I(c)onic Space

With the buzz surrounding the launch of the Chandrayaan-3 spacecraft, a lot seems to be happening with regard to space exploration not only in India, but all around the world.

Speaking of space, there is a meteorite called the Murchison meteorite, which was found in Australia in 1969. A meteorite is a solid piece of debris from space, which falls down to the earth. The Murchison meteorite is rich in organic compounds and is the most studied meteorite in the world.

One of the unique things about the Murchison meteorite is that polycyclic aromatic nitrogen substituted hydrocarbon (PANH) was detected in it. PANH is a cyclic organic compound. PANH is abundant in space and is considered a precursor to complex biological molecules when in space.

In this study, the authors which include Dr. Saroj Barik, Mr. Nihar Ranjan Behera, Mr. Saurav Dutta, and Prof. G. Aravind from the Department of Physics, Indian Institute of Technology Madras, Chennai, India, Dr. Rajesh Kumar Kushawaha from the Physical Research Laboratory, Ahmedabad, India, Dr. Y. Sajeev from the Theoretical Chemistry Section, Bhabha Atomic Research Centre, Mumbai, India, and Dr. Raghunath O. Ramabhadran from the Indian Institute of Science Education and Research, Tirupati, India, seek to answer the question whether a phenomenon called intermolecular coulombic decay (ICD) could lead to the formation of large molecular systems, with PANH as a precursor. This mechanism could critically determine the abundance and growth of biological structure motifs in space.

Intermolecular coulombic decay (ICD), is a process where photoexcited (excited by light) molecules relax by ionizing their neighbouring molecules. Ionization refers to the process by which electrically neutral atoms or molecules gets converted to charged atoms or molecules by losing or gaining an electron.

ICD usually occurs in weakly bound systems. ICD can occur in only one subunit of a weakly bound system, or multiple subunits of a weakly bound system can be simultaneously photoexcited. In this study, multiple subunits was considered.

As ICD only occurs in weakly bound systems, the question the authors of this study ask is, can ICD occur in unbounded systems and with ambient light?

In order to answer this question, ICD was performed on pi-electron systems that are not bound before excitation. Gas-phase pyridine monomers were used in this experiment. Pyridine is the primary unit of the PANH family.

It was found that below-threshold ionization under ambient light was achieved.

With these results, the authors then studied quinoline, which is the smallest PANH. The quinoline monomers were exposed to ultraviolet-photoexcitation. ICD between the associating monomers formed the cations of quinoline dimer. The cations were heavier than quinoline. This proves the efficiency of ambient light induced ICD for the mass growth of PANHs in space. This mechanism also leads to a highly reactive unsaturated PANH-ring via CH loss, which was a previously unknown channel in any photon-driven process.

The formation of complex molecules through this process gives proof of a rich chemistry in dense interstellar clouds. It also points to the role of aromaticity in bottom-up approach in astrochemistry, a subtle concept which they plan to explore more in the future.

Prof. E. Krishnakumar, Senior Professor from the Department of Nuclear and Atomic Physics, Tata Institute of Fundamental Research, Mumbai, India, gave his analysis of the work done by the authors and acknowledged the importance of the findings with the following comments: “Using ion mass spectrometry and electron momentum spectrometry Barik et al.  [Nature Chemistry] have unravelled several new molecular processes which were not in the realm of possibilities considered till now. Intermolecular Coulombic Decay [ICD] is a recently discovered phenomenon of efficient energy transfer between two centres in a loosely bound molecular aggregate. In this work it is shown that three isolated molecules of pyridine on photo-excitation come together forming a single entity and in the process pool their individual excitation energies to ionize and even fragment the molecules forming positive molecular ion of larger mass, small reactive radicals and electrons. This path-breaking discovery of very efficient photo-association of not just two, but three molecules and their subsequent dynamics may have strong implications to our understanding of astrochemistry and radiation induced processes.  In another paper [Science Advances] they have extended this study further to the case of an astrophysically important PANH molecule quinoline, were two molecules on photoexcitation come together and pool the excitation energy forming a molecular ion bigger than the precursor molecule and CH radical, indicating the possibility of such a process in the growth of larger molecules, even biomolecules in astrophysical environments. These findings are truly novel and significant in that they show ionization taking place in a medium where the photon energies are neither big enough to cause single photon ionization nor intense enough to cause multi-photon ionization.” 

Article by Akshay Anantharaman
Click here for the original link to the paper
Click here for the original link to the paper


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