Chemical Physics 269 (2001) 347±355
www.elsevier.nl/locate/chemphys
Diagnosis of a benzene discharge with a
mass-selective spectroscopic technique
Felix G
uthe *, Hongbin Ding, Thomas Pino, John P. Maier
Institute for Physical Chemistry, University of Basel, Klingelbergstrasse 80, CH-4056 Basel, Switzerland
Received 22 December 2000
Abstract
The products formed in a benzene discharge have been probed by resonant two photon ionization spectroscopy. The
molecules formed are cooled in a supersonic expansion and mass-analysis combined with spectroscopic information is
used to identify the species. By this means styrene, phenylacetylene, methylstyrene, indene, ¯uorene and tolane are
recognized. No six-ring polycyclic aromatic hydrocarbon could be detected. With this method the plasma is sampled at
an early stage where the ethynylated and cyclopentafused polycyclic aromatic hydrocarbons are more abundant than
the more stable six-ring ones. The coupling of a discharge source to a REMPI detection system enables electronic
spectra of neutral molecules to be obtained which can be used to study their role in combustion processes, plasmas as
well as in astrophysics. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction
Research on carbon and hydrocarbon molecules has expanded in the last decade due to their
role in combustion processes and interstellar media. In laboratory experiments various discharge
sources and ¯ames have been investigated. Experiments with a laser ablation source led to the
discovery of the C60 -molecule [1]. Kroto et al. [2]
noted the important implications of laboratory
experiments for the existence of carbon species in
the interstellar medium.
Related experiments have been performed in
connection with combustion research. Homann
and coworkers have studied the composition of
¯ames focusing on positive ions [3], negative ions
*
Corresponding author. Fax: +41-61-267-3855.
E-mail address: [email protected] (F. G
uthe).
‡
[4] and neutrals [5±7]. They found Cn H‡
3 and Cn H5
ions to be formed in the oxidation zone of an
acetylene ¯ame [8] as well as larger clusters containing more hydrogen which where attributed to
polycyclic aromatic hydrocarbon cations (PAH‡ )
based on mass spectrometry. To study the neutrals
they used UV radiation between 208 and 300 nm
to ionize the produced species in multiphoton
processes. However they did not record electronic
spectra that could be used for the identi®cation of
the isomers of the selected masses. It has been
believed that it is not possible to record such
spectra from ¯ames because of the large number of
di€erent PAHs present. An assignment to some
molecular structures has been achieved by gas
chromatography (GC) from an aqueous solution
of the residuals of the burning process [9]. Three
classes of hydrocarbons were identi®ed in this
study by comparison with authentic substances:
aliphatic chains, aromatics with side chains and
0301-0104/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 1 - 0 1 0 4 ( 0 1 ) 0 0 3 1 9 - 6
348
F. Guthe et al. / Chemical Physics 269 (2001) 347±355
condensed PAHs. It was pointed out that the side
chains prefer only a few structures, namely the
methyl, vinyl, ethynyl and butadiynyl groups.
Another strategy to identify the molecules
in situ is to use spectroscopic techniques. This
approach also has a problem with spectral assignments due to severe overlap if mass separation
is not available. Most spectroscopic gas phase
work on discharges or with laser ablation sources
used direct absorption methods. Examples include
infrared spectra of pure carbon chains [10,11],
microwave spectroscopy on polar hydrocarbon
chain molecules [12,13] and electronic spectroscopy for both [14,15].
In the UV a large number of aromatic Cn Hm
species has been detected by laser induced ¯uorescence (LIF), or by resonant enhanced 2 photon
ionization spectroscopy (R2PI) [16±18] which is a
mass selective technique. Such studies often use
single precursor molecules which have to be vaporized in a heated source and are subsequently
cooled again by a supersonic expansion. The R2PI
technique can also be used for on-line analysis of
PAHs in emission of industrial combustion processes [19]. Thus it is clear that for a certain assignment of molecules a hyphenation of di€erent
methods is desirable [20].
