of the quantum yield and the detection system. In addition, a calibrated CELIF spectrum requires knowledge of the wavelength-dependent exit mirror transmission. In section \[sec:yield\] we describe a procedure that uses the simultaneously measured ring-down times and Rayleigh scattering collected with the LIF detection setup to determine the wavelength dependence of $T$ based on our unchanged setup. With two sets of UV and visible mirrors we found the transmission to vary by less than 5% over a wavelength range of 1 nm which will allow to approximate the transmission as constant in many applications. We note, however, that the common approximation $T = 1 - R$, with $R(\lambda)$ derived from an empty-cavity ring-down scan, could not be generally applied over the usable wavelength range of each set of mirrors.

Apparatus
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A general CELIF apparatus is shown in fig. \[fig:exp\]. It consists of a standard ring-down cavity including beam-shaping, mode-matching optics and a suitably fast photodetector (x-axis in figure). A typical LIF detection system, including a collimation lens and photodetector, is added at right angles to the cavity axis, preferably at the center of the sample (y-axis in figure). Two experimental setups were used in this study. Setup 1 introduced 