We describe an ultra-sensitive method to record high-resolution mid-IR spectra of hydrocarbon-chain species and other astrophysically relevant molecular transients in a pulsed planar plasma expansion. The method uses an all fiber-laser-based optical parametric oscillator (OPO) light source, in combination with continuous wave cavity ring-down spectroscopy (cw-CRDS) as a direct absorption detection tool. A hardware based multi-trigger concept is introduced to apply cw-CRDS to pulsed plasma. The performance and potential of the method are illustrated and discussed, based on extended and fully rotationally resolved sample spectra of HC4H and HC6H, generated in the discharge of an expanding C2H2/He/Ar gas mixture. (C) 2013 Elsevier B. V. All rights reserved.