by the small sample volume (diameter of the molecular beam), the shot-to-shot variation of sample concentration and the comparatively low reflectivity of the ring-down mirrors. In contrast, the CELIF measurement covers a much larger dynamic range as shown in the right column. With a change in sample density, the amplification of the photodetector was adapted to avoid saturation of the detection system. However, the series of CELIF measurements was internally calibrated each time the amplification of the photodetector was changed and we were able to follow the relative sample density, $\rho / \rho_0$, over a range of more than three orders of magnitude. The very low baseline noise in the CELIF measurements is evident in panels (b) and (d). Even in panel (f) where the sample density is reduced by three orders of magnitude the CELIF baseline noise is comparable to the CRD baseline noise at the highest sample concentration, panel (a).

![Comparison of simultaneously recorded CRD (left column) and CELIF (right column) spectra of jet-cooled BPEB at different relative concentrations, $\rho/\rho_0$. The CRD spectra are shown to the same scale as the absolute noise does not change with concentration. The absolute photon loss per pass of the CELIF spectrum 