Electronic spectroscopy of polycyclic aromatic hydrocarbons (PAHs) at low temperature in the gas phase and in helium droplets

Article abstract of DOI:10.1016/j.molstruc.2005.10.019

The electronic spectroscopy of polycyclic aromatic hydrocarbons (PAHs) is of particular interest for astrophysicists since these molecules are suggested as possible carriers of the diffuse interstellar bands. Here we report on our study of the S₁←S₀ transition of 2,3-benzofluorene (or benzo[b]fluorene) in the gas phase and in He droplets using cavity ring-down spectroscopy in a supersonic jet expansion and molecular beam depletion spectroscopy, respectively. The same techniques were also employed to identify the PAHs contained in the carbonaceous powder which was produced by CO₂ laser pyrolysis of a mixture of ethylene and benzene vapor. Phenanthrene and anthracene were unambiguously confirmed as components of the as-synthesized soot.

Full text of DOI:10.1016/j.molstruc.2005.10.019

Journal of Molecular Structure 786 (2006) 105–111
www.elsevier.com/locate/molstruc
Electronic spectroscopy of polycyclic aromatic hydrocarbons (PAHs)
at low temperature in the gas phase and in helium droplets
a,b
A. Staicu , S. Krasnokutski , G. Rouill e´ , Th. Henning , F. Huisken
a
a
b
a,b,
*
a
Friedrich-Schiller-Universit a¨ t Jena, Institut f u¨ r Festk o¨ rperphysik, Helmholtzweg 3, D-07743 Jena, Germany
b
Max-Planck-Institut f u¨ r Astronomie, K o¨ nigstuhl 17, D-69117 Heidelberg, Germany
Received 25 July 2005; received in revised form 10 October 2005; accepted 13 October 2005
Available online 19 December 2005
Abstract
The electronic spectroscopy of polycyclic aromatic hydrocarbons (PAHs) is of particular interest for astrophysicists since these molecules are
suggested as possible carriers of the diffuse interstellar bands. Here we report on our study of the S )S transition of 2,3-benzofluorene (or
1
0
benzo[b]fluorene) in the gas phase and in He droplets using cavity ring-down spectroscopy in a supersonic jet expansion and molecular beam
depletion spectroscopy, respectively. The same techniques were also employed to identify the PAHs contained in the carbonaceous powder which
2
was produced by CO laser pyrolysis of a mixture of ethylene and benzene vapor. Phenanthrene and anthracene were unambiguously confirmed as
components of the as-synthesized soot.
q 2005 Elsevier B.V. All rights reserved.
Keywords: Polycyclic aromatic hydrocarbons; PAHs; Diffuse interstellar bands; DIBs; Cavity ring-down spectroscopy; CRDS; Helium nanodroplets; Molecular
beam depletion spectroscopy; MBDS
1
. Introduction
studied in the expansion of a supersonic jet using the highly
sensitive photoabsorption technique called cavity ring-down
spectroscopy (CRDS). In this technique, the decay of the
radiation from a pulsed tunable laser is measured at the exit of a
high-quality resonator containing the molecules to be studied.
The decay time can be directly converted to an absorption cross
section. So far, with this technique, we have investigated the
neutral PAHs anthracene [3] and pyrene [4] as well as the
cations of naphthalene and anthracene [5]. Electronic CRDS
studies on PAHs carried out by other researchers in supersonic
jets include the cations of naphthalene [6,7,8], acenaphthene
[7,8], and pyrene [8], as well as the neutrals of perylene [9] and
benzo[ghi]perylene [10].
Neutral and ionized polycyclic aromatic hydrocarbons
PAHs) have been postulated to be present in the interstellar
(
medium [1] and to account for some of the so-called diffuse
interstellar bands (DIBs) [2], still unassigned astrophysical
absorption features observed in the spectra of reddened stars.