The aim of this study is to extend mass spectrometric studies of discharges by recording concurrently electronic spectra and thus obtaining an
unambiguous assignment of the mass peak to
molecular structure(s). In the approach introduced
here a number of molecules produced in a discharge can be measured simultaneously. If the
spectral patterns can be understood and assigned,
this is an ideal and sensitive tool to characterize a
discharge source. This method can also be applied
for analytical purposes in the characterization of
combustion processes. This has been shown in the
identi®cation of the products of reactions of excited butadiyne and benzene or toluene by R2PI
spectroscopy [21].
2. Experimental
The instrument built to record the electronic
spectra of mass-selected clusters produced in dis-
Fig. 1. Schematic diagram of the experimental setup.
charge sources is shown in Fig. 1. The source is
similar to that used for anions in our group [22],
based on a design of Ohshima and Endo [23]. A
pulse of a gas mixture of 0.25±0.5% benzene in Ar
at a backing pressure of 5±10 bar is expanded
through the ceramic body which holds two steel
electrodes with a 1 mm hole in the middle separated by a spacer of 2±10 mm. The gas ¯ow is
computer controlled adjusting the opening time of
the pulsed nozzle to keep the pressure in the expansion chamber constant to ensure stable source
conditions. A 100±200 ls long high voltage pulse
(600±1000 V) from a homebuilt pulse generator is
applied between the electrodes. The current is
limited to 100±200 mA. The emerging Cn Hm cluster beam is shaped in an extender and enters the
ionization region through a 2 mm skimmer. Ions
are removed by an electrical ®eld perpendicular to
the beam before entering the mass spectrometer.
The neutral beam is then ionized by the R2PI
method. The cations are extracted in a two-stage
acceleration setup towards an MCP detector
within a Wiley±McLaren type time-of-¯ight spectrometer [24]. The signal from this detector is
sampled by a fast oscilloscope and transferred to a
computer. The mass resolution R50% of the instrument is 900. Gates are set on the mass
spectrum and the ion signal is recorded versus the
F. Guthe et al. / Chemical Physics 269 (2001) 347±355
349
wavelength enabling the scan of a large number of
masses independently. Up to 140 gates have been
used concurrently.
To record the R2PI spectra the frequency
doubled output of a dye-laser (bandwidth ' 0:15
cm 1 ) pumped by the second harmonic of a
Nd:YAG laser is scanned. The output of an excimer laser running on 193 nm with ArF, or 157 nm
with F2 , or the second (532 nm), third (355 nm), or
fourth (266 nm) harmonics of Nd:YAG laser are
used for ionization. The relative intensities of the
peaks in the spectra are dicult to compare because di€erent dyes have to be used to cover the
range between 278 and 320 nm. No additional
calibration was carried out and therefore the precision for wavelengths of the transitions is estimated to be 6 4 cm 1 . However, an internal
calibration is often achieved by comparison with
known spectra.
3. Results
3.1. Mass spectrometry
The mass spectra reveal that with the discharge a rich mass distribution with species larger
than the precursor evolves. Fragments with masses
smaller than benzene are relatively minor. The
mass distribution also depends on the wavelength
used for the ionization (Fig. 2). The spectrum
observed using an F2 -excimer laser (157 nm) (trace
a) is mostly governed by a one photon process and
shows an enhanced ion yield for species with an
ionization potential (IP) <7.9 eV. The relative intensities in the mass spectrum do not change signi®cantly if the laser power density is varied
between 2 kW/cm2 and '2 MW/cm2 . Species
corresponding to the masses of Cn H3 and Cn H5 to
Cn H10 , with a maximum at Cn H7 for n ˆ 7 up to at
least n ˆ 14 appear. Assuming not too di€erent
ionization probabilities and no signi®cant fragmentation, the Cn H7 group appears to be the most
abundant. Even±odd carbon atom alternation is
not observed.
