A sensitive infrared detection scheme is presented in which continuous wave cavity ring down spectroscopy is used to record rovibrational spectra of molecular ions in direct absorption through supersonically expanding planar plasma. A cw optical parametric oscillator is used as a light source and combines a broad spectral coverage (2100-3600 cm(-1)) and narrow bandwidth (7 kHz) with a high output power (similar to 1 W) and excellent TEM00 mode characteristics. The performance of the technique is demonstrated by recording a rotationally cold spectrum of formyl cation, HCO+, in the CH stretching region. For optimal conditions, absolute ion densities in the plasma expansion amount to 2 x 10(10) HCO+-ions/cm(3) in the detection zone. (c) 2007 Elsevier B.V. All rights reserved.