Although discovered in the twenties of the last century, they
still represent one of the mysteries in astronomy. Despite the
importance of this class of molecules for astrophysics and
nanophysics (PAHs can be regarded as nanoscale fragments of
a sheet of graphite), the spectroscopic characterization of PAHs
under well-defined conditions has remained a challenge. To
improve this situation and to provide more laboratory data that
can be compared with astronomical observations, we have
initiated a research program to study the electronic spec-
troscopy of PAH molecules under astrophysically relevant
conditions, i.e. at low temperature and so low density that the
molecules do not interact. For this purpose, the molecules are
These true gas phase studies are complemented by
absorption experiments of PAH molecules embedded in liquid
helium droplets. In contrast to conventional matrix spec-
troscopy in argon or neon matrices, which suffers from the fact
that the electronic absorption bands are significantly broadened
and shifted with respect to the gas phase bands, the helium
droplet provides a medium in which the interaction between
the molecule and the rare gas atoms is minimized [11]. There is
still a matrix effect, but the shifts are generally much smaller
and the band widths much narrower. On the other hand, some
advantages of matrix spectroscopy are kept, for example the
possibility to collect the molecules for a longer time so that not
so high vapor pressures are required as in gas phase
*
Corresponding author. Address: Friedrich-Schiller-Universit a¨ t Jena, Insti-
r Festk o¨ rperphysik, Helmholtzweg 3, D-07743 Jena, Germany. Tel.: C49
641 947 354; Fax: C49 3641 947 308.
tut fu
¨
3
E-mail address: friedrich.huisken@uni-jena.de (F. Huisken).
0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2005.10.019

106
A. Staicu et al. / Journal of Molecular Structure 786 (2006) 105–111
experiments. Moreover, the helium droplets provide a constant
extremely low temperature of 380 mK for all degrees of
freedom. The PAH spectral studies in He droplets are
performed by molecular beam depletion spectroscopy
by two plano-concave mirrors with a radius of curvature of
250 mm. The cavity length is 420 mm. In the present
experiment, three different sets of mirrors were used. Their
reflectances were 99.84% at 361 nm (Layertec GmbH),
99.76% at 330 nm (Layertec GmbH), and 99.5% at 283 nm
(Laseroptik GmbH). The light transmitted by the rear mirror of
the cavity was detected by a photomultiplier tube (Hamamatsu
H6780-02) and its output signal digitized by a fast oscilloscope
(Tektronix TDS 3052). Typically, at each wavelength, the
signal was averaged over 64 laser shots, and the resulting decay
function was transferred to a computer processing the signals
and calculating the losses per pass. The wavelength calibration
of the dye laser was carried out by using the absorption lines of
metastable Ne observed by the optogalvanic effect in a Fe–Ne
hollow cathode lamp (Hamamatsu L233-26NU). All wave-
lengths are given for vacuum. The laser linewidth is estimated
(
MBDS) or laser induced fluorescence (LIF) spectroscopy.
Up to now, the PAH molecules that we have investigated in He
droplets include anthracene [12,13], tetracene [12], and pyrene
[
4]. Using the same technique, Toennies and co-workers
studied the PAHs tetracene and pentacene [11]. Very recently,
emission spectra of tetracene, pentacene, and perylene in
helium droplets were reported by Lehnig and Slenczka [14].
In this paper, we present our preliminary results on another
PAH molecule, 2,3-benzofluorene or benzo[b]fluorene (Bzf,
C H ), that we have studied both in supersonic jet expansions
1
7 12
and in helium droplets. The S )S transition was investigated
1
0
in the spectral range from 322 to 335 nm, which contains the
origin band and vibrational bands with energies smaller than
K1
to be 0.1 cm in the visible [3].
K1
200 cm . Neither gas phase nor helium droplet studies of
1
Bzf in this spectral range were available up to now.
2.2. Spectroscopy in helium droplets
Besides commercially available PAHs, we have also
investigated the carbonaceous powder that was synthesized
by CO2 laser pyrolysis of a 2:1 mixture of ethylene and
benzene vapor. The resulting soot was expected to contain a
large amount of PAHs [15] as could be verified by FTIR
spectroscopy of the extract obtained by dissolving the soot in
toluene and evaporating the solvent [16]. Moreover, UV
spectra taken from the toluene extract dissolved in hexane
suggested that the 3-ring PAHs phenanthrene and anthracene
should be major components [16]. In order to confirm this
finding, we have investigated the soot as well as the toluene
extract using both spectroscopic techniques discussed before,
i.e. cavity ring-down and helium droplet spectroscopy. The
present investigation has been focused on the detection of the
isomers phenanthrene and anthracene (C H ) as components
The experiments devoted to the investigation of the
absorption properties of PAHs in helium droplets were carried
out in a molecular beam machine that has been described
elsewhere [4,13]. Large helium clusters containing on the
average Nz8000 helium atoms were produced by supersonic
expansion of pure helium gas (Messer Griesheim GmbH,
purity 99.9999%) at high pressure (pZ20 bar) through a 5-mm-
diameter pinhole nozzle, cooled by liquid helium. The
temperature of the nozzle was kept constant at TZ13 K and
stabilized with an accuracy of better than 0.05 K. Since the
helium clusters are liquid they are also referred to as droplets or
nanodroplets. After preparation, the beam of helium nano-
droplets was directed through a heatable cell (oven) containing
3
1
4
10
approximately 1 cm of the PAH sample. Upon collisions with
of the samples.
