Signatures of incoherent mixing pathways in action-detected spectroscopies

Incoherent population mixing creates a large background in ultrafast, action-detected pump–probe and 2D spectroscopies. This background often obscures the desired coherent responses and leads to incorrect assignment of spectral features. In a previous study, we postulated three classes of incoherent population mixing but could only measure one. In this work, we report spectra collected using a two-quantum (2Q) pulse sequence generated by a fully phase stable spectrometer, enabling spectra containing all three classes of incoherent responses to be measured. We test the postulated formalism describing incoherent population mixing by collecting and simulating spectra measured using a 2Q pulse sequence. A 2Q pulse sequence reported here also provides a means of determining the magnitude of these backgrounds relative to the desired coherent response. Using this technique, we obtain results consistent with the existence of three incoherent population mixing responses, reveal their spectroscopic signatures, and establish a formalism for their description, enabling future methods for their identification and elimination.