SubLIME – Experiment
SubLIME – Experimental Setup
Our experiments take place in an ultrahigh vacuum chamber (pressure ~ 10-9 Torr). The ice samples are formed at the center of the chamber on a gold substrate, attached to a three-stage cryostat and closed-cycle helium cryocooler, which can reach temperatures as low as 10 K (-261°C; -438°F). This chamber is equipped with instruments that allow us to combine the techniques of infrared (IR) spectroscopy, mass spectrometry, and millimeter/submillimeter (mm/submm) spectroscopy to study the chemical and physical processes in laboratory analogs of interstellar ices and icy planetary/cometary surfaces. The composition of the ice samples are monitored with Fourier-transform infrared (FTIR) spectroscopy (Thermo Scientific Nicolet™ iS50 spectrometer). Simultaneously, the ice sample can be exposed to ultraviolet (UV) photons from a microwave-discharged hydrogen flow lamp. As the ice sample is photolyzed, molecules leave the ice surface, as an effect of sublimation due to heat or UV photodesorption. These gas-phase products are then probed using direct-absorption millimeter/submillimeter spectroscopy, which allows for the unique identification and quantification of sublimated gas products. Our laboratory-measured millimeter/submillimeter spectra can then be compared to telescope data as a means of elucidating the gas compositions observed in interstellar clouds, the comae of comets, and the atmospheres/exospheres of other icy planetary bodies.
Millimeter/Submillimeter spectrometer system
The gases released during our thermal or photodissociation experiments are probed by sampling the vapor ∼ 1 cm above the ice layer (this distance is constrained by the mm/submm beam size) using direct-absorption mm/submm spectroscopy. The source is an RF signal generator (Agilent Technologies E8257D PSG) coupled with a set of multiplier chains that generate harmonics of the input frequency. The output frequency coverage of our setup is 75-1000 GHz, 1.8-1.9 THz, and 2.5-2.6 THz, and spectra are collected with 0.1 MHz resolution. The radiation passes over the gold substrate and is detected by a cryo-cooled InSb hot-electron THz bolometer (QMC Instruments QNbB/PTC). The signal is processed using a lock-in amplifier (Stanford Research SR830 DSP) to increase the signal-to-noise ratio, resulting in second-derivative line shapes.