the helium droplets, the sample molecules were embedded into
the interior of the droplets and carried by them to the mass
spectrometer detector. Before entering the detector chamber,
the chromophore-containing helium cluster beam interacted
with the pulsed radiation of a tunable dye laser.
2
. Experimental
2
.1. Cavity ring-down laser absorption spectroscopy
In case of resonance, the laser radiation is absorbed by the
PAH molecule in the helium droplet. Two processes are
competing in the relaxation of the excitation energy:
photoemission (fluorescence) and transfer of the electronic
and/or rovibrational energy to the helium cluster. The transfer
of the excitation energy to the helium cluster induces the
evaporation of a few hundred to thousand helium atoms. This
evaporation results in a broadening of the angular distribution
of the cluster beam and in a reduction of the ionization cross
section, both giving rise to a reduction of the molecular beam
signal measured with the mass spectrometer detector on the
mass of the PAH molecule. The latter technique, termed
molecular beam depletion spectroscopy (MBDS), is exclu-
sively used in the present study.
The experimental setup has been described in detail
elsewhere [3]. The PAH source consists of a pulsed 1-mm-
diameter pinhole nozzle driven by a Series 9 valve supplied by
Parker Hannifin Corporation (General Valve Division).
The sample is contained in a reservoir located inside the
valve and close to the nozzle, and can be heated up to 500 8C to
provide a sufficiently high vapor pressure for the PAH under
study. In almost all experiments, argon (Linde, purity
9
9.999%) is used as carrier gas.
The source is placed into a vacuum chamber onto which the
cavity ring-down spectrometer is mounted. It is based on the
original setup described by O’Keefe and Deacon [17]. The
laser source consists of a pulsed tunable dye laser (Continuum
TDL 60) pumped by the second harmonic (lZ532 nm) of a
Nd:YAG laser (Quantel YG-581-20) operated at a repetition
rate of 20 Hz. The dye laser beam, frequency-doubled by a
KDP crystal, is coupled into the CRDS cavity, which is formed
The laser system used for MBDS consists of a tunable dye
laser (Lambda Physik SCANMATE 1) pumped by the second
harmonic of a 10-Hz pulsed Nd:YAG laser (Continuum model
NY81). The dye laser was operated with either DCM or

A. Staicu et al. / Journal of Molecular Structure 786 (2006) 105–111
107
0
K1
state. They are A Z0.0516080 cm , B Z0.0089539 cm
and C Z0.00764095 cm
0
K1
,
for the excited state. The RCIS
Rhodamine 6G, and its output was doubled in a KDP crystal.
K1
0
K1
The laser linewidth was 0.24 cm
before doubling. The
maximum pulse energy was around 500 mJ in the UV. The
wavelength calibration was performed as in the CRDS
experiment.
calculation also yielded the projections of the transition dipole
moment on the principal axis of inertia, indicating that the
vibrationless transition is 81% A-type. Rotational contour
simulations of the S (0))S (0) transition were carried out
1
0
with the SpecView program of Stakhursky and Miller [21]. The
best simulation is obtained for a rotational temperature of 12 K
and is shown in Fig. 1 by the dashed line. The band center of
the S (0))S (0) transition, as determined while optimizing
3
. Results and discussion
3
.1. 2,3-benzofluorene
1
0
K1
the simulated profile, is 29 894.3 cm
.
The molecular structure of 2,3-benzofluorene (Bzf) is
The origin band measured in He droplets is shown in Fig. 2
together with the gas phase spectrum just discussed. It is noted
K1
that the He droplet spectrum is shifted by 5.1 cm to higher
depicted in Fig. 1. It is characterized by 1C2 benzene rings
with a five-fold ring in between giving rise to a bent structure
with only one symmetry element (except of the identity), a
plane of symmetry. Neither gas phase nor helium droplet
spectra were available for this molecule. However, we could
resort to argon matrix isolation studies [18,19] which were
helpful to locate the origin of the S )S transition in the gas
phase.
