Aravind G

A state-of-the-art photoelectron spectroscopy (PES) experimental setup is designed and built to study the structure and dynamics of interstellar medium anions, in particular polyaromatic hydrocarbon chain (PAH) anions.

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.

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.

We report the first experimental probe on resonance states in FeO anion, a molecule of astrophysical relevance. Excitation of FeO through collision with argon was performed to measure the kinetic energy release in the centre of mass frame. Results show access of two anion resonances upon collisional excitation. We discuss our evaluation of these two resonances from kinetic energy release in the centre of mass frame. The astrophysical importance of these anion resonances will be presented.

The stability of FeC- against dissociation in an astrophysical environment was probed by the collisional excitation of FeC-. Two anion resonances yielding Fe- and C- fragments were observed and studied through measurement of the kinetic energy released during fragmentation. The yield of Fe- was found to be nearly 5.5 times more than that of C- indicating the C- fragment to be in the loosely bound (2D) state. The possibility of avoided crossing leading to the observed fragment ion yield is also discussed. The dissociation of FeCn(n = 2 to 4, 6) cluster anions predominantly resulted in the cleavage of Fe-C bond yielding only fragments with similar energy release. The yield of Cn is discussed in the light of the observed abundances of HCn in IRC+10216. The importance of rotational transitions pertaining to both the ground and excited-electronic states of these cluster anions is discussed.

Depletion of FeO in the interstellar medium through resonance states of FeO- was studied through collision-induced dissociation of FeO-. Collisional excitation of FeO- lead to the yield of Fe- and O-, with a higher yield for Fe-. The ground electronic state of FeO- was identified to be 6Δ state and the results are compared with previous theoretical and experimental results. The kinetic energy distributions of the fragments revealed two FeO- anion resonances accessed upon excitation and their potential energy curves were evaluated. The role of the observed resonances in the depletion of FeO and the viable presence of Fe- and O- in Sagittarius B2 are discussed.

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).