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SubLIME – Experiment

SubLIME – Experimental Setup

The sublime experimental setup.

The experiments take place in an ultra-high vacuum chamber (pressure ∼ 10-9 Torr). Ices 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 12 K (-261°C; -438°F). This chamber is equipped with instruments that allow us to combine the techniques of infrared spectroscopy, mass spectrometry, and millimeter/submillimeter spectroscopy to study the chemical and physical processes in laboratory analogs of interstellar ices and icy planetary/cometary surfaces. The composition of the iceis monitored with Fourier-transform infrared (FTIR) spectroscopy (using a Thermo Scientific Nicolet™ iS50 spectrometer). Simultaneously, the ice can be exposed to UV photons from a microwave-discharged hydrogen flow lamp. As the ice is photolyzed, molecules leave the ice surface, as an effect of sublimation due to heat or photodesorption due to the UV photons. These gas-phase species can then be probed using millimeter/submillimeter spectroscopy, which can be used to uniquely identify and quantify the gas components (which can be complex). Our laboratory-measured spectra can then be compared to telescope data as a means of interpreting the gas compositions observed in interstellar clouds, the comae of comets, and the atmospheres/exospheres of other icy planetary bodies.

A schematic diagram of the SubLIME chamber. On the left is a side view and on the right is a top view.

Katarina Yocum working with the Lyman-α lamp settings.

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.

Schematic of the direct-absorption millimeter/submillimeter spectrometer system. The output frequency coverage of our setup is 75-1000 GHz, 1.8-1.9 THz, and 2.5-2.6 THz.

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