Fig. 1 shows the S ( A ))S ( A ) origin band measured by
CRDS in the expansion of a supersonic jet, 4.6 mm down-
stream from the nozzle exit (solid line). The valve was operated
at a temperature of 170 8C, and Ar was used as carrier gas
wave numbers. Although broadened by the phonon wing, the
band observed in helium droplets comes close to the gas phase
K1
data (FWHM is 1.6 cm
K1
for the gas phase spectrum and
2 cm for the phonon wing-broadened helium droplet band).
1
1
0
Compared to other PAHs that we have studied in helium
droplets (e.g. anthracene and tetracene [13]), in the case of Bzf,
the splitting of the zero phonon line shows multiple peaks
1
0
1
0
1
0
K1
separated by 1.5–2.3 cm . This splitting can be explained by
a coupling with the vibrational modes of the first closed shell of
He atoms surrounding the Bzf molecule.
Fig. 3 presents the Bzf spectra that we have recorded in the
(pArZ2.5 bar). The structure in the band profile corresponds to
P and R rotational branches. The S and S states of Bzf
0
1
K1
0
wave number range from 29 800 to 31 000 cm . The gas
phase spectrum measured for a nozzle temperature of 200 8C is
transform according to the A irreducible representation of the
C point group. Therefore the vibrationless transition between
s
displayed in the bottom panel. The bands at higher energies
K1
these two electronic states has to show a hybrid A- and B-type
rotational profile. Rotational constants were determined for the
equilibrium geometry of both electronic states while carrying
out ab initio calculations with the GaussianW03 package [20].
The configurations of Bzf in the S and S states were tightly
than the origin band at 29 894.3 cm
belong to vibronic
transitions from the vZ0 level of the ground state to excited
vibrational levels in the S state.
1
High-resolution matrix spectra of Bzf were first reported by
Nakhimovsky et al. [22] using a n-hexane matrix at 5 K. For
each band, the spectra showed a multiplet structure caused by
the various orientations of the embedded molecule. The newer
high-resolution spectra of Bzf obtained by Geigle and
0
1
optimized in the frozen core approximation, at the RHF/6-31C
G(d,p) and RCIS/6-31CG(d,p) levels, respectively. The
K1
calculated rotational constants are AZ0.0528082 cm
,
K1
K1
BZ0.0089092 cm , and CZ0.0076339 cm for the ground
1
0
1
0
Fig. 1. The S ( A ))S ( A ) origin band of 2,3-benzofluorene in the gas phase
1 0
as obtained by CRDS (solid line). The dashed line corresponds to a numerical
simulation of the rotational contour. A rotational temperature of 12 K is
estimated at 4.6 mm distance from the nozzle exit.
1 0
Fig. 2. The origin band of the S )S transition of 2,3-benzofluorene in He
droplets obtained by MBDS and shown in comparison with the gas phase
spectrum.

108
A. Staicu et al. / Journal of Molecular Structure 786 (2006) 105–111
Table 1
6
5
1
0
1
0
Parameters of the origin band of the S ( A ))S ( A ) transition of 2,3-
1 0
benzofluorene observed in a supersonic jet and various matrix materials
He droplets
Ar matrix
Condition
Band position
K1
Shift
Width (FWHM)
K1
K1
4
3
2
1
0
[cm
]
[cm
]
[cm
]
a
Supersonic jet
a
He droplets
29 894.3
29 899.4
29 661
0
1.6
12
5
C5.1
K233
Ar matrix with site
b
selection
Conventional Ar
c
matrix
29 647
29 304
K247
K590
71
d
n-hexane matrix
w170
a
This work.
4
3
2
000 Gas phase
b
Geigle and Hohlneicher [18].
Banisaukas et al. [19].
c
d
000
000
Nakhimovsky et al. [22].
Using Gaussian ab initio calculations carried out at the same
level as mentioned above, we have determined the energies of
the fundamental vibrations in the electronically excited S1
state. After scaling the values with the factor 0.9, the
1000
corresponding transition frequencies from the S vibrational
0
0
ground state were derived. They are given in Fig. 3 by the
vertical lines plotted below the gas phase spectrum. Only the
totally symmetric modes were taken into account since
transitions to non-totally symmetric levels are not allowed
within the Franck-Condon approximation. Using the MolFC
program developed by Borrelli [23], we made an attempt to
determine the intensities of the vibronic bands, but the
agreement between experiment and calculation was rather
poor. Indeed, as has been pointed out by Geigle and
Hohlneicher [18], due to the vibronic coupling between S1
and higher electronic states, Herzberg-Teller interferences
must be taken into account to correctly predict the band
intensities. Advanced calculations are in progress to properly
assign the vibronic transitions observed in the present study
2
9800
30200
30600
31000
-
1
Wavenumber (cm )
1 0
Fig. 3. S )S absorption spectra of 2,3-benzofluorene measured in the gas
phase (supersonic jet) by CRDS (bottom panel) and in helium droplets by
MBDS (upper panel). The conventional Ar matrix spectrum plotted in the
upper panel was adopted from Banisaukas et al. [19]. The vertical lines given
below the gas phase spectrum mark the frequency positions obtained from ab
initio calculations.
