The theory that polycyclic aromatic hydrocarbons (PAHs) are a constituent of the interstellar medium, and a source of the IR emission bands at 3.3, 6.2, 7.7, 8.6, and 11.3 microns is being studied using PAH containing acid insoluble residue of the Orgueil CI meteorite and coal tar. FTIR spectra of Orgueil PAH material that has undergone thermal treatment, and a solvent insoluble fraction of coal tar that has been exposed to hydrogen plasma are presented. The UV excided luminescence spectrum of a solvent soluble coal tar film is also shown. Comparison of the lab measurements with observations appears to support the interstellar PAH theory, and shows the process of dehydrogenation expected to take place in the interstellar medium

Boyd, David A.

Flickinger, Gregory C.

Wdowiak, Thomas J.

English

NASA Technical Reports Server

N91- 14936
PAH IN THE LABORATORY AND INTERSTELLAR SPACE
Thomas J. Wdowiak, Gregory C. Flickinger, and David A. Boyd
Physics Department, University of Alabama at Birmingham,
UAB Station, Birmingham, Alabama 35294 USA
ABSTRACT. The hypothesis that polycyclic aromatic hydrocarbons (PAH) are
a constituent of the interstellar medium, and a source of the infrared
emission bands at 3.3, 6.2, 7.7, 8.6, and 11.3 microns is being studied using
PAH containing acid insoluble residue of the Orgueil CI meteorite and coal
tar. FTIR spectra of Orgueil PAH material that has undergone thermal
treatment, and a solvent insoluble fraction of coal tar that has been exposed
to hydrogen plasma are presented. The ultraviolet excited luminescence
spectrum of a solvent soluble coal tar film is also shown. Comparison of the
laboratory measurements with observations appears to support the
interstellar PAH hypothesis, and demonstrates the process of
dehydrogenation expected to take place in the interstellar medium.
The presence of polycyclic aromatic hydrocarbon molecules (PAH) in
the interstellar medium is inferred from the observation of infrared emission
bands at 3.3, 6.2, 7.7, 8.6, and 11.3 microns. (for a review of the field see
"Polycyclic Aromatic Hydrocarbons and Astrophysics:, ed. A. Leger, L.
d'Hendecourt, and N. Boccara, D. Reidel, 1987). Demonstration that this
assignment is correct is important because it would mean PAH is a major
form of interstellar carbon and that PAH molecules could play a role in
charge exchange reactions leading to chemical pathways radically different
from what has been considered previously. The latter situation leads to new
estimates of abundances of interstellar molecules, such as formaldehyde in
dark clouds, that are orders of magnitude greater than previous results.
(Lepp and Dalgarno 1988) The difficulty in making definite the PAH
assignment is that the kinds of PAH samples generally available to the
experimenter from supply houses and laboratory sources are probably not of
the type considered to exist in the interstellar medium. The I 1.3 micron
emission band mandates PAH molecules that have single hydrogen atoms per
aromatic ring (Bellamy 1975, Cohen, Tielens, and Allamandola 1985), while
most laboratory samples have 2, 3, or 4 hydrogen atoms per ring. Also,
interstellar PAH molecules must be composed of >20 carbon atoms in order
to survive the interstellar ultraviolet radiation field (Leger and Puget 1984).
Laboratory experiments with PAH candidates having >20 carbon atoms are
difficult because of low solubility and low volatility. While available pure PAH
samples are useful for exploring basic mechanisms, we have turned our
attention toward PAH in carbonaceous chondrites and the PAH products of
the coking process, pyrolysis, plasma chemistry, and combustion with the
intent of determining if the PAH assignment for the UIR bands is reasonable.
The acid insoluble residue of the Orgueil (CI) carbonaceous chondrite
has been shown to have substantial aromatic content (Cronin, Pizzarello, and
Frye 1987). After vacuum treatment it exhibits an infrared absorption