The third harmonic of a Nd:YAG laser (355
nm) was used for ionization in the recording of
trace c in Fig. 2. The laser power had to be in-
Fig. 2. Mass spectra of the neutrals produced in a benzene
discharge recorded with di€erent ionization wavelengths: (a)
157 nm, (b) scanned resonantly 278±290 nm, (c) 355 nm slightly
focused.
creased to 55 mJ per pulse and the laser was
focused with a 250 mm lens (50 MW/cm2 ). The
mass spectral pattern di€ers signi®cantly from the
other spectra: the hydrogenated molecules seen in
Fig. 2a and b are not ionized by this wavelength.
However a number bare carbon clusters (C2 ±C5 )
are present which are not seen in the other spectra.
The same e€ect can be seen more clearly in butadiyne discharges [25], where clusters up to C20
could be observed. The origin of the strong peaks
m=z ˆ 52 and 54 is not clear, though they might be
oil contaminants from the di€usion pump of the
expansion chamber.
The mass spectrum in Fig. 2b was obtained by
integrating the ion signals during a scan of the
laser wavelength from 278 to 290 nm with K 1 mJ
350
F. Guthe et al. / Chemical Physics 269 (2001) 347±355
per pulse ( K 3 MW/cm2 ) and shows only the
species that have an electronic state which is excited resonantly. The peaks Cn H8 for n ˆ 8±13 and
Cn H10 for n ˆ 13±14 are the strongest. These
molecules exhibit strong R2PI electronic spectra in
this region (see Section 3.2).
These traces show typical distributions within
the range of the given parameters. A variation of
these parameters results in instability of the source
or vanishing signal rather than a change of the
relative intensities. For example a richer mixture
(>0.5% benzene/Ar) leads to increased soot formation and signal instability. If the voltage is
higher than 1200 V or a higher current is applied
the signal disappears completely.
It is dicult to derive a conclusion on the
structure of the species present from its mass
alone. The speci®city of R2PI is limited to species
with UV transitions that lie energetically above
half of the IP. Still the most abundant species (i.e.
Cn H7 ) seem not to be detected by this method although they are likely to be aromatic. A similar
looking mass spectrum derived from samples
which have been exposed to interstellar UV radiation on a satellite has recently been observed [26].
Without further evidence a number of the observed masses have been assigned to six-ring
PAHs.
photon ¯uorescence excitation spectroscopy [28].
The R2PI spectra have been recorded in a molecular beam [29,30]. High resolution ¯uorescence
excitation spectroscopy [31] yielded a value of
35 877.18 cm 1 for the position of the 000 band. In
Fig. 3a the measured R2PI spectrum recorded in a
benzene discharge for mass 102 is shown. It can be
clearly identi®ed as that of phenylacetylene.
The R2PI spectrum observed for mass 104
(C8 H8 ) in the range from the origin at 34 760 cm 1
to about 36 000 cm 1 is shown in Fig. 3b. The
spectrum can be identi®ed as that of styrene from
the published LIF [32] and R2PI [33] spectra of the
S1
S0 transition. This spectrum is the strongest of all observed species. The origin band has
been measured in high resolution and a value of
35 758.79 cm 1 has been derived [31]. The features
of the styrene spectrum can also be observed
3.2. Spectroscopic results
The R2PI spectra of about 20 molecules were
recorded in the 278±320 nm range. The molecules which could be identi®ed are focused on.
The species identi®ed spectroscopically are: phenylacetylene (m=z ˆ 102 C8 H6 ); styrene (m=z ˆ
104 C8 H8 ); indene (m=z ˆ 116 C9 H8 ); methylstyrene (m=z ˆ 118 C9 H10 ); ¯uorene (m=z ˆ
166 C13 H10 ) and tolane (m=z ˆ 178 C14 H10 ).
The spectrum observed on the mass 128 (C10 H8 ) is
not that of naphthalene, a common PAH that
could be expected to be formed in a discharge. For
a number of molecules with masses di€erent from
those of the common PAHs new electronic spectra
were observed.
A gas phase spectrum at room temperature of
phenylacetylene m=z ˆ 102 (C8 H6 ) has been observed in direct absorption [27] as well as by two
Fig. 3. R2PI spectra from a benzene discharge. (a) Phenylacetylene, (b) styrene. The assignment and labeling of the
strongest peaks is adopted from Ref. [32]. (c) Indene [35], (d)
tolane [43]. The origin band in the electronic spectrum of phenanthrene lies in this range but is absent [45].
