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ABSORPTION SPECTROSCOPY
OF POLYCYCLIC AROMATIC
HYDROCARBONS

A short review written by Jon Gingell specifically for this website

PAH structures
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  What are Polycyclic Aromatic Hydrocarbons (PAHs)?

Briefly, polycyclic aromatic hydrocarbons are molecules built up of benzene rings which resemble fragments of single layers of graphite.  They have planar structures and come in a wide variety of shapes and sizes.


  Some simple PAHs Some typical PAH structures.
 

On Earth these compounds are quite common, being formed during incomplete combustion of almost any kind of organic material hence they are continually being released into the environment [1].  They are present in coal extracts, internal combustion engine exhaust fumes, cigarette smoke, soil, marine sediments, soot, smoke from wood burning, and even fried or grilled food!  Since some of the PAHs are known carcinogens [2] their presence in the environment (particularly in food) is a cause of concern to health authorities.
 

  How might PAHs be relevant to Astrophysics?

Astronomical observations of the interstellar medium (ISM) have revealed three main sources of mystery [3]:

  1. Unidentified Infra-Red emission bands (UIRs) have been observed at the same wavelengths (i.e. 3.3, 6.2, 7.7, 8.6 and 11.3 microns) along many lines of sight.  These are molecular in origin, but the molecules concerned are unknown.
  2. Diffuse Interstellar Bands (DIBs) are absorption bands observed in the visible part of the spectrum at the same wavelengths (numerous) along many lines of sight.  Again, these are molecular in origin although the nature of the molecules remains unknown.
  3. The UV extinction bump is an intense absorption feature in the UV centred at ~220 nm (5.6 eV) along many different lines of sight; again, the cause of the bump is unknown.

Recently, the PAHs have been shown to be good candidates for each of these observations. Therefore, it has been proposed that PAHs are present in the interstellar medium (ISM) in relatively high abundances (see, for example [4, 5]). However, although the PAHs seem to be good candidates to explain some of these phenomena, not one single PAH molecule has yet been identified in the ISM.  Current thought is that if PAHs are present, they may be in modified form, e.g. ionised or protonated [6].
 

  How could PAHs form in space?

Two main mechanisms have been proposed to account for the possible formation of PAHs in the ISM.  First, collisions between dust grains (thought to consist of graphite and/or silicates) could fracture the graphite planes thus releasing free PAHs [7].  Secondly, PAHs could grow from reactions between smaller unsaturated hydrocarbon molecules and radicals in the remnants of carbon rich stars [8].  Once formed the PAHs would be remarkably stable, and would resist dissociation from UV absorption (unlike most other polyatomic molecules in the ISM) since they are extremely efficient at rapidly re-emitting the absorbed energy at infra-red wavelengths [7].
 

  Electronic Spectroscopy of PAHs

The UV photoabsorption spectroscopy of the PAHs is also known to be related to that of benzene since the delocalised pi-orbitals are dominant, hence they have very strong absorption bands between 4-7 eV.  These bands occur in the region of the astronomically observed UV extinction bump (~220 nm » 5.6 eV), hence it is possible that PAHs may be the cause of this mysterious astronomical feature [9].  Therefore, the accurate measurement of photoabsorption spectra of the PAHs has become an experimental goal in recent years.  However, reliable measurements of absolute cross-sections remain sparse due to the difficulties involved in handling PAH samples.  For this reason, members of the Centre for Cosmic Chemistry and Physics have attempted to measure accurate cross-sections of some simple PAHs in conjunction with the Molecular Physics Laboratory at UCL.
 

  Experimental Apparatus

VUV photoabsorption spectra of some small PAHs were recorded , using a photoabsorption apparatus attached to the synchrotron light source at the UK Daresbury Laboratory.  Details of this instrument may be found in [10].
 

Experimental apparatus
                Experimental apparatus used to record photoabsorption cross-sections of PAHs
 

Briefly, the synchrotron radiation enters and leaves an absorption cell through LiF windows.  Transmitted radiation passes into a photon-monitoring section for detection with a photomultiplier tube. The absorption path length of the cell is 16 cm, and the spectral range available for this experiment extended from about 5 eV (250 nm) to the LiF cut-off at 11.2 eV (110 nm). This was covered in sections of about 20 nm, each scanned in steps of 0.05 nm.