spectrum similar to that observed in emission from the Orion Nebula
(Wdowiak, Flickinger, and Cronin, 1988). The Orion emission spectrum in
the 5 to 14 micron region and the KBr pellet absorption spectrum of Orgueil
163
https://ntrs.nasa.gov/search.jsp?R=19910005623 2020-03-19T19:34:48+00:00Z
acid insoluble residue after vacuum heating to 500C are shown in Figure 1.
The Orgueil FTIR spectrum has had a continuum subtracted to remove the
effect of scattering in the KBr pellet, revealing the maximum of the broad
absorption "hump" is at -7.8 microns in addition to the peaks at 6.2 and
11.3 microns. The coincidence of the wavelength of the heated treated
residue "hump" maximum and the observed 7.7 micron emission band is
interesting.
The Orgueil acid insoluble residue has other interesting IR spectral
features besides those shown in Figure 1. Figure 2 shows the heat treated
Orgueil acid insoluble residue has absorption bands at 16 and 19.6 microns
(625 cm-1 and 510 cm-1). These features are intrinsic to the residue and
are evident in untreated material. They are probably due to bending of the
PAH aromatic structure and are of interest as a candidate for the cause of the
broad 25 micron band emission observed with IRAS (Hauser et al.. 1984;
Lynch et al., 1988). Because of the absence of a I0 micron (I000 cm-I) band
one can conclude the 16 micron (625 cm-I) and 19.6 micron (510 cm-1)
bands are not due to silicate material that has escapedbeing dissolved by HF.
Sandford and Walker (1985) have published an IR spectrum of an
interplanetary dust particle (IDP "Conehead") having bands at 16.8 micron
(595 cm-I) and 23.1 micron (433 cm-I) attributed to an hydrous A1203 or
Fe203. The Orgueil 16 micron (625 cm-l) and 19.6 micron (512 cm-I)
absorption features could therefore be due to microgram amounts of mineral
in the milligram amount of sample residue.
g
m A _ OFIGUEIL j
mic ron
ORGUEIL
r'%. o
micron
Fig. 1 Orion Bar (Allamandola et al. 1987)
and heat treated Orguefl PAH residue.
Fig. 20rgueil heat treated PAH residue
a-11.3, b-16 and c- 1916 micron bands.
164
Coal tar prepared by coking coal at 1400 K in the absence of air is
utilized as a PAH mixture that is an analog to interstellar PAH mixtures. Bulk
coal tar mixed with acetone was filtered and the filtrate evaporated
afterwards to concentrate a solvent soluble fraction used to prepare air dried
films. As the solvent is evaporated away a dramatic spectral shift of -2000A
from blue to orange-red occurs in the near ultra-violet excited luminescence.
The solvent soluble fraction coated on a substrate as a film and excited with
ultraviolet radiation exhibits luminescence having a spectral signature in the
5500A to 7500A range similar to that of the Red Rectangle and other dusty
regions (Figure 3). This result suggests the Red Rectangle emission is due to
excited-dimer (excimer) emission of PAH molecules clumped together
(Wdowiak 1986). In the excimer process which requires adjacent PAH
molecules (as in the film), an excited dimer is formed from an excited singlet
molecule and an unexcited one. This is followed by a radiative transition
emitting light at longer wavelengths than single molecules would, followed by
rapid dissociation into monomers. Duley and Williams (1988) have
demonstrated PAH clumps would have an infrared emission spectrum similar
to that of free molecules, making the excimer process plausible for the
source of the optical emission of objects such as the Red Rectangle which
also exhibits IR band emission (Russell, Softer, and Willner 1978).
T | • • | • • • I
Z
©
o9
o9
|
4000 S000 6000 A 7000
Fig. 3 Luminescence of 3070A excited coal tar in ethyl alcohol (I) and as a film on
quartz (2) corrected for R928 PMT sensitivity shown with the spectrum of the Red
Rectangle (Schmidt, Cohen, and Morgan 1980).
165
The coal tar residue obtained in the filtering process was washed