F. Guthe et al. / Chemical Physics 269 (2001) 347±355
weakly on the product channel of mass 78 ˆ C6 H‡
6
for energies above 35 000 cm 1 with a laser power
1±2 mJ. The IP of styrene is 8.46 eV and
the benzene cation (C6 H‡
6 ) has been measured to
be the lowest (metastable) fragmentation channel
from styrene with an appearance energy of 12.3 eV
[34]. Probably the formation of this fragment results from a 3-photon absorption and the reaction
is fast enough to be an important exit channel only
at about 13 eV (3 35 000 cm 1 ).
In Fig. 3c the spectrum recorded on the mass
116 (C9 H8 ) is shown. Based on the known spectrum of the S1
S0 transition [35] indene is
identi®ed as the carrier. On the other hand the
signal obtained on mass 118 (C9 H10 Fig. 4a) was
weak and more dicult to obtain than for the
other species. However, a number of the observed
Fig. 4. R2PI spectra from a benzene discharge. (a) Methylstyrene. The assignment and labeling of the strongest peaks is
adopted from Ref. [36]. (b) Recorded on m=z ˆ 128. The indicated band positions for naphthalene are taken from Ref. [37]
but are absent in the spectrum. (c) Fluorene [39].
351
S0 system of
bands could be assigned to the S1
trans-b-methylstyrene with an origin at 34 586
cm 1 [36]. Another transition to the red is observed at 33 577 cm 1 which probably belongs to
an isomer.
The spectrum recorded on the mass 128 (C10 H8 )
(Fig. 4b) is not that of naphthalene, the smallest
PAH, which could be expected to be formed in a
discharge from benzene. Comparison with the
S1
S0 R2PI spectrum [37,38] shows that naphthalene can be excluded as the carrier of the observed band system. Thus naphthalene is not
abundant in the discharge source. The positions
for the prominent bands in Fig. 4b are 1 (33 580
cm 1 ), 2 (33 831 cm 1 ), 3 (34 511 cm 1 ) and 4
(35 372 cm 1 ). Possible candidates are discussed in
Section 4.
The spectrum recorded on mass 166 (C13 H10 ) is
shown in Fig. 4c and the carrier could be identi®ed
as ¯uorene by comparison with the ¯uorescence
spectra of the S1
S0 transition [39±41]. Simulation of the pro®le of the origin band around 34 140
cm 1 using the given spectroscopic constants [41,
42] indicates the rotational temperature of the
molecular beam to be 20 K. In the latter paper
the appearance of small bands to the red of the
origin was attributed to hot bands which arise
from incomplete cooling of a low frequency mode.
The intensity of these bands in the present spectrum is similar and therefore it can be concluded
that also the vibrational temperature of the molecules formed in the pulsed discharge source is
comparable to that with a pulsed heated valve
where the pure precursor is introduced. The assignment of the peaks in Fig. 4c by symmetry labels of the C2v point group is according to the
literature.
Mass 178 (C14 H10 ) is known from the investigation of ¯ames and is usually assigned to anthracene or phenanthrene which are stable PAH
structures containing six rings only. However,
neither of these molecules were detected. Instead a
congested spectrum (Fig. 3d) is identi®ed as the
S3
S0 and S4
S0 transition of tolane based on
the analysis of Okuyama et al. [43]. In this spectrum the high number of transitions arises from
the fact that two di€erent electronic states B1u and
B2u (D2h symmetry) have close lying origins and
352
F. Guthe et al. / Chemical Physics 269 (2001) 347±355
both form progressions of torsional vibration
modes. These torsional modes have u symmetry
and therefore only double excitations are allowed
(t20 ˆ 96 and 85 cm 1 ). By comparison with the
analyzed spectrum a good estimate on the vibrational temperature of the molecules formed in the
discharge source can be derived. The spectrum in
Fig. 3d appears to be colder than the spectrum of
Okuyama et al. recorded at an estimated temperature of 19 K.