Hohlneicher [18] at 14 K in an Ar matrix were free of multiplet
structures as they used site-selective laser excitation/fluor-
escence spectroscopy. As far as the vibronic band intensities
and their positions relative to the origin band are concerned,
their excitation spectrum resembles very much our gas phase
spectrum. However, the vibronic bands are approximately
three times broader and the entire spectrum is shifted by
[24].
Some of the bands including the origin band have also been
measured in helium droplets employing the molecular beam
depletion technique with the mass spectrometer tuned to the
mass of Bzf (mZ216 amu). The results are plotted in the upper
frame of Fig. 3. It is important to note that the bands measured
in helium droplets have not been shifted in frequency. In
contrast to other PAH molecules studied before, it appears that,
for Bzf, the frequency shifts induced by the helium matrix are
K1
approximately 240 cm to the red. Although, compared to the
n-hexane matrix, the red shift was reduced by a factor of more
than 2, it is still quite pronounced. The most recent Ar matrix
isolation study was carried out by Banisaukas et al. [19] using a
conventional UV spectrometer. Concentrating on the spectral
range discussed here, we adopted their spectrum and included
K1
very small (blue shifts between 3.2 and 6.5 cm ).
K1
it in the upper panel of Fig. 3 (after it was shifted by 246 cm
to the blue). It is clearly seen that all spectral features of the Ar
matrix spectrum find their counterparts in the gas phase
K1
spectrum. However, with widths around 70 cm , the bands
K1
are much broader than observed in the gas phase (w1.6 cm
and also in the earlier site-selective matrix experiment
Discussing frequency shifts for different molecules as a
function of the matrix material is not an easy task since the total
shift is composed of two contributions, an attractive and a
repulsive term. Obviously an increase of the attraction
experienced by the molecule and the surrounding matrix
during the transition results in a red shift. On the other hand, a
stronger repulsion between the molecule and the matrix after
the excitation will produce a blue shift. It can be predicted that
the change in attraction is proportional to the polarizability. In
case of electronic transitions, the change of the repulsive term
is not predictable. It depends on the electronic structure and on
what electrons are involved in the transition. For an argon
)
K1
(
the argon matrix can be given to be 233.3 cm
w5 cm ) [18]. With our study, the red shift produced by
K1
[18] or
47.1 cm [19], depending on what matrix data is taken for
K1
2
comparison. Table 1 compares the band parameters observed in
the supersonic jet with those measured in the various matrix
materials discussed.

A. Staicu et al. / Journal of Molecular Structure 786 (2006) 105–111
109
matrix, the attraction is in almost all cases the dominant term
yielding a pronounced red shift. In contrast, in helium droplets,
the attractive contribution is much smaller so that the repulsive
term may prevail, finally resulting in a small blue shift.
Apparently, this situation is encountered in the case of 2,3-
benzofluorene.
6
0
50
40
30
1
amu
78
Particularly interesting are the features observed on the red
K1
side of the origin band in both spectra (K22 and K26.5 cm
2
1
0
0
0
in the gas phase and helium droplet spectra, respectively). In
our gas phase studies, we observed that this band did not
change in intensity when the valve temperature was varied
between 175 and 240 8C while all other bands strongly
increased in intensity when the temperature was raised. This
behavior suggested that the band on the red side should be
assigned to the Bzf$Ar van der Waals complex. Indeed, the
proof for this interpretation was given when we used neon
instead of argon as carrier gas, for the peak was absent. This
explanation cannot be given for the small band observed in
helium droplets at the equivalent position. Clearly, the
coincidence must be considered as accidental. Further
investigations are necessary to reveal the nature of this peak
in helium droplets.
