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Simulation of interstellar aromatic hydrocarbons using ion cyclotron resonance. preliminary results.
RAPID COMMUNICATIONS IN MASS SPECTROMETRY, VOL. 11
, 1619-1623 (1997)
Simulation of Interstellar Aromatic
Hydrocarbons Using Ion Cyclotron Resonance.
Preliminary Results +
Christine Joblin1*, Christophe Masselon 2, Pierre Boissel3, Patrick de Parseval3, Suzana
Martinovic2 and Jean-françois Muller2
1CESR, Laboratoire du CNRS, BP 4346, 9 Av. du Colonel Roche, 31028 Toulouse Cedex, France
2LSMCL, Université de Metz, Technopole 2000, 1 Bd Arago, 57078 Metz Cedex 3, France
3LPPM, Laboratoire du CNRS, Université Paris Sud, Bât. 213, 91405 Orsay Cedex, France
Using laser microprobe and Fourier transform ion cyclotron resonance (FTICR) mass spectrometry, we have
produced and trapped aromatic species which might well be laboratory analogues of interstellar polycyclic
aromatic hydrocarbons. These species are produced by laser ablation of a sample of pyrolysed coronene. They
are large (up to two hundred carbon atoms) and can be highly dehydrogenated upon irradiation. More or less
condensed forms have also been identified. Although still preliminary, these results open a new field of
investigation: the study of the photophysics and chemistry of these large reactive species in isolation conditions
close to those found in interstellar space. This will be the first objective of the PIRENEA experiment, a
FTICRMS set-up devoted to astrophysics. 1997 by John Wiley & Sons, Ltd.
Received 13 June 1997; Accepted 18 August 1997
Rapid. Commun. Mass Spectrom. 11,
No. of Figures: 6 No. of Tables: 0 No. of Refs: 39
atoms. They might be large planar structures, i.e.
graphitic layers with surrounding hydrogen atoms
The European satellite, Infrared Space Observatory
(more or less dehydrogenated PAHs of large size), or
(ISO), launched in November 1995 is currently provid-
3D graphitic islands such as the so-called ‘basic
ing a large amount of spectral information regarding
structural units' found in coals.14 The inte resting point
interstellar matter. One exciting result concerns a
now is how to produce and study such species in the
family of infrared emission bands at 3.3,6.2,7.7,8.6 and
11.3 ¹m which are easily observed in the regions of high
Dramatic changes were observed at about 600 °C
UV field. The observations of the balloon experiment
when measuring the IR spectra of various gas-phase
AROME1,2 have suggested that these features are
PAHs at high temperatures.15 At this temperature,
present everywhere in our galaxy and in other galaxies,
making their carriers an important and widespread
24H12) was pyrolysed into a pitch-like
brown residue mixed with red needles. Interestingly,
component of interstellar matter. This is beautifully
the IR spectrum of the pyrolysed coronene appeared to
confirmed by the results of the ISO misson. The IR
provide a better match to the astronomical spectra than
features are now well observed in regions where the
did the spectrum of the precursor PAH (Fig. 1).
radiation field intensity differs from the average by
Although it is clear that the sample contains species
orders of magnitude.3-5 This strongly supports an
which might well be analogues of the interstellar
excitation mechanism by transient heating (up to about
species, there are also other products which are of less
1000 K) after absorption of single UV photons.6,7 This
interest. In particular, we arbitrarily subtracted a
mechanism implies that the carriers are small (typically
power-law (¸-l) continuum from the laboratory spec-
1-2 nm in size according to Sellgren7). Certainly,
trum displayed in Fig. 1. This continuum is most likely
classical interstellar grains (0.1 ¹m size) are not able to
dominated by the solid phase but a contribution from
account for this mid-IR emission since they are at an
large aromatic molecules is not excluded and has to be
equilibrium temperature of 10-20 K in most places in
the interstellar medium.
The observed emission bands at 3.3, 6.2, 7.7, 8.6 and
11.3 ¹m are characteristic of hydrogenated aromatic
Analysis of the pyrolysed coronene sample
species. Polycyclic aromatic hydrocarbons PAHs) havenaturally been proposed as their carriers.8,9 However,
Samples of pyrolysed coronene (C24H12) were pre-
no PAH studied to date in the laboratory has provided
pared by sealing under vacuum a quartz tube
a convincing spectral match with the astrophysical
(V = 9 cm3) containing 25 mg of coronene (Aldrich).
spectrum (Fig. 1). Other candidates such as coal grains
The tube was heated in an oven up to 660 °C and then
seem to give a better spectral agreement12,13 but they
cooled down. The heating rate was typically 3°C/min
are too large to account for the emission mechanism.
between 500 and 660 °C.
