($R>99.99\%$). For the fluorescence measurements, the same set of mirrors was used as in setup 1 and the drop-off in their reflectivity to about 99.7% shortened the ring-down time to $\approx 800$ ns. The output of the (doubled) dye laser (Quanta-Ray PDL-2, pumped by Continuum Minilite II, 10 Hz repetition rate) was $\approx 300~\mu$J at 583.5 nm and $\approx 100~\mu$J at 313 nm, both with a bandwidth of 0.3 cm$^{-1}$. The LIF optics imaged a 3 mm long probe volume onto the photodetector.

In both setups we use two identical photomultipliers (Hamamatsu, H7732-10 module with R928 tube), the signals of which were simultaneously recorded using a two-channel digitizer (National Instruments NI PCI-5124, 12-bit for setup 1 and AlazarTech ATS460, 14-bit for setup 2).

Data acquisition and analysis {#sec:daq}
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For each laser shot, the LIF transient and the CRD transient are measured simultaneously as shown in fig. \[fig:transients\] (a) and (b), respectively. The CRD transient follows the typical exponential decay (fig. \[fig:transients\] (b)) from which the ring-down time, $\tau$, is extracted by a non-linear least squares fit. In combination with the empty-cavity ring-down time, $\tau_0$, the absorption coefficient, $\alpha$, is determined as in a typical CRD experiment. In the following 