x 25
600
100
200
300
400
500
700
Mass (amu)
Fig. 4. Mass spectrum recorded for laser pyrolysis soot. The sample was
evaporated at 140 8C and deposited into helium droplets.
anthracene and phenanthrene (both molecules have the same
mass), other mass peaks corresponding to aromatic hydro-
carbons with masses up to w650 amu are detected. Interest-
ingly, two alternations with strong and weak peak intensities
are observed. The average distance between two peaks in each
alternation corresponds to 25 amu, suggesting that, during the
growth of the PAHs, the addition of two carbon and one
hydrogen atom was favored (on the average). This successive
growth mechanism complies with the hydrogen-abstraction-
C H -addition (HACA) route widely used in modelling studies
3
.2. Carbonaceous powder
The general idea of the following experiment results from
the fact that one has already measured the spectra of several
commercially available PAH molecules, but up to now no
correspondence with any DIB was found. Perhaps some DIBs
are due to not so common PAHs and possibly such molecules
can be synthesized in free chemistry processes. It is known that
PAH-containing soot can be produced in flames and in laser
pyrolysis experiments using hydrocarbons as precursors. In the
present experiment, we have studied the carbonaceous soot
obtained by laser pyrolysis of a 2:1 mixture of ethylene and
2
2
carried out to explain the PAH formation in combustion
environments [25].
To identify the species giving rise to the signal on mass mZ
78 amu, we have measured depletion spectra with the mass
1
spectrometer fixed to this mass and the laser beam tuned
around the absorption band of phenanthrene. For a proper
comparison and prior to this experiment, we have determined
the absorption profile of phenanthrene (Aldrich, purity 98%) as
a pure substance employing the same conditions. Up to now, no
other studies of phenanthrene in He droplets were reported in
benzene vapors using a continuous-wave CO laser with a
2
power of 600 W [15]. The conditions were adjusted so as to
obtain high concentrations of PAHs. Indeed, gas chromatog-
raphy mass spectra of the samples revealed high concentrations
of PAHs and UV spectra taken from the toluene extract in
hexane suggested that the 3-ring PAHs phenanthrene and
anthracene should be a major component [16].
1
1
the literature. In Fig. 5, the S ( B ))S ( A ) origin band of
2
2
0
1
phenanthrene as measured in helium droplets is plotted by the
solid curve. The dashed and dotted curves represent the spectra
as obtained from the soot and extract, respectively. The close
resemblance of the absorption bands clearly reveals that
phenanthrene is a constituent of all samples. The smaller
depletion signals measured for the soot and extract (19%
depletion compared to 45% for the pure substance) indicate
that only 42% of the total count rate at mass mZ178 amu
originates from phenanthrene. The remaining 58% must be due
to other molecules with the same mass (for example
anthracene) or fragments of larger PAHs to this mass.
Compared to the respective gas phase transition to be reported
Employing the CRDS and MBDS techniques described in
the experimental part and applied to 2,3-benzofluorene, we
have investigated both the carbonaceous powder as it was
synthesized in the laser pyrolysis process as well as the extract
which was obtained by dissolving the carbonaceous powder in
toluene, filtering the solution, and finally evaporating the
solvent.
Our first studies were devoted to the investigation of the
samples by incorporating the volatile molecules into helium
droplets. In Fig. 4, we present the mass spectrum that we could
measure when the extract was evaporated close to the helium
droplet beam at a temperature of 140 8C. It reveals the mass
distribution of the molecules which were actually carried by
the helium droplets to the mass spectrometer detector. Besides
the rather intense mass peak at 178 amu, probably caused by
below, the helium droplet spectra reveal a matrix shift of
K1
K62 cm
however difficult to quantify since the helium droplet spectra
were measured at higher laser energy so that power broadening
is effective.
and exhibit substantial broadening. This is

110
A. Staicu et al. / Journal of Molecular Structure 786 (2006) 105–111
1
1
1 0
g
Fig. 7. The origin band of the S ( B2u))S ( A ) transition of anthracene
1
1
2 1
2 0
Fig. 5. The S ( B ))S ( A ) origin band of phenanthrene measured by
measured by CRDS in supersonic jets for the pure chemical substance (solid
line) and the extract of the soot obtained by laser pyrolysis (dashed lines).
MBDS in helium droplets. The solid curve refers to the pure substance of
phenanthrene while the dashed and dotted lines correspond to phenanthrene
spectra obtained from laser pyrolysis soot and the extract thereof, respectively.
