Optical Properties of Cyanoacetylene Ices in the Far- to Near-infrared with Direct Relevance to Titan's Stratospheric Ice Clouds

Cyanoacetylene (HC3N) ice has been observed in Titan's stratosphere by both Voyager 1's InfraRed Interferometer Spectrometer (IRIS) and Cassini's Composite InfraRed Spectrometer (CIRS), and it is likely prevalent in other objects in our solar system and exoplanetary systems as well. While previous experimental studies targeting Titan's stratospheric clouds have determined the optical properties of HC3N ice in the infrared (IR) spectral range, those thin ice films were formed by annealing processes, which contradicts the formation mechanism of Titan's stratospheric ice clouds. As a result, optical constants of HC3N ices, experimentally created in a similar manner to the way they are formed in Titan's stratosphere, are crucial. Here we experimentally measured absorbance spectra of HC3N thin ice films from the near- to far-IR spectral region (50-8000 cm(-1); 200-1.25 mu m) formed via direct vapor deposition at 30, 50, 70, 90, 110, and 113 K. The corresponding optical constants at all temperatures were also computed, resulting in the largest continuous IR spectral range available for HC3N ice. New tentative peak assignments for spectral features in the near-IR are also reported, thereby further enhancing the inventory of optical constants available for HC3N ice spanning the near- to far-IR spectral range.

Automated summary

Observations have shown the presence of HC3N ice in the stratosphere of Titan and it is believed to be common in other celestial bodies. Previous experimental studies on HC3N ice have focused on thin ice films formed by annealing processes, which contradicts Titan's stratospheric ice clouds formation mechanism. Therefore, experimental measurement of HC3N thin ice films formed in a similar manner to those in Titan's stratosphere is crucial for understanding the optical properties of HC3N ice.