Spectroscopic Investigation of Polycyclic Aromatic
Hydrocarbons Trapped in Liquid Helium Clusters
Friedrich Huisken and Serge Krasnokutski
Max-Planck-Institut für Strömungsforschung, Bunsenstr. 10, D-37073 Göttingen, Germany
Abstract. Polycyclic aromatic hydrocarbons (PAHs) are believed to be responsible for the Diffuse Interstellar Bands (DIBs),
a set of absorption bands observed in Interstellar Space. To provide further information on this issue, we have started a series
of experiments devoted to the electronic spectroscopy of some neutral PAH molecules. In order to overcome the difficulty
in achieving densities high enough for spectroscopic studies, we have embedded the PAHs in liquid helium droplets which
are considered to represent an ideal nanomatrix with very small interaction between the trapped molecule and host. Here we
report on first results obtained for the two molecules anthracene (C14 H10 ) and tetracene (C18 H12 ). Using both laser-induced
fluorescence (LIF) and molecular beam depletion (MBD) spectroscopy, we have characterized the first electronic transition
S0 ) of these species when they are solvated in liquid helium clusters.
(S1
INTRODUCTION
The so-called Diffuse Interstellar Bands (DIBs), a series of absorption bands in the visible and near infrared spectrum
observed in space, constitute a long-standing problem in astronomy. Today, polycyclic aromatic hydrocarbons (PAHs)
in their neutral and ionized states are believed to be the most promising candidates to account for this phenomenon [1].
In order to prove this conjecture, the electronic spectra of these species must be investigated in the laboratory. For a
proper comparison, the temperature should be very low, and thus, one would like to study the PAHs in molecular
beams. Unfortunately, due to their low vapor pressure, it is very difficult to prepare these molecules in the gas phase.
Therefore, up to now, only some rather light PAHs could be studied in molecular beams. Another possibility is to
prepare the PAH molecules in low-temperature rare gas matrices. This technique, however, has the disadvantage that
the measured spectra are affected by rather large matrix shifts which make the comparison with the astrophysical
observations very difficult.
During the last few years, another technique has emerged that has been proven to be very useful. It is based on the
incorporation of the molecule of interest into liquid helium clusters (droplets) using molecular beam techniques [2, 3].
The spectroscopy of the embedded molecules can be studied employing for example laser-induced fluorescence (LIF)
or molecular beam depletion (MBD) spectroscopy. Advantages of the novel technique are manyfold: (1) The systems
are prepared at a unique extremely low temperature (0.4 K). (2) Molecular species with very low vapor pressure
(10;6 mbar) can be successfully studied in molecular beams. (3) Although the measured spectra are not interactionfree, the corresponding matrix shifts are considerably smaller than found in conventional rare gas matrices, neon and
argon. (4) Due to the directivity of the molecular beam, the spectroscopy can be combined with a mass spectrometer,
thus facilitating the assignment of the spectral features and avoiding spectral contamination.
We have started a series of measurements aimed at studying the electronic absorption spectra of a number of
neutral and charged PAH molecules trapped in liquid helium droplets. First results were obtained for the two species
anthracene (AC), C14 H10 , and tetracene (TC), C18 H12 . For both molecules, we present LIF and MBD spectra of
their first electronic transition (S1 S0 ). While the LIF spectrum of TC@HeN (the “@” denotes the fact that the
molecule is residing inside the helium cluster) has been reported before by Toennies and coworkers [4], new data will
be presented that result from monomer and dimer MBD studies. The smaller PAH molecule, AC, was studied by Even
et al. [5] when it was successively surrounded by up to 17 He atoms, but data for AC completely solvated by helium
is still missing.
CP663, Rarefied Gas Dynamics: 23rd International Symposium, edited by A. D. Ketsdever and E. P. Muntz
© 2003 American Institute of Physics 0-7354-0124-1/03/$20.00
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FIGURE 1.
Schematic view of the experimental setups employed in the present study.
