Molecular beam gas-phase IR absorption spectra of (a) naphthalene, (b) anthracene, (c) tetracene and (d) pentacene (black). Predicted IR spectra using the harmonic and anharmonic approximation are given as blueand green traces, respectively. The harmonic spectra have been obtained from the NASA Ames PAH spectral database (Bauschlicher et al. 2018). The predicted spectra are multiplied by a factor five, truncating the strongest bands to enable a better visualization of the low-intensity bands (normalized spectra can be found in the appendix).