The soot and extract samples were heated to 100 8C while the pure substance
was kept at K7 8C.
soot/extract samples were evaporated at the same temperature
of 130 8C.
The next experiment was devoted to the identification of
anthracene in our laser pyrolysis samples. Due to the same
mass of anthracene and phenanthrene and the large abundance
of phenanthrene, we were not able to observe a depletion signal
on mZ178 amu in the helium droplet experiment when the
laser was tuned to the absorption frequency of anthracene [13].
This failure is putting an upper limit of 5% for the
concentration of this species in our samples. In contrast, our
CRDS gas phase investigation unambiguously confirms the
presence of anthracene in the extract sample. Fig. 7 shows the
The presence of phenanthrene in the soot and extract
samples was also confirmed by gas phase studies carried out
with our CRDS setup. As for the He droplet studies, the S )S0
2
origin band of phenanthrene was used for the investigation in
the gas phase. The position of this band was previously
determined in a LIF study of phenanthrene in a supersonic jet
K1
K1
(
35 375 cm ) [26] and we found it at 35 378.2 cm . Fig. 6
shows the S ) S origin band of phenanthrene as recorded for
2
0
the pure chemical substance (solid line) as well as for the soot
dashed curve) and extract (dotted curve) samples. The smaller
amplitudes of the soot and extract peaks in the CRDS spectra
compared to the intensity of the pure substance) merely reflect
1
1
S ( B ))S ( A ) origin band of anthracene measured by
1
2u
0
g
(
CRDS for the pure chemical substance (Aldrich, purity 97%)
solid line) and the extract (dashed lines). The signal from the
(
(
extract is 30 times lower than that obtained from chemically
pure anthracene at the same valve temperature of 180 8C.
Based on the approximation that the same vapor pressure is
achieved for both samples, we can estimate that the
concentration of anthracene in the extract is approximately
the fact that the phenanthrene densities were lower when the
3.3%, which is consistent with the 5% upper limit determined
in the MBDS experiment.
4. Conclusions
In this paper, we have presented new results on the
absorption spectroscopy of 2,3-benzofluorene for the transition
from the S electronic ground state to the first singlet excited
0
state, S . The studies were conducted in the gas phase using
1
cavity ring-down spectroscopy in combination with supersonic
jets and in helium droplets employing molecular beam
depletion spectroscopy. The position of the S (0))S (0)
1
0
K1
transition was located at 29 894.3 cm (334.5 nm). While the
K1
same transition in helium droplets is only shifted by 5.1 cm
Fig. 6. Phenanthrene absorption spectra measured by CRDS in supersonic jets.
The meaning of the solid, dashed, and dotted curves is the same as in Fig. 5. The
soot and extract samples were heated to 130 8C while the pure substance was
kept at 100 8C.
to the blue, the matrix shift for argon was determined to be
K1
around 240 cm to the red. Various yet unassigned
vibrational bands were measured on the blue side of the origin

A. Staicu et al. / Journal of Molecular Structure 786 (2006) 105–111
111
band. For all of them, the blue shift caused by the helium
K1
droplets was smaller than 6.5 cm . It is concluded that helium
References
[
1] F. Salama, G.A. Galazutdinov, J. KreŁowski, L.J. Allamandola,
F.A. Musaev, Astrophys. J. 526 (1999) 265.
droplet spectra are much better suited for comparison with
astronomical data than neon or even argon matrix data, if gas
phase spectra are not available or too difficult to obtain. Since
the absorption features of 2,3-benzofluorene are in the UV this
molecule cannot be responsible for any of the major diffuse
interstellar bands.
[
2] P. Jenniskens, F.-X. D e´ sert, Astron. Astrophys. Suppl. Ser. 106 (1994) 39.
3] A. Staicu, G. Rouill e´ , O. Sukhorukov, Th. Henning, F. Huisken, Mol.
Phys. 102 (2004) 1777.
[
[
[
4] G. Rouill e´ , S. Krasnokutski, F. Huisken, Th. Henning, O. Sukhorukov,
A. Staicu, J. Chem. Phys. 120 (2004) 6028.
5] O. Sukhorukov, A. Staicu, E. Diegel, G. Rouill e´ , Th. Henning,
F. Huisken, Chem. Phys. Lett. 386 (2004) 259.
In a completely different approach, we have investigated the
carbonaceous soot produced by laser pyrolysis of a mixture of
hydrocarbons and benzene vapor. Mass spectra revealed that
the soot contained various PAHs with masses up to 650 amu
which are difficult to identify. The presence of phenanthrene
was confirmed by measuring its absorption spectrum in the gas
phase and in helium droplets. Because of the low concentration
achieved in the gas phase, the stronger S )S transition was
[6] D. Romanini, L. Biennier, F. Salama, A. Kachanov, L.J. Allamandola,
F. Stoeckel, Chem. Phys. Lett. 303 (1999) 165.
[
7] L. Biennier, F. Salama, L.J. Allamandola, J.J. Scherer, J. Chem. Phys. 118
2003) 7863.
