Spectroscopic Studies of Large PAH Molecules.

Although the spectroscopic studies of PAHs carried out to date have provided an important "proof of concept" that free molecular PAHs can account for the interstellar infrared emission features, the current database is limited almost exclusively to PAHs having fewer than about 30 C atoms. The size of the typical interstellar PAH, on the other hand, is likely to be much larger. For example, the infrared spectra of PAH cations - now believed to be the main PAH form in the most energetic emission zones - usually have a strong feature in the vicinity of, but somewhat redshifted from, the interstellar 6.2 µm emission band. However, as illustrated in the figure below, the position of this feature is dependant on the size of the PAH carrier, steadily decreasing in wavelength with increasing molecular size. Extrapolation of this trend indicates that PAHs in the 40 to 80 carbon atom size range are consistent with the position of the observed interstellar feature at 6.2 µm, indicating that PAHs of this size dominate the emission at these wavelengths.

Figure. A Stick representation of the spectra between 5.9-8.3 µm (1700 - 1200 cm-1) for three different sized PAHs in our spectral database. The position of the canonical 6.2 µm (1610 cm-1) and 7.7µm (1310 cm-1) interstellar features are indicated by the vertical shaded regions.

Thus, while the studies to date do provide valuable insight into the general spectroscopic characteristics of PAHs, it is difficult to assess how these characteristics extrapolate to larger molecules. What are the spectroscopic properties of really large molecular PAHs? To what extent do the spectroscopic properties of PAHs vary with increasing molecular size? What trends are there in these variations which could be used to probe such molecular properties as structure and stability? To what extent can one reasonably infer the nature of a population of large molecular PAHs armed only with spectroscopic data from a collection of smaller molecules? These are the questions we hope to address by extending our laboratory studies to significantly larger PAH species. Our current inventory of large PAHs is shown in the table below, and we are always on the lookout for new samples of such rare and precious species.

The Astrochemistry Lab's Inventory of Large PAHs.

 
Structure
Formula
    
Structure
Formula
1
C78H26
  
8
C44H20
2
C66H26
  
9
C42H18
3
C60H22
  
10
C42H18
4
C50H22
  
11
C42H22
5
C48H22
  
12
C40H18
6
C48H20
  
13
C40H18
7
C46H20
  
14
C38H16

PAH samples 1, 2, and 3 were provided by Prof. Dr. K. Muellen of the Max Planck Institute for Polymer Research.

PAH samples 4 - 14 were purchased from Prof. W. Schmidt of the PAH Research Institute, Greiffenberg am Ammersee, Germany.

Updated January 2001.