Cavity ring-down spectroscopy (CRDS) is a well-established, highly sensitive absorption technique whose sensitivity and selectivity for trace radical sensing can be further enhanced by measuring the polarization rotation of the intracavity light by the paramagnetic samples in the presence of a magnetic field. In this paper, we highlight the use of this Faraday rotation cavity ring-down spectroscopy (FR-CRDS) for the detection of HO2 radicals. In particular, we use a cold atmospheric pressure plasma jet as a highly efficient source of HO2 radicals and show that FR-CRDS in the near-infrared spectral region (1506 nm) has the potential to be a useful tool for studying radical chemistry. By simultaneously measuring ring-down times of orthogonal linearly polarized light, measurements of Faraday effect-induced rotation angles (θ) and absorption coefficients (α) are retrieved from the same data set. The Faraday rotation measurement exhibits better long-term stability and enhanced sensitivity due to its differential nature, whereby highly correlated noise between the two channels and slow drifts cancel out. The bandwidth-normalized sensitivities are and . The latter corresponds to a minimum detectable (circular) birefringence of . Using the overlapping qQ3(N = 4–9) transitions of HO2, we estimate limits of detection of 3.1 × 108 cm−3 based on traditional (absorption) CRDS methods and 6.7 × 107 cm−3 using FR-CRDS detection, where each point of the spectrum was acquired during 2 s. In addition, Verdet constants for pertinent carrier (He, Ar) and bulk (N2, O2) gases were recorded in this spectral region for the first time. These show good agreement with recent measurements of air and values extrapolated from reported Verdet constants at shorter wavelengths, demonstrating the potential of FR-CRDS for measurements of very weak Faraday effects and providing a quantitative validation to the computed rotation angles.
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28 September 2019
Research Article|
September 30 2019
Sensitive detection of HO2 radicals produced in an atmospheric pressure plasma using Faraday rotation cavity ring-down spectroscopy
Michele Gianella
;
Michele Gianella
a)
1
Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford
, South Parks Rd., Oxford OX1 3QZ, United Kingdom
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Sioned A. Press;
Sioned A. Press
1
Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford
, South Parks Rd., Oxford OX1 3QZ, United Kingdom
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Katherine M. Manfred
;
Katherine M. Manfred
b)
1
Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford
, South Parks Rd., Oxford OX1 3QZ, United Kingdom
2
Department of Earth Sciences, University of Oxford
, South Parks Rd., Oxford OX1 3AN, United Kingdom
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Helen C. Norman;
Helen C. Norman
1
Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford
, South Parks Rd., Oxford OX1 3QZ, United Kingdom
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Meez Islam
;
Meez Islam
3
School of Science, Engineering and Design, Teesside University
, Borough Road, Middlesbrough TS1 3BA, United Kingdom
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Grant A. D. Ritchie
Grant A. D. Ritchie
c)
1
Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford
, South Parks Rd., Oxford OX1 3QZ, United Kingdom
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a)
Now at: Laboratory for Air Pollution and Environmental Technology, Empa, 8600 Dübendorf, Switzerland.
b)
Now at: Wolfson Atmospheric Chemistry Laboratories, Department of Chemistry, University of York, York YO10 5DD, United Kingdom.
c)
Electronic mail: [email protected]
J. Chem. Phys. 151, 124202 (2019)
Article history
Received:
July 09 2019
Accepted:
September 10 2019
Citation
Michele Gianella, Sioned A. Press, Katherine M. Manfred, Helen C. Norman, Meez Islam, Grant A. D. Ritchie; Sensitive detection of HO2 radicals produced in an atmospheric pressure plasma using Faraday rotation cavity ring-down spectroscopy. J. Chem. Phys. 28 September 2019; 151 (12): 124202. https://doi.org/10.1063/1.5119191
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