INFERENCE OF A 7.75 EV LOWER LIMIT IN THE ULTRAVIOLET PUMPING OF INTERSTELLAR POLYCYCLIC AROMATIC HYDROCARBON CATIONS WITH RESULTING UNIDENTIFIED INFRARED EMISSIONS
Ms. Robinson et al., INFERENCE OF A 7.75 EV LOWER LIMIT IN THE ULTRAVIOLET PUMPING OF INTERSTELLAR POLYCYCLIC AROMATIC HYDROCARBON CATIONS WITH RESULTING UNIDENTIFIED INFRARED EMISSIONS, The Astrophysical journal, 474(1), 1997, pp. 474-478
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.