A precursor gas is bombarded with an electron beam to create molecular ions, which are accelerated out of the source by an electric field applied to the source region. Mass selection of the `parent' species occurs at a magnetic sector. The ions are focused into a beam and irradiated coaxially by a tunable dye laser, causing dissociation. The fragment (or `daughter') ion is selected with a second magnetic sector, and detected at an electron multiplier. Lock-in detection is used to reduce noise due to unimolecular and collisional dissociation in the beam.