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

Abstract

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 O₃ 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 O₃ in the Chappuis band (λ ∼ 400–700 nm) depends strongly on the timescales for energy transfer from the nascent photoproducts O(³P) and O₂(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(³P) and O₂(X, v > 0). These timescales enable a full assessment of the accuracy of PAS calibrations using O₃-laden gas samples, dependent on the sample pressure, bath gas composition and PAS laser modulation frequency.