The emitting species are, therefore, probably inter-
The red needles were identified with an IR micro-
mediate compounds containing hundreds of carbon
scope as crystals of dicoronene (C48H20) suggesting an
efficient polymerization process.15 Lewis16 and Lewis
and Singer17 have studied the pyrolysis products in the
Presented at the Fourth European Workshop on Fourier
liquid phase around 500 °C of small PAHs, anthracene
Transform Mass Spectrometry. Pont-à-Mousson, France,28-30 April 1997
(C14H10) and naphthalene (C10H8). They pr oposed that
* Correspondence to: Christine Joblin
the pyrolysis process is dominated by polymerization
CCC 0951-4198/97/141619-05 $17.50
1997 by John Wiley & Sons, Ltd.
SIMULATION OF INTERSTELLAR AROMATIC HYDROCARBONS
which produces mixtures of polymers with varyingdegrees of condensation.
Analysis of the sample using standard chemical
separation methods18 led to the extraction of coroneneand dicoronene but not of larger oligomers. Tricor-onene and tetracoronene were first observed usingtime-of-flight secondary-ionization mass spectrome-try.19 In these experiments,the species were desorbedwith an infrared CO2 laser and further ionized by a
pulsed UV laser beam. 20 The next goal was clearly to beable to isolate these large molecules and to study theirphysico-chemical properties and spectroscopy in condi-tions close to those found in astronomical environ-
ments, in particular in high vacuum conditions.
In the present paper, we report some experiments
The emission spectrum of the interstellar PAHs observed in
performed with the microprobe laser Fourier transform
the Ml7 object by the European ISO satellite. The spectrum wasmeasured at the interface between a dense molecular cloud and a
ion cyclotron resonance (FTICR) mass spectrometer
highly UV-excited medium produced by surrounding young stars (see
available at the LSMCL (Metz University). This tech-
Ref. 5). The astronomical spectrum is compared with the laboratory
nique is widely used for analytical purposes but
spectrum of: (i) gas-phase coronene measured at 820 K10 and (ii) the
presents further advantages for simulation experiments
solid residue (pitch) obtained by pyrolysis of coronene at 660 °C. The
on interstellar matter. In particular, very reactive
spectrum of pyrolysed coronene was measured in absorption at roomtemperature through a pellet of the solid residue mixed with CsI salt
species can be trapped in the ICR cell for long
and the contribution of large dust particles ( ¸-l continuum) was
durations, which enables further studies of their
subtracted. An emission temperature of 820 K and 500 K was
assumed for coronene and pyrolysed coronene respectively. Thisemission temperature depends on the size of the emitters (cf. Léger etal.11 for details). Note that the 3.3 ¹m band emitted by coronene at
820 K is much more intense than the astronomical feature.
The experimental set-up consists of a modified, differ-entially pumped dual cell FTMS 2000 (Finnigan, SanJose, CA, USA) which is operated in a 3 T magnetic
field and is coupled with a reflection laser interface and
special sample manipulation hardware. Both 266 nm(Nd-YAG laser) and 193 nm (excimer laser chargedwith ArF) wavelengths were used throughout this work.
A focusing telescope allowed the adjustment of thelaser spot diameter on the sample from 5 ¹m to about1 mm, corresponding to a power density ranging from
105 to 109 W cm-2.
All experiments were performed in the source cell at
a 2 V trapping voltage. After a relaxation time of100 ms following the ionization step, the ions wereexcited by a radiofrequency chirp (110 Vp-p). The
excitation and detection bandwidths were typtcally inthe range 1.0 to 2.6 MHz.
2.5 10' W cm
RESULTS AND DISCUSSION
At low irradiance (between 106 and 107 W cm-2), themass spectrum of the pyrolysed coronene sample is
dominated by coronene and its oligomers (Figs 2(a), 3and 4). At higher irradiance new species generatedduring the laser ablation process are observed (Fig.
2(b), 5 and 6).
