C-H Stretch Vibrational Modes: Tracers of Interstellar PAH Geometries?

Polycyclic aromatic hydrocarbon (PAH) molecules havelong beenadjudged as carriers of the frequently detected interstellar emissionfeatures in the 3-20 & mu;m region. In the present work,PAHs with straight edges having solo-duo (PAH(D))and solo-duo-trio (PAH(T)) C-H modesalong with PAHs with irregular edges (PAH(I)) have been studiedtheoretically to understand the effect of molecular geometry on theinterstellar C-H stretch vibrations at 3.3 & mu;m. The C-Hout-of-plane bending vibrations at 11.2 and 12.7 & mu;m are alsoincluded for completeness. Using the NASA Ames PAH IR SpectroscopicDatabase, the mid-infrared spectra have been studied for125 PAH molecules of varying molecular geometries, sizes, charge states,and symmetries. Results show that the individual solo, duo, and trioC-H stretches follow an order in the peak wavelength (& lambda;(3.3 (solo)) > & lambda;(3.3 (duo)) >& lambda;(3.3 (trio))) and intensity (I (3.3 (solo)) < I (3.3 (duo)) < I (3.3 (trio))). If only PAH(D)'s areconsidered, the contribution of each charge state is required to accountfor the observed peak wavelength of the 3.3 & mu;m band, or if onlyneutrals are contributors, PAH(D) and PAH(T) neutralscan explain the 3.3 & mu;m band variations. The observed emissionat 11.2 and 12.7 & mu;m is found to match effectively with PAH(D) with increasing size, and the 11.2 & mu;m band is presentat longer wavelengths for PAH(T) contributing to the redwing. When the solo to duo hydrogen ratio is nearly equal to or greaterthan 1.0, PAH(D) neutrals yield better 3.3 & mu;m peakpositions. The ratio has a lower limit of 0.8 for the 11.2 & mu;mband and converges at 1.5, indicating a size range of PAH(D) neutrals with 80 to larger numbers of carbon atoms. The presentwork examines the presence of solo, duo, and trio modes in the C-Hstretching band, which must be taken into consideration when interpretingaccurate data from James Webb Space Telescope (JWST) to further explain the observed variations in the interstellar 3.3 & mu;m.