empty-cavity ring-down times of $1.2-2.5~\mu$s (cavity length $\approx 84$ cm). The doubled output of the dye laser (Sirah Cobra-Stretch, pumped by Continuum Surelite I-10, 10 Hz repetition rate) ranged from 30 to $100~\mu$J with a bandwidth of 0.045 cm$^{-1}$ and a pulse length of $\approx 5$ ns. The LIF collection optics imaged the full probe volume (overlap of molecular and laser beams) onto the photodetector.

With setup 2 we carried out N$_2$ Rayleigh scattering (at 583.5 nm) and acetone fluorescence (at 313 nm) measurements to verify the methodology as described in section \[sec:method\]. We note that the same detector setup was used for the Rayleigh scattering and fluorescence measurements. In general, CELIF signal is referred to signal collected by the LIF detection setup normalized by the CRD intensity. Therefore CELIF signal in the course of this paper refers to Rayleigh scattering or fluorescence signals. The gas pressure of the filled cavity (length $\approx 81$ cm) was monitored over a range of $0.01-1000$ mbar with capacitance manometers (Leybold CR090/CTR100 and MKS 626A) to cross reference the spectroscopically determined sample densities. For the Rayleigh scattering measurements, empty-cavity ring-down times in excess of 40 $\mu$s were achieved with the corresponding set of mirrors 