Laser-induced plasmas in air for pulsed broadband cavity-enhanced absorption spectroscopy

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dc.contributor.advisor Ruth, Albert A. en Keary, Bryan P. 2020-04-27T12:15:26Z 2020-04-27T12:15:26Z 2020-01-07 2020-01-07
dc.identifier.citation Keary, B. P. 2020. Laser-induced plasmas in air for pulsed broadband cavity-enhanced absorption spectroscopy. PhD Thesis, University College Cork. en
dc.identifier.endpage 141 en
dc.description.abstract A pulsed laser-induced plasma (LIP) was generated in ambient air inside a high-finesse near-concentric optical cavity. The broadband optical plasma emission was successfully sustained within the cavity and the light leaking from the cavity was used to measure broadband absorption spectra of gaseous azulene through (i) time-dependent cavity ring-down (CRDS), and (ii) intensity-dependent incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) methodologies. Based on the broadband ring-down time of light within the cavity, cavity mirror reflectivities may be calibrated in situ, thus facilitating measurements of absolute absorption coefficients with the intensity-dependent (IBBCEAS) approach. Each of the two spectroscopic approaches are compared in terms of their overall performance and suitability to the experimental requirements. The IBBCEAS approach was found to possess numerous advantages over the CRDS approach in terms of measurement speed, and achievable signal-to-noise ratios and minimum detectable absorption coefficients. However, the IBBCEAS approach is limited by the presence of an artificial offset in the measured absorption that arises due to the absorption of light occurring between LIP formation and the commencement of data collection. This delay prior to data collection is necessitated by the non-Lambert-Beer conditions during plasma equilibration. For the given experimental conditions, the cavity output typically takes ~ 1.5 μs to begin mono-exponential decay. The time-dependence of the cavity output intensity was also investigated. Light propagating within the cavity is subject to direct absorption by the intra-cavity LIP, and a lensing effect (f ≈ −3.1 cm) by the LIP as a result of a localised nonlinear refractive index change induced by the high temperatures and strong electric fields associated with the LIP. This time-dependence was observed to be dependent on both the mirror separation and the pulse duration of the LIP formation laser. These dependencies come about as a consequence of the LIP lensing temporarily affecting both the cavity stability and the imaging of the light that is output from the cavity, with both of these aspects being dependent on the cavity geometry. In the case of fs-lasers, the increased pulse power density expectedly yields a more significant lensing effect when compared to LIP formation with ns-pulsed lasers. en
dc.format.mimetype application/pdf en
dc.language.iso en en
dc.rights © 2020, Bryan P. Keary. en
dc.rights.uri en
dc.subject Laser-induced plasma en
dc.subject Cavity-enhanced absorption spectroscopy en
dc.subject CEAS en
dc.subject IBBCEAS en
dc.subject LIBS en
dc.subject Laser spectroscopy en
dc.subject Azulene en
dc.subject Cavity-ringdown spectroscopy en
dc.subject CRDS en
dc.subject Mirror reflectivity en
dc.subject Optical cavity en
dc.subject Intra-cavity lensing en
dc.title Laser-induced plasmas in air for pulsed broadband cavity-enhanced absorption spectroscopy en
dc.type Doctoral thesis en
dc.type.qualificationlevel Doctoral en
dc.type.qualificationname PhD - Doctor of Philosophy en
dc.internal.availability Full text not available en
dc.description.status Not peer reviewed en Physics en
dc.internal.conferring Summer 2020 en
dc.internal.ricu UNEP GEMS/Water Capacity Development Centre en
dc.availability.bitstream embargoed 2020-12-01

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© 2020, Bryan P. Keary. Except where otherwise noted, this item's license is described as © 2020, Bryan P. Keary.
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