potentially to a level which makes a measurement impossible. In addition, quenching, predissociation and other non-radiative processes complicate a straightforward relationship between signal and concentration, due to possible dependences of these processes on the excited state. Nevertheless, LIF measurements have been carried out in different media from collision-free environments, *e.g.* molecular beams, to liquids and solids.[@Brecher1976]

Over the last two decades, cavity ring-down spectroscopy (CRDS) has become a well-established and widely applied spectroscopic technique.[@Berden2009; @O'Keefe1988] CRDS is based on the Beer-Lambert law and performs a direct absorption measurement. Consequently, fluorescence of the sample is not a detection requirement. In a CRD experiment, light enters a cavity formed by two highly reflective mirrors (typically $R > 0.999$). During each pass a small fraction of light leaks out through the mirrors and is detected as an exponential decay behind the exit mirror. The inverse of the decay rate of the signal is referred to as ring-down time, which for an empty cavity is given by $$\tau_0 = \frac{d}{c (1 - R)},$$ where $d$ is the distance between the mirrors of reflectivity $R$ and $c$ is the speed of light. Depending on the reflectivity of the mirrors, several thousand round trips can be 