8] L. Biennier, F. Salama, M. Gupta, A. O’Keefe, Chem. Phys. Lett. 387
2004) 287.
(
[
(
[9] X. Tan, F. Salama, J. Chem. Phys. 122 (2005) 084318.
[
[
[
10] X. Tan, F. Salama, J. Chem. Phys. 123 (2005) 014312.
2
0
11] J.P. Toennies, A.F. Vilesov, Angew. Chem. Int. Ed. 43 (2004) 2622.
12] F. Huisken, S. Krasnokutski, in: E.P. Muntz, A. Ketsdever (Eds.),
Proceedings of the 23rd International Symposium on Rarified Gas
Dynamics, Whiley, New York, 2002, p. 678.
chosen to monitor the molecule (instead of S )S ).
1
0
Anthracene was analyzed in the samples employing cavity
ring-down spectroscopy and probing the S )S transition.
1
0
[
13] S. Krasnokutski, G. Rouill e´ , F. Huisken, Chem. Phys. Lett. 406 (2005)
It was found that the laser pyrolysis soot provides samples
which could be very interesting for laboratory astrophysics
applications. While the present study focused only on the three-
ring PAHs phenanthrene and anthracene, further investigations
are in progress to identify the larger PAHs in the soot, which
are expected to have their absorption bands in the visible, i.e. in
the same region where the DIBs are observed, and to study
their spectroscopy.
3
86.
[
[
14] R. Lehnig, A. Slenczka, J. Chem. Phys. 122 (2005) 244317.
15] I. Voicu, private communication, 2005.
[16] C. Jaeger, I. Voicu, A. Staicu, S. Krasnokutski, F. Huisken, in preparation.
[
[
[
17] A. O’Keefe, D.A.G. Deacon, Rev. Sci. Instrum. 59 (1988) 2544.
18] K.P. Geigle, G. Hohlneicher, J. Mol. Struct. 480-481 (1999) 247.
19] J. Banisaukas, J. Szczepanski, M. Vala, S. Hirata, J. Phys. Chem. A 108
(
2004) 3713.
[20] M.J. Frisch, G.W. Trucks, H.B. Schlegel, et al., GAUSSIAN 03W, Revision
C. 02, Gaussian, Inc., Pittsburgh, PA, 2003.
[
21] V. Stakhursky, T.A. Miller, SpecView: Simulation and fitting of
rotational structure of electronic and vibronic bands, 56th Molecular
Spectroscopy Symposium; http://molspect.mps.ohio-state.edu/goes/spec-
view.html
Acknowledgements
This work was supported by a cooperation between
the Max-Planck-Institut f u¨ r Astronomie and the Friedrich-
Schiller-Universit a¨ t Jena as well as by the Deutsche
Forschungsgemeinschaft in the frame of the Forschergruppe
Laborastrophysik. The authors are grateful to I. Voicu for the
synthesis of the PAH-containing powder by laser pyrolysis, to
C. Jaeger for analyzing this powder, and to M. Vala for
providing computational results for the vibrations of 2,3-
benzofluorene in its electronic ground state.
[
[
22] L.A. Nakhimovsky, M. Lamotte, J. Joussot-Dubien, Handbook of Low
Temperature Electronic Spectra of Polycyclic Aromatic Hydrocarbons,
Elsevier, Amsterdam, 1989.
23] R. Borrelli, A. Peluso, J. Chem. Phys. 119 (2003) 8437http://pcdual.chem.
unisa.it/; A. Peluso, F. Santoro, G. Del, Int. J. Quant. Chem. 63 (1997)
2
33.
[
[
24] A. Staicu, S. Krasnokutski, G. Rouill e´ , F. Huisken, Th. Henning, R.
Scholz, S. Rastogi, in preparation.
25] H. Wang, M. Frenklach, Combust. Flame 110 (1997) 173.
[26] N. Ohta, H. Baba, Mol. Phys. 59 (1986) 921.