EXPERIMENTAL
In Fig. 1, we present schematic views of the different configurations used to realize MBD (Fig. 1a) and LIF (Fig. 1b)
spectroscopy. Since the apparatus has been described recently in context with an infrared spectroscopic study on
the smallest amino acid, glycine [6], we will focus on the most important aspects. Large helium clusters containing
between N = 2 000 and 15 000 helium atoms are produced by supersonic expansion of helium gas at high pressure
(p = 20 atm) through a cooled (T = 11 ; 16 K) pinhole nozzle (d = 5 m). The larger clusters (N = 15000) are
obtained at the lower temperature (T = 11 K). Since the helium clusters are liquid they are also referred to as
nanodroplets. The beam of helium nanodroplets is then directed through a cell containing the PAH molecules at
low vapor pressure (TAC = 25 C; TTC = 10;170 C). Upon collisions with the helium droplets, the PAH molecules
are embedded into their interior and carried by them to the mass spectrometer detector. Further downstream, the
chromophore-containing helium cluster beam is crossed with the radiation of a pulsed tunable dye laser ( = 360;450
nm) using either a counterpropagating (Fig. 1a) or perpendicular (Fig. 1b) geometry. In case of resonance, the laser
radiation is absorbed by the PAH molecule embedded 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 latter process results in the evaporation of a few hundred or thousand helium atoms
and leads to a reduction of the beam intensity measured with the mass spectrometer on the mass of the chromophore.
Thus, we have two possibilities to study the absorption behavior of the PAH molecules: laser-induced fluorescence
(LIF) and molecular beam depletion (MBD) spectroscopy. Both techniques have been employed in the present study.
The laser system consists of a Nd:YAG-laser-pumped dye laser operated with Pyridine (for AC) and Coumarine 120
(for TC). In the former case, the dye laser was pumped with the 532 nm radiation while its output was doubled in a
KTP crystal and, in the latter case, we used the third harmonic (355 nm) for pumping the dye laser. The pulse energies
were 100;500 J and 300;700 J, respectively.
RESULTS
Anthracene
As our first candidate, we chose anthracene that is composed of three aromatic rings arranged in series. Figure 2
shows a mass spectrum that is obtained when anthracene is evaporated at room temperature and incorporated into
helium clusters with an average size of hN i = 15 000. The large peak at m = 178 amu results from the incorporation
of anthracene molecules into the helium clusters. In contrast, the small peak at m = 356 amu is due to anthracene
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FIGURE 2. Mass spectrum of anthracene trapped in helium nanodroplets. The peak at 178 amu corresponds to the anthracene
monomer while the peak at 356 amu is related to anthracene dimers trapped in the helium droplets.
FIGURE 3. Spectrum of the electronic S1
S0 transition of anthracene trapped in helium droplets. The solid line represents the
LIF spectrum while the data points, that are connected by straight lines, are obtained by measuring the depletion of the signal on
m
amu.
= 178
dimers. The fact that we observe this peak indicates that, at room temperature conditions, the density of anthracene
molecules is already high enough that two molecules may be picked up by the helium clusters, giving rise to the
+
formation of anthracene dimers. The peak at m = 152 amu can be assigned to C 12 H+
8 and is obtained from C14 H10
after the abstraction of an acetylene molecule. Apparently, this fragmentation is induced by the ionization of the
chromophore-containing helium droplet by 100 eV electrons. The series of small peaks at the lower masses are due to
small helium cluster fragments.
Figure 3 displays two spectra revealing the first electronic transition (S1 S0 ) of anthracene trapped in liquid
helium clusters. The LIF spectrum that was obtained with the setup shown in Fig. 1b is represented by the solid line.
A depletion spectrum measured on the mass of anthracene (m = 178 amu) is given by the circular data points that
are connected by straight lines. Both spectra reveal a splitting of 0.85 cm ;1 of the main peak whose lower-frequency
component (27,627.4 cm ;1 ) was arbitrarily set to zero. Comparing the two spectra, it is noticed that the blue wing of
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FIGURE 4. Spectrum of the electronic S1
S0 transition of tetracene trapped in helium droplets. The spectra are obtained at
different helium cluster source temperatures yielding different average helium cluster sizes. The spectrum given by the circular data
points that are connected by straight lines has been obtained by measuring the depletion on the mass of tetracene. The other two
spectra are LIF spectra.
the absorption profile is clearly more pronounced in the depletion spectrum.