Building of large PAHs
Whereas dicoronene, tricoronene and tetracoroneneare easily observed, higher mass oligomers (up to theoctamer) have only been clearly observed in some
. Positive ion mass spectra obtained from laser ablation of the
experiments using the 266 nm Nd-YAG laser at an
products of pyrolysis of coronene (C24H12) at 600 °C using (a) an
irradiance of 5 x 106 W cm-2 (Fig. 2(a)). Condensed
irradiance of 5 x 106 W cm-2 at 266 nm and (b) an irradiance of
forms of the trimer and tetramer were identified at m/z
2.5 x 107 W cm-2 at 193 nm. In (a) the mass peaks correspond to the
888 and 1180 (M = 888 and 1180) respectively (Fig. 3).
oligomers of coronene (C24pH8p+4, with p=0,.,8). In (b) even
These species result from the condensation of the most
carbon-number clusters dominate the high-mass range. The close-upview in the C
bent forms (Fig. 4).
84 to C124 range shows that the maxima observed in the
mass distribution of Cn clusters fall close to the masses of fully
Millon et al.21 showed that dicoronene and tricor-
dehydrogenated coronene oligomers.
Rapid Communications in Mass Spectrometry, Vol. 11, 1619-1623 (1997) 1997 by John Wiley & Sons, Ltd.
SIMULATION OF INTERSTELLAR AROMATIC HYDROCARBONS 1621
Laser 193 nm lrradiance
Laser 193 nm lrradiance 2.5 10 W
890.00 9 0 0 . 0 0
1140.0 1160.0 1160.0
A close-up view of the mass peaks corresponding to the coronene oligomers C48H20, C72H28, and C96H36 observed in positive-ion
mode using 193 nm laser ablation. Increasing the irradiance (b) clearly induces molecular dissociation, essentially by loss of pairs ofhydrogen atoms (H2?) but also by C2 loss Note that the condensed forms of the oligomers (Fig. 4) are observed even at low
onene can be formed directly by UV laser ablation of
generated plasma can induce saturation of carbon
pure coronene at low irradiance (106 W cm-2 at
bonds. In particular, the dominant mass 670 is probably
266 nm). However, the observation of higher mass
a hydrogenated derivative of mass 668 obtained by
oligomers as well as the results of the previous studies
addition of three C2 to dicoronene (Fig. 6).
summarized above strongly support the idea that mostof the observed oligomers are present in the initial
Building of large carbon clusters
pyrolysed coronene sample.
On the contrary, at high irradiance (typically
At high irradiance, the high-mass range (m/z 1000 to
2 x 107 W cm-2 at 193 nm), new species appear in the
3000) is dominated by pure carbon clusters. The
mass spectrum which are very probably products of the
observed 24 u spacing is typical of fullerene-like species
chemistry induced by the laser ablation process. In the
which are easily produced by laser ablation of carbona-
range m/z 600 to 700 (Fig. 5), there is strong evidence of
ceous materials (see for instance Refs 21-25). These
accretion of C2 by dicoronene leading to larger PAHs
species were not observed in negative-ion mode, which
(Fig. 6). Starting from dicoronene (M = 596), new
supports the proposal that they are not present in the
hexagonal cycles can be built by addition of C2 (Fig. 6).
initial pyrolysed coronene sample but produced during
The accretion of C2 is best observed in experiments on
the laser ablation process.21 In order to obtain the
pure coronene at high irradiance (108 W cm-2) as
curvature necessary for the formation of fullerene
shown in Fig. 5. In these experiments a high concentra-
cages, five-membered rings are required.26 The forma-
tion of C2 is provided by the dissociation of coronene,
tion of these rings might be related to a growth
e.g. m/z 276 is observed instead of m/z 300 which
mechanism by ion/molecule reactions in the laser-
corresponds to the molecular mass of coronene.
as suggested above (Fig. 6).
