The effect of ionization on the infrared absorption spectra of PAHs: A preliminary report

The emission lines observed in many interstellar IR sources at 3.28, 6.2, 7.7, 8.7, and 11.3 microns are theorized to originate from polycyclic aromatic hydrocarbons (PAHs). These assignments are based on analyses of lab IR spectra of neutral PAHs. However, it is likely that in the interstellar medium that PAHs are ionized, i.e., are positively charged. Besides, as pointed out by Allamandola et al., although the IR emission band spectrum resembles what one might expect from a mixture of PAHs, it does not match in details such as frequency, band profile, or relative intensities predicted from the absorption spectra of any known PAH molecule. One source of more information to test the PAH theory is ab initio molecular orbital theory. It can be used to compute, from first principles, the geometries, vibrational frequencies, and vibrational intensities for model PAH compounds which are difficult to study in the lab. The Gaussian 86 computer program was used to determine the effect of ionization on the infrared absorption spectra of several small PAHs: naphthalene and anthracene. A preliminary report is presented of the results of these calculations

Carbon in the Galaxy: Studies from Earth and Space

Presented here is the text of the invited papers presented during a meeting entitled, Carbon in the Galaxy: Studies from Earth and Space, that was held at NASA Ames Research Center on November 5 and 6, 1987. For completeness, abstracts from all of the poster papers and the text of a paper summarizing what was learned during the course of the meeting are also included. The underlying premise for the meeting was that there is much to be gained by bringing together scientists from very different disciplines, all of whom study carbon in different ways for different reasons. The interchanges took place during the meeting and the contents of the enclosed papers validate that premise