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Quantitative description of Faraday modulation spectrometry in terms of the integrated linestrength and 1st Fourier coefficients of the modulated lineshape function
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics.
Institute of Information Optics of Zhejiang Normal University, Jinhua, China.
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2010 (English)In: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 111, no 16, 2415-2433 p.Article in journal (Refereed) Published
Abstract [en]

A quantitative description of the strength and shape of Faraday modulation spectrometry (FAMOS) signals is given. It is first shown how the signal can be expressed in terms of the integrated linestrength for the targeted transition, Si,j. Secondly, since the technique relies on a periodic modulation of the transition frequency induced by an alternating magnetic field, it is explicitly shown that it is possible to express the FAMOS signal concisely in terms of 1st Fourier coefficients of a magnetic-field-modulated dispersive lineshape function for left- and right-handed circularly polarized light. Expressions for the FAMOS signal in terms of the integrated linestrength and such Fourier coefficients are given for three cases: (i) for transitions between two arbitrary types of states, (ii) for transitions between two states that both belong to Hund’s coupling case (a), as is the case for rotational–vibrational transitions of NO, and finally (iii) for the commonly used Q-transitions between such states. It is finally shown that the FAMOS signal from a Q-transition can be expressed succinctly solely in terms of one 1st Fourier coefficient. A general analysis of FAMOS addressing an arbitrary Q-transition as well as the most sensitive Q3/2(3/2) transition in NO is given. The conditions for maximum signal are specifically identified.

Place, publisher, year, edition, pages
Elsevier , 2010. Vol. 111, no 16, 2415-2433 p.
Keyword [en]
FAMOS, Faraday rotation spectrometry, Magnetic rotation spectrometry, Integrated linestrength, Fourier coefficient; Nitric oxide (NO)
National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
URN: urn:nbn:se:umu:diva-36153DOI: 10.1016/j.jqsrt.2010.06.017ISI: 000282252500006OAI: oai:DiVA.org:umu-36153DiVA: diva2:352394
Available from: 2010-09-20 Created: 2010-09-20 Last updated: 2017-12-12Bibliographically approved
In thesis
1. Development of diode laser-based absorption and dispersion spectroscopic techniques for sensitive and selective detection of gaseous species and temperature
Open this publication in new window or tab >>Development of diode laser-based absorption and dispersion spectroscopic techniques for sensitive and selective detection of gaseous species and temperature
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The main aim of this thesis has been to contribute to the ongoing work with development of new diode-laser-based spectroscopic techniques and metho­do­lo­gies for sensitive detection of molecules in gas phase. The techniques under scrutiny are tunable diode laser absorption spectrometry (TDLAS) and Faraday modulation spectrometry (FAMOS). Conventional distributed-feedback (DFB) tele­communication diode lasers working in the near-infrared (NIR) region have been used for detection of carbon monoxide (CO) and temperature in hot humid media whereas a unique frequency-quadrupled external-cavity diode laser producing mW powers of continuous-wave (cw) light in the ultra violet (UV) region have been used for detection of nitric oxide (NO).

A methodology for assessment of CO in hot humid media by DFB-TDLAS has been developed. By addressing a particular transition in its 2nd overtone band, and by use of a dual-fitting methodology with a single reference water spectrum for background correction, % concentrations of CO can be detected in media with tens of percent of H2O (≤40%) at T≤1000 °C with an accuracy of a few %. Moreover, using an ordinary DFB laser working in the C-band, a technique for assessment of the temperature in hot humid gases (T≤1000 °C) to within a fraction of a percent has been developed. The technique addresses two groups of lines in H2O that have a favorable temperature dependence and are easily accessed in a single scan, which makes it sturdy and useful for industrial applications.

A technique for detection of NO on its strong electronic transitions by direct absorption spectrometry (DAS) using cw UV diode laser light has been deve­loped. Since the electronic transitions are ca. two or several orders of magnitude stronger than of those at various rotational-vibra­tional bands, the system is capable of detecting NO down to low ppb∙m concentrations solely using DAS.

