Investigation of an energy-gap model for photoacoustic O2A-band spectra: H2O calibration near 7180 cm−1

Highlights: ► We investigate an energy transfer model for photoacoustic measurements of the O2A-band. ► We measure the response of a photoacoustic spectrometer for known quantities of H2O and O2. ► We fit multiple theoretical spectral line profiles to the data. ► We bind the relative uncertainty of the energy transfer model to less than 1%. ► We demonstrate that speed-dependence is an important line shape effect for these experiments. - Abstract: A photoacoustic spectrometer is used to evaluate the accuracy of an energy-gap model for collisional energy transfer. For photoacoustic measurements involving the b1Σg+←X3Σg- transition of molecular oxygen the conversion of photon energy to thermal energy is inefficient and proceeds through the a1Δg state. This results in attenuation of the photoacoustic signal. The magnitude of the attenuation can be predicted with an energy-gap model whose accuracy has been previously confirmed to within 3 ± 5%. However, this prior result does not rule out incomplete rotational relaxation of O2 in the a1Δg state. In this study, high-resolution spectra of H2O in air are used to calibrate the photoacoustic spectrometer. This work binds the relative uncertainty in the energy-gap relaxation factor for O2A-band photoacoustic signals to be approximately 1%. During one acoustic cycle, this result implies negligible collisional relaxation to the X3Σg- state of O2 and nearly complete collisional relaxation to the a1Δg state.