A new absorption spectrometer operating in the submillimeter wave region has been designed. The apparatus combines the previously reported fast scan submillimeter wave spectroscopic technique (FASSST) and a pulsed supersonic jet sample. It is specially designed for the rapid searching for unknown molecular transitions of weakly absorbing and/or low abundance species. Vibration-rotation transitions of the Ar . CO and Ar . ND3 van der Waals molecules have been observed using this apparatus. Transitions in Ar . CO involve the j=1 -->j=2 hindered rotor transition in the complex. Transitions in Ar . ND3 have been assigned as the rotational structure of the fundamental of the lowest frequency bending mode involving the two moieties. Transitions have been observed emanating from the levels correlating to the two lowest energy inversion components of ND3 yielding a determination of the inversion splitting in the complex.