Experimental infrared spectra and theoretical harmonic and anharmonic IR spectra of naphthalene (C₁₀H₈), anthracene (C₁₄H₁₀), and tetracene (C₁₈H₁₂). The dotted lines indicate the characteristic positions of the interstellar AIBs. The experimental spectra are based on matrix-isolated PAHdb data and are reconstructed from transition intensities (A-values) using a Gaussian line profile with a uniform FWHM (=12 cm⁻¹), ensuring consistent broadening across all spectra.