A quantum cascade laser cw cavity ringdown spectrometer coupled to a supersonic expansion source

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 $CH2Br2$ have been acquired at $∼1197cm−1$⁠. 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 $CH2Br2$ 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 $1.4×10−8cm−1$⁠. Spectral simulations of the band indicate that the supersonic source produces rotationally cold $(∼7K)$ molecules.