setup is shown in fig. \[fig:exp\]. It is a straightforward combination of a classical LIF and pulsed CRDS setup: the sample is intersected by the laser beam that is confined in the cavity and the laser-induced fluorescence is collected at right angles. In our setup the wavelength-integrated fluorescence is recorded. Without loss of generality, the technique can be used in a setup where the dispersed fluorescence spectrum is measured. In principle, the technique should be widely applicable to any gas-phase, liquid and even solid samples which have been used in previous CRDS studies as long as the fluorescence light can be extracted from the sample volume.

In the following subsection we derive how in our combined CRD/LIF setup the cavity ring-down signal is used to normalize and absolutely calibrate the LIF signal.

CELIF methodology {#sec:method}
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In a general LIF measurement, the time-integrated LIF signal, $S{^\text{LIF}}$, is proportional to the light intensity, $I{^\text{LIF}}$, that has interacted with the sample within the LIF probe volume: $$\label{eq:SLIF}
    S{^\text{LIF}}(\lambda) = \alpha(\lambda) \cdot \Gamma(\lambda) \cdot g \cdot I{^\text{LIF}},$$ where $\lambda$ is the excitation wavelength, $\Gamma$ is the fluorescence quantum yield and $g$ is a geometry dependent factor of the detection system. In principle, $g$ 