Speed-dependent effects (SDEs) in both the absorption and dispersion modes of detection have been detected and scrutinized by the noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS) technique. The present paper achieves four objectives: (i) it provides the first demonstration of SDEs detected in dispersion, (ii) it validates the expression for a speed-dependent Voigt (SDV) dispersion line-shape function that is derived in an accompanying paper, (iii) it illustrates the influence of SDEs on the NICE-OHMS technique, and (iv) it gives the first experimental comparison of SDEs for the absorption and dispersion modes of detection. Experiments were performed using an isolated transition in the band of acetylene [ at ] in the 100–250 Torr range at room temperature. It is shown that SDEs appear in both the absorption and dispersion modes of detection, that they can be well described by the suggested SDV dispersion line-shape function, and that they need to be taken into account if NICE-OHMS signals detected under optimal pressures are to be properly assessed.
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