Aravind G

Nitrogen-bearing polycyclic aromatic hydrocarbons (PANHs) are ubiquitous in space. They are considered precursors to advanced biomolecules identified in meteorites. However, their chemical evolution into biomolecules in photodestructive astrophysical mediums remains a paradox. Here, we show that light can efficiently initiate the molecular mass growth of PANHs. Ultraviolet-photoexcited quinoline monomers, the smallest PANH, were observed to associate and intermolecular Coulombic decay between the associating monomers formed the cations of quinoline-dimer. Molecular rearrangements in the dimer cation lead to a dominant formation of cations heavier than quinoline. The enrichment of these heavier cations over all the other cations reveals the efficiency of this route for the mass growth of PANHs in space. This mechanism also leads to a highly reactive unsaturated PANH-ring via CH loss, a hitherto unknown channel in any photon-driven process. The occurrence of this efficient pathway toward complex molecules points to a rich chemistry in dense interstellar clouds.

We probe a new pathway for the formation of smaller anions from the temporary negative ion states (anion resonances) of C n N- (n = 1-3, 5-7) in the circumstellar envelope of IRC+10216. C n N- (n = 1-3, 5-7) anions were collisionally excited to their resonance states and were observed to decay into a variety of smaller anions. The measured kinetic-energy-release distributions for the anionic fragments arising from each of the parent anions indicate a concerted manner of occurrence of these fragments, implying rich dissociation dynamics. and C2N- were found to be dominant fragments of these anions, suggesting their presence in the external layers of IRC+10216 where UV photons penetrate. C n N- (n = 1-3) were also observed to undergo dissociative and nondissociative double-electron detachments, with the former being dominant. The significance of this new pathway in determining the stability and abundance of anions in IRC+10216 is discussed.

A low-lying resonance in FeCN- anion was identified through abrupt changes in the spectral dependence of the photoelectron angular distribution. Non-Franck-Condon transitions from the resonance to the neutral FeCN (4 "), and the corresponding photoelectron angular distributions revealed that the resonance is a dipole scattering state. Significant thermionic electron emission was observed in the resonant photoelectron spectra, indicating a strong coupling of the resonance with the ground state of this triatomic anion and its competition over autodetachment. This low-lying resonance is identified to be an efficient pathway for the formation of FeCN- anion in the outer envelope of IRC+10216. The results in general reveal formation pathways in space for anions with low-lying resonances and large permanent dipole moment.

Intermolecular Coulombic decay (ICD) is a process whereby photoexcited molecules relax by ionizing their neighbouring molecules. ICD is efficient when intermolecular interactions are active and consequently it is observed only in weakly bound systems, such as clusters and hydrogen-bonded systems. Here we report an efficient ICD between unbound molecules excited at ambient-light intensities. On the photoexcitation of gas-phase pyridine monomers, well below the ionization threshold and at low laser intensities, we detected the parent and heavier-than-parent cations. The isotropic emission of slow electrons revealed ICD as the underlying process. π–π* excitation in unbounded pyridine monomers triggered an associative interaction between them, which leads to an efficient three-centre ICD. The cation resulting from the molecular association of the three pyridine centres relaxed through fragmentation. This below-threshold ionization under ambient light has implications for the understanding of radiation damage and astrochemistry. [Figure not available: see fulltext.]

The photoprocessing of astrophysically relevant small polycyclic aromatic hydrocarbon (PAH) molecules, namely, anthracene, phenanthrene, and pentacene, under intense UV field is experimentally studied to explore the formation of smaller ions upon their dissociative ionization and the effect of their size and structure on the dynamics of dissociative ionization. The molecules are UV processed in the multiphoton regime using a 266-nm nanosecond laser pulse. The dissociative ionization spectra at different laser intensities reveal distinct dissociation dynamics of these PAHs based on their size and structures.

Time delay in photodetachment has been investigated for few negative ions, Cl-, Br-, I-, and Li-. Very few studies on photodetachment time delay for negative ions are present till date and have been reviewed briefly in the present article. Different many body formalisms, like RRPA, RRPA-R and RMCTD, have been employed to obtain new results on photodetachment time delay, accounting for important electron correlations present in the negative ions studied here. Results on time delay show significant angle dependence near the threshold region, at shape resonance and at Cooper minimum region.

This work reports an effort to introduce a few advanced concepts in quantum physics to undergraduate students who have only a minimal familiarity with wave mechanics. The geometric phase is not introduced in most introductory courses on quantum mechanics. It is of pivotal significance for the understanding of topological phase transitions which are becoming increasingly important in a large number of physical phenomena, including those that involve platforms for robust quantum computing. This article would provide a gentle introduction to the geometric phase. Toward this goal, Feynman's path integral approach to quantum mechanics and its application in the interpretation of the wondrous Aharonov-Bohm effect is introduced in a gentle manner. A computer simulation of this interesting phenomenology is presented to help gain an early insight into some advanced concepts.

Photoionization and dissociation of anthracene, phenanthrene, and pentacene in the multiphoton regime at 266 nm wavelength were performed. CHn+ (n = 4-6) ions were observed as fragments, and their yields as a function of laser intensity were studied. H-migration and "roaming" mechanisms leading to the formation of these fluxional ions from polycyclic aromatic hydrocarbons (PAHs) are proposed. The present results show higher photostability of PAHs with bent structure and suggest PAHs in the UV regions of interstellar medium as a prominent source for CHn+ (n = 4-6).