In Fig. 3d the position of the origin of the
S2
S0 transition of phenanthrene at 35 375 cm 1
[44±46] is indicated. The transition is not visible in
this spectrum. Anthracene cannot be excluded by
this direct spectroscopic method because it does
not absorb in this wavelength range.
4. Discussion and conclusion
In addition to mass spectrometry, which is often
used to characterize ¯ames, discharges or other
hydrocarbon samples, the in situ recording of the
electronic spectra of the constituents of known
mass has been used to analyze a benzene discharge. The mass spectrometric data show a strong
dependence of the produced ions on the wavelength of the ionizing laser. If a 157 nm wavelength
is used a large number of species with an IP below
7.9 eV are observed with a maximum for Cn H7 ,
while for 355 nm only pure carbon clusters Cn
(probably chains) are apparent. The resonance
enhanced spectra show a di€erent distribution and
reveal the existence of substituted benzenes (phenylacetylene, styrene and methyl styrene). As in
the work of Bockhorn et al. [9] only a few side
chains are clearly observed and identi®ed up to
now: ethynyl (C2 H) and vinyl (C2 H3 ). The methyl
and butadiynyl substituent (C4 H) could not be
assigned here yet, but the CH3 group appears in
form of the propenyl (CH@CHCH3 ) attached to a
vinyl group.
The presence of trans-b-methylstyrene (C9 H10 )
is proven although the spectrum is the weakest of
all presented. It is surprising because the ring disubstituted isomers (methylstyrene) would be expected to be present. The cis and trans isomers of
the m-substituted isomer have been observed by
LIF spectroscopy [47] and are absent. The amethylated isomer and a cis isomer are also possible for the monosubstituted isomers. The R2PI
spectrum of a-methylstyrene has been measured
[48] and shows absorptions in the range studied
here. However, the transitions are not discernible.
The spectroscopy of cis-b-methylstyrene has not
been investigated. The structure of this cis isomer
suggests that it could close the second ring to form
indene and is therefore removed from the sampled
plasma.
The carrier of the spectrum recorded on mass
128 (C10 H8 ) in Fig. 4b cannot be unambiguously
identi®ed. Naphthalene can be excluded from the
absence of its transitions. Azulene is another possible carrier and its spectrum has been measured
by LIF [49]. The band systems to the origins of the
S3 and S4 states lie in the studied range but all the
bands are broad and appear at di€erent positions.
Therefore, it is probable that the carrier is a substituted benzene. Because only closed shell species
have been identi®ed here the carrier of the transitions in Fig. 4b is likely to be one as well. The
plausible molecules are the disubstituted o-, m- or
p-ethynylvinylbenzenes or the monosubstituted 4phenyl-1-buten-3-yne and cis and trans 1-phenylbuten-3-yne.
Electronic spectra of these compounds are not
available for a direct comparison. However an
analogy can be drawn to an investigation of the
reaction of excited butadiyne with toluene [21].
There a product with mass 116 was shown to be a
5:1 mixture of o- and p-ethynyltoluene, with only
a minor contribution from the m isomer. Thus the
spectrum shown in Fig. 4b is most likely a mixture
of o- and p-ethynylvinylbenzene. The oscillator
strength …f † for the transitions of these compounds can only be estimated. Assuming that f is
between the value for styrene (S1
S0 , f ˆ 0:02
[50]) and phenylacetylene (S1
S0 , f ˆ 0:0005
[27]) the relative abundance of naphthalene
(S1
S0 , f ˆ 0:003 [51]) in this discharge must be
at least one order of magnitude lower, considering
the fact that no transitions could be detected.
A similar comparison can be made for the ratio
of phenanthrene and tolane, where both oscillator
strengths are relatively high. For phenanthrene f
has been found to be 0.2 for the S2
S0 transition
F. Guthe et al. / Chemical Physics 269 (2001) 347±355
[52]. In the case of tolane the oscillator strength for
the whole absorption band (S3 , S4
S0 ) around
290 nm has been determined to be 0.82 in solution
[53]. Therefore, the single origin band of phenanthrene, which carries most of the oscillator
strength should be prominent compared to the
tolane features which spread out over many bands.
