Browsing by Author "Izumi Endo"

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C-H Stretch Vibrational Modes: Tracers of Interstellar PAH Geometries?
(American Chemical Society, 2023) Akant Vats; Amit Pathak; Takashi Onaka; Itsuki Sakon; Izumi Endo
Polycyclic aromatic hydrocarbon (PAH) molecules have long been adjudged as carriers of the frequently detected interstellar emission features in the 3-20 μm region. In the present work, PAHs with straight edges having solo-duo (PAHD) and solo-duo-trio (PAHT) C-H modes along with PAHs with irregular edges (PAHI) have been studied theoretically to understand the effect of molecular geometry on the interstellar C-H stretch vibrations at 3.3 μm. The C-H out-of-plane bending vibrations at 11.2 and 12.7 μm are also included for completeness. Using the NASA Ames PAH IR Spectroscopic Database, the mid-infrared spectra have been studied for 125 PAH molecules of varying molecular geometries, sizes, charge states, and symmetries. Results show that the individual solo, duo, and trio C-H stretches follow an order in the peak wavelength (λ3.3 (solo) > λ3.3 (duo) > λ3.3 (trio)) and intensity (I3.3 (solo) < I3.3 (duo) < I3.3 (trio)). If only PAHD’s are considered, the contribution of each charge state is required to account for the observed peak wavelength of the 3.3 μm band, or if only neutrals are contributors, PAHD and PAHT neutrals can explain the 3.3 μm band variations. The observed emission at 11.2 and 12.7 μm is found to match effectively with PAHD with increasing size, and the 11.2 μm band is present at longer wavelengths for PAHT contributing to the red wing. When the solo to duo hydrogen ratio is nearly equal to or greater than 1.0, PAHD neutrals yield better 3.3 μm peak positions. The ratio has a lower limit of 0.8 for the 11.2 μm band and converges at 1.5, indicating a size range of PAHD neutrals with 80 to larger numbers of carbon atoms. The present work examines the presence of solo, duo, and trio modes in the C-H stretching band, which must be taken into consideration when interpreting accurate data from James Webb Space Telescope (JWST) to further explain the observed variations in the interstellar 3.3 μm. © 2023 American Chemical Society.
Theoretical study of infrared spectra of interstellar PAH molecules with N, NH, and NH2incorporation
(Oxford University Press, 2022) Akant Vats; Amit Pathak; Takashi Onaka; Mridusmita Buragohain; Itsuki Sakon; Izumi Endo
This work presents theoretical calculations of infrared spectra of nitrogen (N)-containing polycyclic aromatic hydrocarbon (PAH) molecules with the incorporation of N, NH, and NH2 using density functional theory (DFT). The properties of their vibrational modes in 2-15 μm are investigated in relation to the Unidentified Infrared (UIR) bands. It is found that neutral PAHs, when incorporated with NH2 and N (at inner positions), produce intense infrared bands at 6.2, 7.7, and 8.6 μm that have been normally attributed to ionized PAHs so far. The present results suggest that strong bands at 6.2 and 11.2 μm can arise from the same charge state of some N-containing PAHs, arguing that there might be some N-abundant astronomical regions where the 6.2 to 11.2 μm band ratio is not a direct indicator of the PAHs' ionization. PAHs with NH2 and N inside the carbon structure show the UIR band features characteristic to star-forming regions as well as reflection nebulae (Class A), whereas PAHs with N at the periphery have similar spectra to the UIR bands seen in planetary nebulae and post-AGB stars (Class B). The presence of N atoms at the periphery of a PAH may attract H or H+ to form N-H and N-H2 bonds, exhibiting features near 2.9-3.0 μm, which are not yet observationally detected. The absence of such features in the observations constrains the contribution of NH and NH2 substituted PAHs that could be better tested with concentrated observations in this range. However, PAHs with N without H either at the periphery or inside the carbon structure do not have the abundance constraint due to the absence of 2.9-3.0 μm features and are relevant in terms of positions of the UIR bands. Extensive theoretical and experimental studies are required to obtain deeper insight. © 2021 The Author(s).