loss per pass using eq. \[eq:alpha\_CRD\]. The Rayleigh scattering signal obtained by the CELIF setup was calibrated to match the CRD photon loss at a N$_2$ pressure of 1 bar according to eq. \[eq:alpha\_LIF\]. The slopes of the independent linear least squared fits to the data in fig. \[fig:powerdep\]b are indistinguishable proving the validity of eqs \[eq:CELIF\_norm\]–\[eq:cal\_LIF\].

Moreover, we found that during these experiments with variable cavity pressures, movement of the mirrors could be observed by the ring-down measurement. Only after we improved the mechanical stability of our cavity and mirror mounts, we were able to set up a cavity unaffected by the change from vacuum to atmospheric pressure. This was verified by comparison of the CRD density measurement to the reading of the capacitance manometer. However, the simultaneous CELIF measurement is immune to these cavity misalignments as these are fully compensated for by the shot-to-shot normalization procedure leading to higher quality data.

Figure \[fig:BPEBspectra\] shows a series of simultaneously recorded CRD and CELIF spectra of jet-cooled BPEB as a function of relative sample density $\rho / \rho_0$. The dynamic range of this particular CRD experiment is rather poor as shown in the left column. The main limitations are set 