the electronic noise of the detector and the digitization error, increases the error in the exponential fit. All these sources of noise/error contribute to the shot-to-shot fluctuations in the measured ring-down time and hence the absorption coefficient, $\alpha$. A changing mirror alignment, *e.g.* due to pressure changes inside the cell, will lead to a systematic error in $\alpha$, as we observed in our early measurements of the pressure dependence of the N$_2$ Rayleigh scattering (*cf.* fig. \[fig:powerdep\] (b)). Saturation of the photodetector (non-linear amplification) may also introduce a systematic error as the ring-down transient may be distorted. Although, this can easily be avoided by adjusting the CRD signal level via the laser pulse energy and the PMT voltage.

In contrast, mode fluctuations or poor mirror alignment do not affect the CELIF measurement as only the integrated LIF and CRD signals, $S{^\text{LIF}}$ and $I{^\text{CRD}}$ respectively, are analyzed and not the shape of their transients. Thus, the noise/error in the integrals is only due to the noise of the detectors and the digitization errors. Saturation of the photodetectors needs to be avoided as described above. The strength of the CELIF technique is the precise determination of the laser intensity in the cavity, 