the sample via an unskimmed molecular beam (z-axis in figure) whereas setup 2 was used for cell measurements where the entire cavity was filled with the sample gas.

![Experimental setup. The sample volume is situated at the center of the ring-down cavity and the fluorescence is collected at right angles.[]{data-label="fig:exp"}](Fig01){width="8.6cm"}

Setup 1 was based on our CRD spectrometer that was used to study the torsional motions of jet-cooled 1,4-bis(phenylethynyl)benzene (BPEB), the details of which are described in ref. . Briefly, solid BPEB was sublimated in a heated oven attached to the front of a pulsed solenoid valve (Parker, General Valve Series 9). The gaseous BPEB was picked-up by the argon carrier gas in the channel of the oven and cooled in the subsequent supersonic expansion. The BPEB sample density could be controlled over more than three orders of magnitude by varying the oven temperature. The cavity axis crossed the molecular beam approximately 5 mm downstream of the oven orifice. The $\text{S}_1 \leftarrow\text{S}_0$ transition of BPEB is very strong[@Beeby2002; @CrossSectionNote] and was excited over a wavelength range of 317–321.5 nm. Over this range, the reflectivity of the cavity mirrors (Layertec, center wavelength 330 nm) varied from 99.8 to 99.9% leading to 