Multipass cell for molecular beam absorption spectroscopy.

We report a two-spherical mirror multipass cell which allows efficient coupling of a laser beam to a collimated molecular beam at the center of the cell. With near-concentric alignment and the laser beam coupled into and out of the cell at the edges of opposite mirrors, an approximately parabolic pattern of reflections is observed on each mirror. Aligned for forty to fifty passes, the cell is used for direct absorption spectroscopy of jet-cooled acetylene, propyne, and 1-butyne in the 3-microm region. Sub-Doppler linewidths as narrow as 12 MHz have been recorded.