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We report new analyses of spectra of the $3.2-3.3~\mu$m absorption featureobserved in the diffuse interstellar medium toward three Milky Way sources:2MASS $J17470898-2829561$ (2M1747) and the Quintuplet Cluster, both located inthe Galactic center, and Cygnus OB2-12. The $3.2-3.3~\mu$m interval coincideswith the CH-stretching region for compact polycyclic aromatic hydrocarbons(PAHs). We focus on the 2M1747 spectrum. Its published optical depth spectrumcontains residual telluric transmission features, which arise from the 0.06difference in mean airmasses between the observations of the source and itstelluric standard star. We corrected the published spectrum by adding theairmass residual optical depth spectrum. The corrected spectrum is well fit bya superposition of four Gaussians. The absorption spectra of the other twosources were also fit by four Gaussians, with similar central wavelengths,widths, and relative peak opacities. We associate the three longer wavelengthGaussians covering the $3.23-3.31~\mu$m interval with compact PAHs in positive,neutral, and negative charge states. We identify the shortest wavelengthGaussian, near 3.21 $\mu$m, with irregularly-shaped PAHs. Constraints imposedby spectral smoothness on the corrected 2M1747 spectrum, augmented by a PAHcluster formation model for post-asymptotic giant branch stars, suggests that$> 99$\%\ of the PAHs in the diffuse interstellar medium reside in smallclusters. This study supports the PAH hypothesis, and suggests that a family ofprimarily compact PAHs with a C$_{66}$H$_{20}$ (circumvalene) parent isconsistent with the observed mid-infrared and ultraviolet interstellarabsorption spectrum.

Analysis of the 3.2-3.3 $\mu$m Interstellar Absorption Feature on Three Milky Way Sightlines