Construction of a Highly Sensitive Spectrometer in the Visible Region

A sensitive spectrometer was constructed in order to observe very weak visible absorption lines of molecules using a ring dye laser, a multiple-pass absorption cell, and a Stark modulation technique. The minimum detectable absorption coefficient is 3×10-9/cm and is limited by excess laser-radiation noise of the ring dye laser. This system has been successfully applied for the observation of very weak overtone transitions of NH3 with Δv=5.