where c is the speed of light, τ 0 ≈ 25 µs is the ringdown time for an empty cavity, L is the distance between the cavity mirrors, and d is the absorber path length of the buffer-gas cell. Cyanoacetylene was prepared using a modified procedure of an existing method using propiolamide, phosphorous pentoxide, and sand . All reagents were ground together using a mortar and pestle and transferred to an oven-dried reaction flask. The reaction vessel was kept under active vacuum and connected to a U-shaped trap that is immersed in liquid nitrogen. The reaction mixture was heated up to 180 • C in an oil bath over the course of 1 hour. After this, the reaction flask was closed off from the trap, whereupon it was removed from the liquid nitrogen bath. The collected product was allowed to sublime into a stainlesssteel gas cylinder before being diluted (∼ 2 -4% of total cylinder pressure) in ultra-high purity helium. Sample purity was confirmed using standard FTIR spectroscopy.