In situ infrared absorption spectroscopy of cryogenic-specific reddish coloration produced by cryoplasma irradiation on an ice surface

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Abstract
Recently, cryoplasma has been demonstrated to form reddish coloration on methanol- and water-containing ice surfaces below 150 K. This cryogenic-temperature-only reddish coloration could provide a clue for understanding the composition of the red organic materials observed in the outer solar system. In this study, ice with reddish coloration was analyzed by IR absorption spectroscopy (2500–1700 cm−1) during cryoplasma irradiation at 85 K and subsequent increase of the temperature from 85 to 200 K without cryoplasma. The intensities of the IR peaks, differing from those of the raw materials, increased with the irradiation time during reddish coloration. When the reddish coloration disappeared at 120–150 K, the IR peak intensities at 2170 and 1790 cm−1 decreased. These peaks are thought to originate from C- and/or N-containing triple bonds or cumulative double bonds and carbonyl groups. Therefore, the reddish materials are considered to contain these functional groups.

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