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Context. As revealed by high-resolution spectral investigations in thewavelength range between 300 and 400 nm, the interstellar extinction curve doesnot display any of the sharp electronic absorption bands that arecharacteristic for large polyatomic molecules, such as polycyclic aromatichydrocarbons (PAHs), which belong to the most abundant interstellar molecules.Aims. We aim to verify whether the absorption curves of mixtures ofmedium-sized PAHs produced in the laboratory can explain the astronomicalobservations. Methods. The PAH mixtures were synthesized by infrared laserpyrolysis and subsequent chemical extraction and size separation. The matrixisolation technique was used to study the absorption spectra of isolatedmolecules at low temperature. Results. Our experimental results demonstratethat the UV-visible absorption curves of PAH mixtures can be very smooth,displaying no sharp bands, if the molecular diversity is sufficiently high.Conclusions. In view of the absence of sharp electronic features on theinterstellar extinction curve for 300 < {\lambda} < 400 nm, we conclude fromour experimental findings that the interstellar PAH population must be verydiverse. The low fractional abundances of individual species prevent theirdetection on the basis of spectral fingerprints in the UV.

The smoothness of the interstellar extinction curve in the UV. Comparison with recent laboratory measurements of PAH mixtures