The matrix-isolation spectrometer has been described in detail previously. Briefly, the matrix-isolated sample is prepared in the center of the chamber on a 1-inch KBr salt-glass substrate that is cooled by a CTI Model 22 cryocooler. The temperature of the sample holder is monitored and controlled through both a silicon diode and resistive heaters connected to a Lakeshore Model 331 temperature controller. A differentially pumped rotary platform enables the sample to rotate 360° inside the chamber without breaking the vacuum. In practice, the sample mount either faces towards the deposition axis where sample/matrix gases are deposited or is rotated into alignment with the optical axis for spectroscopic analysis. The chamber maintains a base pressure of ~8×10 Torr, and reaches ~8×10 Torr with cryopumping.