Matrix isolation and molecular beam gas-phase IR absorption spectra of naphthalene, anthracene, tetracene, and pentacene in red and black, respectively. The MIS spectra of naphthalene and anthracene were obtained from Roser & Allamandola (2010); Roser et al. (2014), while the MIS spectra of tetracene and pentacene were obtained from the NASA AmesPAH spectral database Bauschlicher et al. (2018). Predicted IR spectra using the harmonic and anharmonic approximation are in blue and green, respectively. The harmonic spectra were obtained from the NASA Ames PAH spectral database (Bauschlicher et al. 2018).