Also the FAMOS technique has been further developed. A new theoretical description expressed in terms of both the integrated line strength of the transition and 1st Fourier coefficients of a magnetic-field-modulated dispersive lineshape functions is presented. The description has been applied to both ro-vib Q-transitions and electronic transitions in NO. Simulations under different pressures and magnetic field conditions have been made that provide the optimum conditions for both cases. A first demonstration and characteri­zation of FAMOS of NO addressing its electronic transitions in the UV-region has been made, resulting in a detection limit of 10 ppb∙m. The characterization indicates that the technique can be significantly improved if optimum conditions can be obtained, which demonstrates the high potential of the UV-FAMOS technique.

Place, publisher, year, edition, pages
Umeå: Umeå University, Department of Physics, 2011. 118 p.
Keyword
Absorption Spectrometry (AS), Wavelengh Modulation Absorption Spectrometry (WMAS), Faraday Modulation Spectrometry (FAMOS), Tunable Diode Laser Absorption Spectrometry (TDLAS)
National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
urn:nbn:se:umu:diva-43799 (URN)978-91-7459-190-3 (ISBN)
Public defence
2011-06-08, Naturvetarhuset, N420, Umeå universitet, Umeå, 10:00 (English)
Opponent
Supervisors
Available from: 2011-05-13 Created: 2011-05-10 Last updated: 2011-05-10Bibliographically approved
2. Faraday modulation spectroscopy: Theoretical description and experimental realization for detection of nitric oxide
Open this publication in new window or tab >>Faraday modulation spectroscopy: Theoretical description and experimental realization for detection of nitric oxide
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Faraday modulation spectroscopy (FAMOS) is a laser-based spectroscopic dispersion technique for detection of paramagnetic molecules in gas phase. This thesis presents both a new theoretical description of FAMOS and experimental results from the ultra-violet (UV) as well as the mid-infrared (MIR) regions. The theoretical description, which is given in terms of the integrated linestrength and Fourier coefficients of modulated dispersion and absorption lineshape functions, facilitates the description and the use of the technique considerably. It serves as an extension to the existing FAMOS model that thereby incorporates also the effects of lineshape asymmetries primarily originating from polarization imperfections. It is shown how the Fourier coefficients of modulated Lorentzian lineshape functions, applicable to the case with fully collisionally broadened transitions, can be expressed in terms of analytical functions. For the cases where also Doppler broadening needs to be included, resulting in lineshapes of Voigt type, the lineshape functions can be swiftly evaluated (orders of magnitude faster than previous procedures) by a newly developed method for rapid calculation of modulated Voigt lineshapes (the WWA-method). All this makes real-time curve fitting to FAMOS spectra feasible. Two experimental configurations for sensitive detection of nitric oxide (NO) by the FAMOS technique are considered and their optimum conditions are determined. The two configurations target transitions originating from the overlapping Q22(21=2) and QR12(21=2) transitions in the ultra-violet (UV) region (227nm) and the Q3=2(3=2)-transition in the fundamental rotational-vibrational band in the mid-infrared (MIR) region (5.33 µm). It is shown that the implementations of FAMOS in the UV- and MIR-region can provide detection limits in the low ppb range, which opens up the possibility for applications where high detection sensitivities of NO is required.

Place, publisher, year, edition, pages
Umeå: Umeå universitet, 2013. 152 p.
Keyword
Faraday modulation spectroscopy (FAMOS), Westberg-Wang-Axner (WWA) method, Fourier coefficients, Lineshape asymmetries, Nitric oxide (NO)
National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
urn:nbn:se:umu:diva-68649 (URN)978-91-7459-616-8 (ISBN)
Public defence
2013-05-14, Naturvetarhuset, N420, Umeå universitet, Umeå, 10:00 (English)
Opponent
Supervisors
Available from: 2013-04-23 Created: 2013-04-22 Last updated: 2013-04-23Bibliographically approved

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Westberg, JonasLathdavong, LemthongDion, Claude M.Axner, Ove
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