At each wavelength in the limited (20 nm) spectral range, the radiation transmitted through the sample, the sample pressure, and the synchrotron beam ring current were recorded. The sample cell was then evacuated, and the radiation transmitted through the empty cell was measured along with the ring current across the same spectral range. The transmitted radiation was normalised to a constant ring current before analysis using the Beer-Lambert law.
 

  Results

Several different PAHs were investigated using the spectrometer with a variable degree of success.  The main difficulty experienced was that the PAHs, being solids at room temperature, had very low vapour pressures hence it was difficult to obtain sufficient sample pressures within the gas cell.  When the samples were heated to increase the vapour pressure, the hot vapours evolved from the crystals were found to condense within the cell and subsequently resisted attempts to remove them.  However, despite these problems reliable data were obtained for some of the smaller compounds.
 
 

                                         The simple PAHs for which data have been obtained
 

The spectra obtained for these compounds are shown below; absolute cross-sections were obtained for all of the compounds except anthracene, for which relative cross-sections were obtained. Due to difficulties in measuring low pressures accurately within the gas cell, the estimated error in the cross-sections recorded for these molecules was ±30%. Some of the data shown below have been published [11].
 
 

                        VUV photoabsorption spectra of some simple PAHs and related compounds
 

The shapes of the observed spectra are in agreement with those of previous measurements (where available: naphthalene [12]; anthracene [13]).  The spectra of naphthalene, 2-methyl naphthalene and anthracene were all observed to have strong absorption bands around 5-6 eV; this was in the energy region of the astronomically observed UV extinction bump, hence these molecules may be contributors to that feature.  The strong bands represented transitions between the p-orbitals of the molecules, and were reminiscent of the benzenoid 1E1u transition.  However, the spectrum of fluorene was observed to be different to the others, showing that absorption spectra of the PAHs are sensitive to the presence of non-aromatic groups. Nevertheless, although the overall shape of the fluorene spectrum was different, it still showed strong absorptions in the region of the UV extinction bump hence it may be present in the interstellar medium.  Fluorene is particularly interesting, since its structure can be thought of as a fragment of a fullerene molecule, C60, popularly known as a "buckyball".
 

  Conclusion

The data recorded for naphthalene in this work are in good agreement with previous measurements, giving confidence in the data recorded for the other PAHs studied.  This suggests that reliable photoabsorption spectra could be obtained for other small PAHs using this apparatus in the future.  However, to obtain data for the larger species would require modifications to the gas cell to allow the entire system to be heated to increase the vapour pressures of the samples without their condensation inside the spectrometer.  Designs for such a device are currently being contrived so that further investigation into the spectroscopy of the PAHs can be performed to provide more data of astrophysical interest.
 

  References

[1]    W. Schmidt, in Polycyclic Aromatic Hydrocarbons and Astrophysics, eds. A. Leger et al (D. Reidel Publishing Co., Dortrecht, 1987) pp. 149-164.
[2]    H. Busch (ed.), The Molecular Biology of Cancer (Academic, New York, 1974).
[3]    D.A. Williams and S.D. Turner, Q. J. Roy. Astron. Soc. 37 (1996) 565.
[4]    L.J. Allamandola, A.G.G.M. Tielens and J.R. Barker, Astrophys. J. 290 (1985) L25.
[5]    D.J. Cook et al, Nature 380 #21 (1996) 227.
[6]    D.M. Hudgins, S.A. Sandford and L.J. Allamandola, J. Phys. Chem. 98 (1994) 4243.
[7]    A. Leger and L. D'Hendecourt, in Polycyclic Aromatic Hydrocarbons and Astrophysics, eds. A. Leger et al (D. Reidel Publishing Co., Dortrecht, 1987) pp. 223-254.
[8]    D.A. Howe, J.M.C. Rawlings and D.A. Williams, Adv. At. Mol. Opt. Phys. 32 (1994) 187.
[9]    C. Joblin, A. Leger and P. Martin, Astrophys. J. 393 (1992) L29.
[10]  N.J. Mason, J.M. Gingell, J.A. Davies, H. Zhao, I.C. Walker and M.R.F. Siggel, J. Phys. B 29 (1996) 3075.
[11]  J.M. Gingell, Faraday Discussion 109 (1998).
[12]  F. Salama and L.J. Allamandola, Astrophys. J. 395 (1992) 301.
[13]  T. Kitagawa, J. Mol. Spectrosc. 26 (1968) 1.

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