It then follows that phenanthrene is not very
abundant in this discharge source.
Apart from the substituted benzenes (phenylacetylene, styrene, methylstyrene, ethynylvinylbenzene) and tolane, two PAH containing ®ve
membered rings (indene and ¯uorene) have been
identi®ed in the benzene discharge. The more stable six-ring PAHs are apparently not formed in
high abundance. The conditions used were varied
over large range of parameters (pressure, discharge
voltage, timing) but no signi®cant di€erences could
be seen in the mass distribution. The absence of
naphthalene and phenanthrene is somewhat surprising, because similar species to the ones detected here were identi®ed in ¯ames except that
these two species were found to be abundant [9]. A
possible reason for this di€erence apart from the
di€erent source is the analysis of the formed
compounds. In the latter work a GC analysis was
carried out after collection of the samples in a trap.
In the extraction process a number of reactive
species might have isomerized to more stable
compounds while direct sampling and cooling in a
molecular beam followed by spectroscopic identi®cation might leave the more reactive species intact. The similarity between our discharge and the
¯ame sources used in other studies can only be
inferred by the similarity of the distribution of the
identi®ed species and the insensitivity to variation
of discharge parameters.
The species identi®ed in this work are known as
intermediates in several chemical models for
combustion. The cyclopentafused PAH (CP-PAH)
have been proposed to be intermediates in fullerene formation [54]. The buildup of larger carbon
species is suggested to go through ethynyl-PAHs
(E-PAH), formed by C2 and C2 H addition and
their rearrangement to CP-PAH [55,56]. In another mechanism for the growth of PAHs molecular structures similar to the substituted benzenes
found here were considered as intermediates [57].
353
Thus the direct sampling of the benzene discharge
in the molecular beam re¯ects the pyrolysis process
in an early stage, where the most stable isomers
have not yet been formed. Their formation might
occur at later stage at higher temperatures as has
been shown by ¯ash vacuum thermolysis [55].
The formation of the neutral species seen in the
spectra can be assumed to occur by two stages in
analogy to cation chemistry known from electron
impact work in high pressure sources [58]: ®rst
is the formation of aliphatic primary fragments
(Cn Hm‡;; ; n, m < 6) [59] in the electric discharge
(Eq. (1)) followed by formation of aromatic secondary fragments by substitution reactions to
the aromatic ring (Eq. (2)). If the substitution
mechanism is of radical (SR ˆ neutral fragment),
electrophilic (SE ˆ cationic fragment) or nucleophilic (SN ˆ anionic fragment) nature cannot be
determined here. The formed E-PAHs and CPPAHs (Eq. (3)) are intermediates, which are
quenched in the supersonic expansion of the molecular beam and are detected here (Eq. (4)).
C6 H6 ‡ EI ! Cn Hm‡;; ;
aliphatic primary fragments n; m < 6
…1†
C6 H6 ‡ Cn H‡;;
! Cn Hm‡;; ;
m
aromatic secondary products n > 6
…2†
Cn Hm‡;; ; n > 6 ! E-PAH ! CP-PAH
…3†
E-PAH ‡ CP-PAH !
quenching by supersonic cooling
…4†
The presented results give an interesting view to
the processes occurring in a benzene discharge and
can be regarded as a model system for hydrocarbon ¯ames. The similarity to related work on
¯ames is emphasized. However the spectroscopic
investigation clearly suggest that care has to be
taken in interpreting mass spectra without further
information. This work shows the results obtained
using an instrument combining a plasma source
with the REMPI technique and focuses only on
the species with known spectra. Future work will
need to involve theory and spectroscopic characterization of the various isomers to identify the
354
F. Guthe et al. / Chemical Physics 269 (2001) 347±355
other species. The wavelength of these could help
to identify important intermediates to give a deeper insight into discharge and combustion processes.
Acknowledgements
The authors would like to thank Danielle Furio
(LPPM, Orsay France) for her help in developing
the labview programs. This work has been supported by the Swiss National Science Foundation
no 20-55284.98.
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