Device for high-resolution frequency-modulated absorption spectroscopy of jet-cooled molecules

A device for high-resolution frequency-modulated absorption spectroscopy of jet-cooled molecules is described and demonstrated. Direct absorption spectra of jet-cooled molecules are recorded by multipassing an infrared laser beam through a free-jet expansion with all light rays perpendicular to the nozzle axis. In sliced-jet spectroscopy sub-Doppler resolution is obtained by inserting a small blade upstream from the laser crossing to obstruct molecules that would otherwise absorb at the center of the Doppler profile. Here the sliced-jet technique is extended by replacing the blade by a stainless-steel string vibrating at a frequency of about 10 kHz. Phase sensitive detection of the frequency-modulated absorption signal is accomplished with a lock-in amplifier. A resolution of 12 MHz is demonstrated using the modulated sliced-jet technique