) cations and its Astrophysical Implications*

Polycyclic aromatic hydrocarbons (PAH) have become a prime focus in Astrochemistry ever since the Unidentified InfraRed (UIR) bands were postulated to arise from aromatic molecules.[i] Unified efforts from experimentalist and theoreticians till date to assign these UIR features have only gotten more interesting with the launch of the James Webb Space Telescope (JWST), that has the Mid-Infrared Instrument (MIRI) onboard to deliver the mid-IR spectrum with greater nuances. PAHs in space can exist in either neural or ionic forms, absorb UV photons and undergo fragmentation, producing a rich source of smaller hydrocarbons.[ii] This top-down mechanism of larger PAHs fragmenting into smaller species is of utmost importance in the photo-dissociation regions (PDR) in space.

In this work, we experimentally investigate the photo-fragmentation pathways of two astronomically significant PAH cations and dications – Corannulene (C₂₀H₁₀) and Sumanene (C₂₁H₁₂), that are structural motifs of C₆₀, using a Dye laser integrated to the i-PoP setup[iii] to understand their sequential fragmentation pathways. The photo-fragmentation experiments with these bowl-shaped PAHs exhibit loss channels that are much different from other planar PAHs. The breakdown of carbon skeleton is found to have different pathways for C₂₀H₁₀ and C₂₁H₁₂ because of the number and positioning of pentagon rings, yet the most abundant low mass cations produced by these two species are found to be similar. The low mass cations showcased in this work could be of utmost interest for its astronomical detections. In addition, a theoretical investigation using DFT calculations for the formation and isomerization of the most abundant photo-fragments will also be discussed in this talk.      

 

[i] Tielens, A. G. (2005). The physics and chemistry of the interstellar medium, Cambridge University Press.

[ii] Peeters, E., et al. (2021). "A spectroscopic view on cosmic PAH emission." Accounts of Chemical Research 54(8): 1921-1933.

[iii] Zhen, J., et al. (2014). "Quadrupole ion trap/time-of-flight photo-fragmentation spectrometry of the hexa-peri-hexabenzocoronene (HBC) cation." Chemical Physics Letters 592: 211-216.