room-temperature spectra of formaldehyde (H212C16O) in the range of 1250 to 1390 cm−1. Through line-by-line fitting, we retrieve line positions and intensities of 747 rovibrational transitions: 558 from the ν6 band, 129 from the ν4 band, and 14 from the ν3 band, as well as 46 from four different hot bands. We incorporate the accurate and precise line positions (0.4 MHz median uncertainty) into the MARVEL (measured active vibration-rotation energy levels) analysis of the H2CO spectrum. This increases the number of MARVEL-predicted energy levels by 82 and of rovibrational transitions by 5382, and substantially reduces uncertainties of MARVEL-derived H2CO energy levels over a large range: from pure rotational levels below 200 cm−1 up to multiply excited vibrational levels at 6000 cm−1. This work is an important step toward filling the gaps in formaldehyde data in the HITRAN database.

Ort, förlag, år, upplaga, sidor
Elsevier, 2024
Nyckelord
Empirical line list, Formaldehyde, Fourier transform spectroscopy, Frequency comb spectroscopy, High-resolution spectroscopy, MARVEL
Nationell ämneskategori
Atom- och molekylfysik och optik
Identifikatorer
urn:nbn:se:umu:diva-215854 (URN)10.1016/j.jqsrt.2023.108782 (DOI)2-s2.0-85174165539 (Scopus ID)
Forskningsfinansiär
Knut och Alice Wallenbergs Stiftelse, KAW 2015.0159Knut och Alice Wallenbergs Stiftelse, KAW 2020.0303Vetenskapsrådet, 2016-03593Vetenskapsrådet, 2020-00238EU, Horisont 2020, 883830
Tillgänglig från: 2023-11-02 Skapad: 2023-11-02 Senast uppdaterad: 2023-11-10Bibliografiskt granskad
7. Accurate measurement and assignment of high rotational energy levels in the 9150 - 9370 cm−1 range of methane using optical frequency comb double-resonance spectroscopy
Öppna denna publikation i ny flik eller fönster >>Accurate measurement and assignment of high rotational energy levels in the 9150 - 9370 cm−1 range of methane using optical frequency comb double-resonance spectroscopy
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(Engelska)Manuskript (preprint) (Övrigt vetenskapligt)
Nationell ämneskategori
Atom- och molekylfysik och optik
Identifikatorer
urn:nbn:se:umu:diva-216402 (URN)
Tillgänglig från: 2023-11-10 Skapad: 2023-11-10 Senast uppdaterad: 2023-11-10

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