INFERENCE OF A 7.75 EV LOWER LIMIT IN THE ULTRAVIOLET PUMPING OF INTERSTELLAR POLYCYCLIC AROMATIC HYDROCARBON CATIONS WITH RESULTING UNIDENTIFIED INFRARED EMISSIONS

The discrete infrared features known as the unidentified infrared (UIR ) bands originating in starburst regions of other galaxies, and in H I I regions and planetary nebulae within the Milky Way, are widely thoug ht to be the result of ultraviolet pumped infrared fluorescence of pol ycyclic aromatic hydrocarbon (PAH) molecules and ions. These UIR emiss ions are estimated to account for 10%-30% of the total energy emitted by galaxies. Laboratory absorption spectra including the vacuum ultrav iolet region, as described in this paper, show a weakening of the inte nsity of absorption features as the population of cations increases, s uggesting that strong pi <-- pi transitions are absent in the spectra of PAH cations. This implies a lower energy bound for ultraviolet pho tons that pump infrared emissions from such ions at 7.75 eV, an amount greater than previously thought. The implications include size and st ructure limitations on the PAH molecules and ions which are apparent c onstituents of the interstellar medium. Also, this might affect estima tions of the population of early-type stars in regions of rapid star f ormation.