A new instrument has been constructed that couples a supersonic expansion source to a continuous wave cavity ringdown spectrometer using a Fabry-Perot quantum cascade laser (QCL). The purpose of the instrument is to enable the acquisition of a cold, rotationally resolved gas phase spectrum of buckminsterfullerene (C60). As a first test of the system, high resolution spectra of the ν8 vibrational band of CH2 Br2 have been acquired at ∼1197 cm-1. To our knowledge, this is the first time that a vibrational band not previously recorded with rotational resolution has been acquired with a QCL-based ringdown spectrometer. 62 transitions of the three isotopologues of CH2 Br2 were assigned and fit to effective Hamiltonians with a standard deviation of 14 MHz, which is smaller than the laser frequency step size. The spectra have a noise equivalent absorption coefficient of 1.4× 10-8 cm-1. Spectral simulations of the band indicate that the supersonic source produces rotationally cold (∼7 K) molecules.
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