A Buffer Gas Cooling experiment coupled to Cavity Ring Down Spectroscopy to explore complex spectra in the Near-Infrared range

Buffer gas cooling relies on the thermalization of a buffer gas with a surface brought to cryogenic temperatures, which in turn thermalizes the target molecules through collisions. Because this process does not rely on any particular energy pattern, any molecule can be brought to the temperature of the buffer gas. Advantages of buffer gas cooling are numerous: it is a continuous source of slow laboratory frame velocities, allowing for long observation times. Moreover, in contrast to supersonic expansion, it does not require important pumping infrastructure because it relies on small gas throughput and cryogenic pumping (Changala et al., Appl. Phys. B 122 (2016) 292). Finally, buffer gas cooling is applicable to nearly all molecules and is very efficient in terms of sample density (Santamaria et al., ApJ 801 (2015) 50). The technique requires continuous injection of helium atoms and the species under study inside a vacuum chamber. We developed a cavity ringdown spectroscopy setup to seek the first cold molecules obtained with our apparatus.

One of our first molecular targets is a six-atoms asymmetric top molecule and the smallest molecule to present internal rotation: methanol (CH₃OH).
The size of this molecule and the presence of this large amplitude motion lead to a dense and disordered rotational structure. This structure gets even more complicated when one goes up in energy with vibrational excitations. Due to its complicated spectrum, this molecule remains poorly known, especially in the NIR. This frequency range was recently explored by Svoboda et al. (Phys. Chem. Chem. Phys., 17 (2015) 15710), probing the 2ν₁ vibration overtone around 7200 cm^-1. In this report, the authors were able to assign on the order of a few percent of the observed lines. It thus seemed to be a promising candidate to challenge our ability to record and understand the spectral signature of large molecules in the overtone range using the cooling efficiency of the buffer gas cooling setup and the sensitivity of the cavity ringdown spectrometer.

The experiment and the spectra of CH₃OH will be discussed. The floor will be open for discussion to identify new targets of astrophysical or atmospheric interest.