| Name: Binduja Panja |
| Affiliation: Indian Institute of Space Science and Technology |
| Conference ID: ASI2026_918 |
| Title: Influence of Methyl Substitution on the Dissociation Dynamics of PAHs: Experimental Observation of Metastable Decay Routes |
| Abstract Type: Oral |
| Abstract Category: Stars, Interstellar Medium, and Astrochemistry in Milky Way |
| Author(s) and Co-Author(s) with Affiliation: Binduja Panja(Indian Institute of Space Science and Technology, Thiruvananthapuram-695547,India), Arupara Antony Joseph(Indian Institute of Space Science and Technology, Thiruvananthapuram-695547,India), Arjun Jaikrishna M(Indian Institute of Space Science and Technology, Thiruvananthapuram-695547,India), Devika V S(Amrita Vishwa Vidyapeetha,Coimbatore-641112,India), Malavika S(Indian Institute of Space Science and Technology, Thiruvananthapuram-695547,India), Rejila J(Indian Institute of Space Science and Technology, Thiruvananthapuram-695547,India), Umesh R Kadhane(Indian Institute of Space Science and Technology, Thiruvananthapuram-695547,India) |
| Abstract: The presence of aliphatic substituents such as methyl groups plays a crucial role in determining the stability of polycyclic aromatic hydrocarbons (PAHs) in interstellar environments. Methyl addition to PAHs is chemically feasible through reactions of neutral aromatic molecules with abundant methyl radicals, leading to methylated analogues as proposed in earlier studies. The incorporation of a methyl-substituent strongly influences the stability and post-ionization dissociation dynamics of aromatic molecules. Although the CH₃-group is externally attached to the PAH backbone, its presence introduces additional low-frequency vibrational degrees of freedom that efficiently participate in intramolecular vibrational energy-redistribution (IVR). Following multiphoton absorption, the excess internal energy is rapidly channelled through IVR into the methyl side chain, effectively lowering the barriers for competitive fragmentation routes. This promotes prompt H-loss and CH₃-loss prior to backbone fragmentation, thereby enhancing the overall photostability of the PAH framework.
The experiments were performed at AMP Lab, IIST, using a high-resolution time-of-flight mass spectrometer. Comparative studies on benzene and its methylated analogue toluene, as well as on naphthalene and 1- and 2-methylnaphthalene under 266 nm multiphoton ionization, show that methyl-incorporation significantly enhances hydrogen-loss from the parent ion. All systems exhibit C₂H₂ loss as a metastable decay channel, while 1- and 2-methylnaphthalene show enhanced C₂H₂ yield together with CH₃ and C₃H₃ channels. DFT calculations indicate that 1H loss occurs efficiently from the external CH₃ group rather than from the aromatic ring. However, at higher internal energies, hydrogen migration processes induce ring expansion and ring contraction, enabling the molecule to evolve through several intermediate stages involving structural rearrangements before final dissociation. The experimental approach allows efficient discrimination between prompt and metastable decay processes, providing direct insight into fragmentation timescales. Combined with DFT calculations using Gaussian, this enables construction of dissociation pathways and a consistent interpretation of decay mechanisms in aromatic and methylated aromatic systems. |
