measured by the ring-down detector ($I{^\text{CRD}}$). Data has been recorded using the fluorescence of acetone at the excitation wavelength of 313 nm at two pressures of 0.1 mbar ($\circ$) and 0.3 mbar ($\bullet$) leading to losses per pass of $(1.42\pm 0.02)\cdot 10^{-3}$ and $(4.31\pm 0.04)\cdot 10^{-3}$ respectively (linear least-squared fit). Error bars are significantly smaller than the symbol size. (b) Pressure-dependent N$_2$ Rayleigh scattering at 583.5 nm. The photon loss is derived from the ring-down times of CRD measurement (eq. \[eq:alpha\_CRD\]) and the CELIF data (shifted for clarity) is calibrated according to eq. \[eq:alpha\_LIF\] at 1 bar. Each data point is based on an average of 311 laser shots in a 10 mbar pressure range. The error bars represent the spread of the data. The solid lines are the linear least-squared fits to the data.[]{data-label="fig:powerdep"}](Fig03 "fig:"){width="8.6cm"}\

In order to assess detection limits and signal-to-noise ratios, we also measured off-resonance transients of BPEB at 321 nm shown in the right column of fig. \[fig:transients\]. As BPEB has an extended $\pi$-system along its major axis leading to a large polarizability, we can detect its Rayleigh scattering in a seeded molecular beam (fig. \[fig:transients\]c) with our CELIF setup. The corresponding CRD transient (fig. 