recorded (c) Rayleigh scattering (measured with the LIF detector) and (d) CRD transients of BPEB at 321.0 nm. The vertical scale in panel (c) is magnified to show the presence of Rayleigh scattering. The absolute noise is equivalent to the baseline noise in panel (a).[]{data-label="fig:transients"}](Fig02 "fig:"){width="8.6cm"}\

Based on our previous CRD work,[@Greaves2006] we chose BPEB as our model system. One of its characteristic properties is a fluorescence lifetime of $<1$ ns following the $\text{S}_1 \leftarrow \text{S}_0$ electronic excitation.[@Fujiwara2008] The left column of fig. \[fig:transients\] shows samples of simultaneously recorded LIF and CRD transients on resonance. As the fluorescence lifetime is negligible compared to the 1–2 $\mu$s ring-down times, the LIF transient follows the exponential decay of the ring-down signal. Based on this fact, we chose the same integration boundaries for the LIF signal ($S{^\text{LIF}}$) and ring-down intensity ($I{^\text{CRD}}$), indicated by the hashed areas in the figure, to derive the normalized CELIF signal ($S{^\text{CELIF}}$). In the more general case, where the fluorescence lifetime is no longer small in comparison to the ring-down time, both the entire LIF and CRD transients need to be integrated to ensure correct normalization.

![(a) Dependence of the LIF signal ($S{^\text{LIF}}$) on the incident laser intensity as 