The instrument used here is described in detail in a previous work , and will only be described briefly, with particular attention paid to the modifications made to the instrument for the present work. Central to the instrument is a cryogenic buffergas cell that is thermally anchored to the second stage of a closed-cycle helium cryostat (Janis RDK-415D2). The buffer-gas cell has two holes, 0.156" in diameter, that create a spectroscopic axis and allow a laser to probe species of interest within the cell. Gas is injected into the cell using a 1/8" O.D. capillary tube that passes through the cell entrance. Newly integrated into the instrument is an insulated temperature-controlled heating sheath that is used to maintain an elevated temperature on the capillary tube to ensure the species of interest does not condense onto the inner tube walls before entering the buffer-gas cell. The copper heating sheath is heated by a resistive heating element, with temperature control achieved by monitoring its temperature with a thermocouple and manually adjusting the supplied current. As before, mixtures of the species of interest are prepared in pre-mixed cylinders with helium buffer gas and the gas flow into the cell is controlled using a mass flow controller (Alicat), with flow rates on the order of 1-10 standard cubic centimeters per minute (sccm). The buffer-gas cell assembly is held within a 6-way Conflat cross that is pumped using a Leybold 90iX turbomolecular pump. The ultimate chamber pressure is 10 -7 Torr (10 -8 to 10 -9 Torr with cryopumping) and is typically on the order of 10 -4 Torr when gas is flowing.