Oxygen rotational Raman linewidth determination considering nonmonoexponential decoherence behavior

The rotational Raman linewidth is a vital empirical parameter in determining temperature and concentration in combustion environments from pure rotational anti‐Stokes Raman spectroscopy (RCARS). The Raman linewidth is regularly determined from the coherence dephasing time constant on a picosecond time scale. Oxy‐fuel combustion processes cannot be studied applying the well‐described nitrogen evaluation but are typically based on oxygen evaluation. Therefore, the dephasing behavior of the rotational Raman coherences of oxygen was studied by picosecond time resolved RCARS. In opposite to nitrogen, the dephasing in oxygen shows distinct quantum beating rendering monoexponential fitting functions unsuited for the evaluation. Therefore, collisional dephasing time constants were obtained from a coherent beating model function. The acquired Raman linewidths show comparable trends with increasing temperature and rotational quantum number as nitrogen but differ from O2‐linewidths used for evaluations up to now. Therefore, an improvement of temperature and concentration evaluations based on oxygen RCARS measurements by application of the S‐branch linewidth can be expected. S‐branch rotational Raman linewidths of oxygen have been determined from picosecond pure rotational coherent anti‐Stokes Raman spectroscopy, which necessitated the application of a novel model function that properly describes the quantum beating in the time domain. The obtained linewidths deviate from the Q‐branch linewidths used for rotational coherent anti‐Stokes Raman spectroscopy temperature and concentration evaluations. Using the new S‐branch linewidths may improve the accuracy of these evaluations.