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Soumya Radhakrishnan at NASA-JPL.
Figure 1. Benzhydryl cation in water ice (Water-ice turns yellow).
Scheme 1: 1a = Benzhydryl radical, 2a = Benzhydryl cation

Solvation Science in outer space conditions helps to understand the origin of life

Easy photoionization of organic radicals in interstellar ice analogues studied in laboratory.

1. Research Summary

My PhD research's aim was to understand the fate of the organic matter in interstellar ices due to the cosmic rays in space. We could demonstrate the easy photoionization of the organic radicals and polyaromatic hydrocarbon (pyrene) studied, since the ionization potential of the compounds is greatly reduced by the water-ice matrix. In light of this and other studies reported earlier, it is safe to conclude that organic chemistry in ice environment is strongly mediated by ionization-whether it is in Earth's cryosphere or in our solar system or in the interstellar space. For this study, we have created interstellar ice analogues, doped them with organic molecules like the polyaromatic hydrocarbon pyrene, radicals (benzhydryl radical, tropyl radical) and subjected them to UV irradiation under laboratory conditions. The common ice analogues studied are H2O ice and varying mixtures of H2O/CO2 ice. Photoionization of these radicals/PAHs in water-ice matrix various UV lights and electron sources prdouces the respective cations and are identified by IR and UV-vis spectroscopy.

 

2. Significance of the research

Prerequisites for the evolution of life on Earth are the coexistence of organic matter and water, and the presence of energy minerals. Astrochemistry research in the past few decades shows that all these conditions may be traced back to primordial interstellar ice grains. These ice grains are micron-sized agglomerates that form the dense molecular clouds that collapse into protostars, protoplanetary disks and ultimately generate solar systems like ours, with planets, moons and asteroids. Interstellar ice grains contain minerals, water, organic molecules like carbon monoxide polyaromatic hydrocarbons (PAHs), and are constantly subjected to energy from irradiation by cosmic rays in outer space. In our research we have mainly mimicked the conditions of outer space under the laboratory environment, to understand how interstellar ice analogues (H2O ice, H2O/CO2 etc.) containing organics form and change upon photoionization.

 

3. Relation to Solvation Science

We studied the effect of solvation by water on the photoionization of reactive intermediates found in the universe like the radicals and PAHs in presence of water-ice. We also addressed some of the important questions as to how water aides in solvating the ions and electrons formed upon irradiating the organics in ice.1 To this end, matrix isolation technique was used. The technique involves the isolation of reactive species in an unreactive host matrix at low temperatures (3-10 K). These experiments were jointly performed at Ruhr University Bochum, Germany (RUB) and NASA JET Propulsion Laboratory, USA (NASA-JPL). The benzhydryl radical (reactive species, organic matter) was trapped in water ice matrix and was exposed to various light sources (see Scheme 1) to reversibly form the cation as the major product. The water ice helps in stabilizing the cation and the electron formed. All the changes to the reactive intermediates and water ice were monitored using Infrared (IR) and Ultraviolet-visible (UV-vis) spectroscopy.


Literature

(1) Radhakrishnan, S; Mieres-Perez, J.; Gudipati, M. S.; Sander, W., Photoinduced Reversible Electron Transfer Between the Benzhydryl Radical and Bezhydryl Cation in Amorphous Water-Ice. J. Phys. Chem. A 2017, 121 (34), 6405-6412.
(2) Jewitt, D.; Luu, J., Discovery of the candidate kuiper belt object 1992 gb(1). Nature 1993, 362, 730-732.
(3) Mcnaughton, N. J.; Pillinger, C. T., Comets and the origin of life. Nature 1980, 288, 540-540
(4) Brou, M.; Morbidelli, A.; Bottke, W. F.; Rozehnal, J.; Vokrouhlicky, D.; Nesvorny, D., Constraining the cometary flux through the asteroid belt during the late heavy bombardment. Astron. Astrophys. 2013, 551, A117.
(5) Napier, W. M., Evidence for cometary bombardment episodes. Mon. Not. R. Astron. Soc. 2006, 366, 977-982.
(6) Shoemaker, E. M., Asteroid and comet bombardment of the earth. Annu. Rev. Earth Planet. Sci. 1983, 11, 461-494.
(7) Radhakrishnan, S., Matrix isolation and solvation studies of reactive intermediates, 2018, doctoral thesis, Ruhr University Bochum

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About the author

Soumya Radhakrishnan from Kerala, India, earned her PhD from RUB in July 2018 under the supervision of Prof. Dr. Wolfram Sander. Part of her research was carried out at the Science Division of Jet Propulsion Laboratory, California Institute of Technology, Pasadena, USA under the supervision of Dr. Murthy S. Gudipati. She is now a postdoc at the University of Gothenburg, Sweden.

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