repeatedly with benzene to remove soluble material and then air dried prior
to pressing it into KBr pellets or subjecting it to further processing. The
solvent insoluble fraction of coal tar has infrared spectral characteristics
similar, but not identical to that of the heat treated acid insoluble residue of
the Orgueil meteorite. In particular the presence of 3.3 micron (3030 cm -I)
absorption and other features longward of 11.3 micron (885 cm -1) suggests
it is more hydrogenated. An experiment was performed to try to simulate in
a gross, but rapid manner the effect of H atom, H ion, and ultraviolet
exposure on interstellar PAH. The benzene insoluble coal tar residue was
placed in a boat, and subjected to a microwave excited hydrogen plasma at a
pressure of 1.5 Torr for 35 minutes. The processing reduced the strength of
the 13.4 micron (746 cm -I) band, probably associated with aromatic rings
having 3 or 4 adjacent hydrogens per ring, relative to the 11.3 micron (885
cm -I) band associated with I hydrogen per ring (Figure 4). This experiment
may demonstrate why the 11.3 micron interstellar emission band dominates
the 10 to 15 micron region, by simulating the dehydrogenation expected to
take place in the interstellar environment.
g
BEFORE
AFTER
I I I !
II 12 13 14
micron
Fig. 4 FTIR spectra in the 10 to 15 micron region of the solvent insoluble fraction of
coal tar before and after being subjected to a hydrogen plasma environment.
166
Comparison of the laboratory measurements discussed here and
observations indicates the linking of the IR emission bands to a PAH source is
not inconsistent with our laboratory experience. However a laboratory analog
that exhibits precisely all of the spectral features of a source such as the Red
Rectangle (HD 44179) still eludes the laboratory worker and is troubling. It
may be that the PAH species have a special character, or that there is
stratification of various populations of PAH species in a unique manner in the
source of IR emission and no single analog or analog system will suffice. Still
to be exploited are the structured features superimposed upon the Red
Rectangle 5500A - 7500A emission "hump" (Schmidt, Cohen, and Margon
1980, Warren-Smith, Scarrott, and Murdin 1981). Those features may be
due to simple radicals or ions trapped in relatively inert ices such as CO or
N2 (Wdowiak 1981) or are the signatures of complex molecules. Perhaps
even stimulated emission may be involved. What is need are observations of
the Red Rectangle quality of other similar sources. This work was supported
by NASA grant NAGW-749.
Allamandola, L.J., Tielens, A.G.G.M., and Barker, J.R.: 1987 in Polycyclic
Aromatic Hydrocarbons and Astrophysics (ed. Leger, A., d'Hendecourt,
L., and Boccaro, N, D. Reidel).
Bellamy, L.J.: 1985, The Infra-red Spectra of Complex Molecules (Chapman
and Hail, London).
Cohen, M., Tielens, A.G.G.M., and Allamandola, L.J.: 1985, Ap.J. 299, 193.
Cronin, J.IL, Pizzarello, S., and Frye, J.S.: 1987, Geochim. Cosmoschim Acta
5 I, 299.
Duley, W.W. and Williams, D.A.: 1988, M.N.ILA.S. 231,969.
Hauser, M.G. et al.: 1984, Ap.j. 278, L 115.
Leger, A. and Puget, L.L.: 1984, Astron.. Astrophys. 137, L5.
Lepp. S. al Dalgarno, A.: 1988, Ap.J. 324, 553.
Lynch, D.K., Hack'well, J.A., Wesselius, P., Oiling, R., and Isreal, F.P.: 1988,
Bull. Am. Astron. soc. 20, 732.
Sandford, S_A. and Walker, ILM.: 1985, Ap.J. 291, 838.
Schmidt, G.D., Cohen, M., and Margon, B.: 1980, Ap.J. 239, L133.
Russell, R.W., Soifer, B.T., and Williams, S.P.: 1978, Ap.J. 220, 568.
Warren-Smith, R.F., Scarrott, S.M., and Murdin, P.: 1981, Nature 292, 317.
Wdowiak, T.J.: 1981, Nature 293, 724.
Wdowiak, T.J.: 1986, in Interrelationships Among Circumstellar,
Interstellar, and Interplanetary Dust (NASA CP 2403), A41.
Wdowiak, T.J., Flickinger, G.G., and Cronin, J.R.: 1988, Ap.J. 328, 175.
167



PAH in the laboratory and interstellar space

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