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Lineshape asymmetries in Faraday modulation spectroscopy
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics.
2014 (English)In: Applied physics. B, Lasers and optics (Print), ISSN 0946-2171, E-ISSN 1432-0649, Vol. 116, no 2, 467-476 p.Article in journal (Refereed) Published
Abstract [en]

Faraday modulation/rotation spectroscopy (FAMOS/FRS) is a spectroscopic technique for detection of paramagnetic species in gas phase. Although the prevailing theoretical description predicts fully symmetrical lineshapes, experiments do not in general provide such. This work shows that asymmetries in FAMOS can have at least two origins; (i) a frequency dependent laser intensity and (ii) polarization imperfections, which both are scrutinized here. A general analytical description for the latter, derived under the assumption that both the polarization imperfections and the relative absorption are small, is presented, conveniently expressed in terms of 1st Fourier coefficients of modulated dispersion and absorption lineshape functions. The resulting expression, which is thus an extension to the conventional FAMOS expression, can thereby be swiftly evaluated and allows for on-line fitting to measured asymmetric FAMOS signals. Curve fits to experimentally obtained data from nitric oxide measured both in the ultra-violet and the mid-infrared region demonstrate the applicability of the methodology.

Place, publisher, year, edition, pages
Berlin/Heidelberg: Springer, 2014. Vol. 116, no 2, 467-476 p.
National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
URN: urn:nbn:se:umu:diva-68500DOI: 10.1007/s00340-013-5721-xISI: 000339727000025OAI: oai:DiVA.org:umu-68500DiVA: diva2:617161
Available from: 2013-04-22 Created: 2013-04-22 Last updated: 2017-12-06Bibliographically approved
In thesis
1. 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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