Gianella, Michele, Press, Sioned A., Manfred, Katherine M. orcid.org/0000-0002-6850-1560 et al. (3 more authors) (2019) Sensitive detection of HO 2 radicals produced in an atmospheric pressure plasma using Faraday rotation cavity ring-down spectroscopy. Journal of Chemical Physics. 124202. ISSN 1089-7690
Abstract
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 αmin=2.2×10-11 cm-1 Hz-1/2 and θmin=0.62 nrad Hz-1/2. The latter corresponds to a minimum detectable (circular) birefringence of Δnmin=5×10-16 Hz-1/2. 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.
Metadata
Item Type: | Article |
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Authors/Creators: |
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Copyright, Publisher and Additional Information: | © 2019 Author(s). Uploaded in accordance with the publisher’s self-archiving policy. Further copying may not be permitted; contact the publisher for details |
Dates: |
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Institution: | The University of York |
Academic Units: | The University of York > Faculty of Sciences (York) > Chemistry (York) |
Depositing User: | Pure (York) |
Date Deposited: | 11 Nov 2019 15:00 |
Last Modified: | 16 Dec 2024 00:11 |
Published Version: | https://doi.org/10.1063/1.5119191 |
Status: | Published |
Refereed: | Yes |
Identification Number: | 10.1063/1.5119191 |
Related URLs: | |
Open Archives Initiative ID (OAI ID): | oai:eprints.whiterose.ac.uk:153352 |