Details

Autor(en) / Beteiligte

• Cotterell, Michael I
• Szpek, Kate
• Tiddeman, David A
• Haywood, Jim M
• Langridge, Justin M

Titel

Photoacoustic studies of energy transfer from ozone photoproducts to bath gases following Chappuis band photoexcitation

Ist Teil von

• Physical chemistry chemical physics : PCCP, 2021-01, Vol.23 (1), p.536-553

Ort / Verlag

England: Royal Society of Chemistry

Erscheinungsjahr

2021

Quelle

Alma/SFX Local Collection

Beschreibungen/Notizen

• Photoacoustic spectroscopy (PAS) is a sensitive technique for the detection of trace gases and aerosols and measurements of their absorption coefficients. The accuracy of such measurements is often governed by the fidelity of the PAS instrument calibration. Gas samples laden with O 3 of a known or independently measured absorption coefficient are a convenient and commonplace route to calibration of PAS instruments operating at visible wavelengths ( λ ), yet the accuracy of such calibrations remains unclear. Importantly, the photoacoustic detection of O 3 in the Chappuis band ( λ ∼ 400-700 nm) depends strongly on the timescales for energy transfer from the nascent photoproducts O( 3 P) and O 2 (X, v > 0) to translational motion of bath gas species. Significant uncertainties remain concerning the dependence of these timescales on both the sample pressure and the bath gas composition. Here, we demonstrate accurate characterisation of microphone response function dependencies on pressure using a speaker transducer to excite resonant acoustic modes of our photoacoustic cells. These corrections enable measurements of photoacoustic response amplitudes (also referred to as PAS sensitivities) and phase shifts with variation in static pressure and bath gas composition, at discrete visible wavelengths spanning the Chappuis band. We develop and fit a photochemical relaxation model to these measurements to retrieve the associated variations in the aforementioned relaxation timescales for O( 3 P) and O 2 (X, v > 0). These timescales enable a full assessment of the accuracy of PAS calibrations using O 3 -laden gas samples, dependent on the sample pressure, bath gas composition and PAS laser modulation frequency. We quantify the pressure dependent energy transfer timescales from Chappuis band photoproducts to bath gas mixtures of N 2 and O 2 .