The formation of five-membered rings could also be
Interestingly, the size distribution of the carbon
involved. In particular, the series of peaks between m/z
clusters that are produced by ablation of the pyrolysed
602 and 608 (Fig. 5), which differ from dicoronene by
coronene sample (Fig. 2(b)) presents some maxima
only one C atom, can be explained by the formation of
which often fall very close to the carbon content of the
five-membered rings as shown in Fig. 6(d). The dom-
coronene oligomers (72, 96,120, 144, 168, 192 and 216
inance of odd-mass species such as m/z 607,605 and 603
carbon atoms for the trimer up to the nonamer). This is
further indicates that the precursors are not fully
different from the rather flat distribution which has
aromatic. Indeed, for PAHs, only pairs of hydrogen
been previously reported for clusters larger than C70, in
atoms are involved (Fig. 3).
particular in the laser ablation of graphite and of
Finally, hydrogen which is present in the laser-
various small PAHs, including anthracene, chrysene,
1997 by John Wiley & Sons, Ltd. Rapid Communications in Mass Spectrometry, Vol. 11
, 1619-1623 (1997)
1622 SIMULATION OF INTERSTELLAR AROMATIC HYDROCARBONS
pyrene29,30 and coronene21 (see also the present work).
This suggests that the large coronene oligomers couldcontribute to the formation of the observed carbonclusters provided that they are able to lose all theirhydrogen atoms to form PA networks, as was pointedout by Creasy and Brenna.23 The formation of full-erenes by curling up graphitic sheets in a collisionalenvironment was initially proposed by Kroto et al.31 butthis mechanism has not received much experimentalevidence.23 Still, Campbell et al.32 suggested thatfullerene-like species might be formed by rearrange-ment of polyimide ‘pieces'.
Figure 3 illustrates that, at high irradiance, series of
peaks separated by 2 u are indeed observed which arecharacteristic of successively dehydrogenated PAHs.
For example, the peak at m/z 586 (Fig. 3(b)) corre-sponds to dicoronene which has lost ten hydrogenatoms. Larger oligomers also appear to be highlydehydrogenated, although the most dehydrogenatedspecies cannot be identified, because their mass peaksfall within the distribution corresponding to C2 loss. De
Parseval33 has reported the production of C24 by
irradiation of the coronene cation isolated in an ICRtrap; note that cyclocarbon has been shown to beone precursor of fullerenes.34,35 Coronene oligomersmight then be fully dehydrogenated, provided that theyabsorb enough energy. The UV spectra of various PAHmixtures present a broad absorption band at 6 eV.36
The 6.44 eV photons from the excimer laser are
A close-up view of the m/z 600 to 730 range corresponding
to the laser ablation at 193 nm of (a) pyrolysed coronene and (b) pure
therefore probably very efficiently absorbed by these
coronene from Aldrich at an irradiance of 2.5 x 107 W cm-2 and
molecules. After full dehydrogenation, the PA net-
108 W cm-2 respectively . The observed mass peaks are interpreted in
works obtained may rearrange to fullerene cage-like
terms of carbon and hydrogen addition to dicoronene (see Fig. 6).
clusters to saturate their dangling bonds.26,31,32 The
This effect is strengthened in the coronene experiment where the
excited clusters might then relax by evaporation of C
plasma is certainly denser in hydrogen atoms and small carbon
fragments. The spectrum between m/z 717 and 725 in (b) is
fragments37,38 and further grow by addition of C2.39.
contaminated by electronic noise and should not be considered.
Building of larger PAHs from dicoronene. The accretion of
C2 leads to the growing of the graphitic plane (a) and (b) but could
also induce the formation of five-membered rings (c). These rings arelikely to account for the peaks around m/z 607 which can be derivedfrom dicoronene by addition of one carbon (d).
Large aromatic species which might be laboratoryanalogues of interstellar PAHs have been isolated andtrapped in an ICR cell. The experimental apparatus ofthe LSMCL is not equipped with diagnostic tools which
would enable further studies on the properties of the
Coronene and its oligomers. There are three possible forms
trapped species and their IR signatures. Still, the first
of tricoronene, the most bent form giving the condensed form at
encouraging insights into the physical-chemistry of
M = 888. The dotted bonds illustrate the condensation process which
these species have been obtained, viz. facile dehydroge-
is accompanied by hydrogen toss. For tetracoronene, one isomer ofmass 1188 and the condensed form at M = 1180 have been
nation (with possible rearrangement into fullerene-like
clusters), addition of carbon (C2, C) and hydrogen
Rapid Communications in Mass Spectrometry, Vol. 11
, 1619-1623 (1997)
1997 by John Wiley & Sons, Ltd.
SIMULATION OF INTERSTELLAR AROMATIC HYDROCARBONS
atoms. Further studies on these compounds are the first
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