Tetracene
Our next candidate was tetracene, a molecule whose S1 S0 transition was already studied in helium clusters by
Hartmann et al. [4] using LIF. Our results are summarized in Fig. 4. The upper LIF spectrum, recorded at a helium
source temperature of 11 K, is almost identical to the one obtained by Hartmann et al. even if the details of the
spectroscopic features are compared. The electronic transition observed at 22,295.8 cm ;1 shows a splitting into two
components separated by 1.04 cm ;1 (see the left two peaks at 0 and 1.04 cm;1 ) and a broad phonon wing with the
maximum blueshifted by 5 cm ;1 . The lower trace in Fig. 4 drawn as solid line represents a LIF spectrum of tetracene in
somewhat smaller helium clusters (hN i = 3 100). This spectrum is characterized by a lower signal-to-noise level. The
spectrum represented by the circular data points connected by straight lines represents, as in the case of anthracene,
the result of the depletion experiment carried out on the mass of the mother molecule, tetracene. It reveals the same
splitting of the zero-phonon band into the two components. However, the intensity of the phonon wing is much more
pronounced. It features a rather strong band that is blueshifted by 6.2 cm ;1 relative to the origin.
DISCUSSION
The effect of solvation by up to 17 helium atoms was studied before by Even et al. [5]. They observed a saturation
of the redshift already starting at N = 10 and achieving a final value of – 34 cm ;1 for N = 13; 17. This seems to
suggest that, for N = 17, the anthracene molecule is already fully solvated by the helium atoms. Our measurements,
however, reveal a redshift of – 61 cm ;1 indicating that the solvation is definitely not terminated with 17 helium atoms.
Moreover, while the measurements of Even et al. reveal only single peaks for the S 1 S0 transition, we observe a
splitting of 0.8 cm;1 . This splitting is quite similar to the one observed for tetracene and will be discussed below.
If we compare the MBD and LIF spectra displayed in Fig. 3 we note that both spectra reveal the same intensity
profile as far as the dip in the main peak is concerned. This indicates that the two spectra are not affected by laser power
saturation (at least not in a different way) and that they can be readily compared. We interpret the enhanced intensity
on the blue side of the absorption peak in the MBD spectrum as evidence for the excitation of vibrational modes
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FIGURE 5. Comparison of the two LIF spectra of anthracene and tetracene measured in this study. The origins of the two spectra
are at 27,627.4 cm;1 (AC) and 22,295.8 cm;1 (TC), respectively.
between the surrounding helium atoms which happen to be in close contact with the chromophore. The excitation of
combinational modes involving helium will result in a faster energy transfer to the helium cluster and, consequently,
to an enhanced depletion signal, compared to the main peak which is due to a pure electronic transition.
As already stated, our tetracene LIF spectrum measured at a helium source temperature of T = 11 K is almost
identical to the tetracene spectrum reported by Hartmann et al. [4]. Using hole burning experiments with two lasers,
these authors find that the observed splitting in the zero-phonon band results from a level splitting in both the electronic
ground and excited states. The origin of the ground state splitting is not yet completely settled but it is believed to result
from two different environments that the tetracene molecule may find in the helium cluster. This assumption is based
on the fact that some helium atoms in the close neighborhood of the chromophore are more tightly bound and that
different local structures may be adopted. Another explanation could be the assumption of a tunelling motion of a
helium atom between two positions above or below the inner aromatic rings of the tetracene molecule, giving rise to
the respective splitting.
While it is more favorable to carry out the LIF experiments at lower helium source temperature, i.e. for larger
helium cluster size (since the inhomogeneous broadening due to the size distribution is getting less important), the
contrary is true for the depletion experiments. This technique is more sensitive if the chromophore molecules reside in
smaller helium clusters since the relative change in cluster size after heat dissipation and helium evaporation is more
pronounced. This is the reason that the depletion spectrum was measured at higher temperature (14 K) than the LIF
spectrum (11 K). Nevertheless, the two spectra can be readily compared. In particular, it is noted that the splitting of
the zero-phonon line is practically the same. Thus, it follows that it is not possible that one of the two components
originates from another species (impurity, dissociation product of anthracene, or anthracene dimer) since the signal
measured with the mass spectrometer is associated with the anthracene molecule. We have also measured a depletion
spectrum with the mass spectrometer tuned to the dimer mass [7], but this spectrum was very broad and depletion was
found in the entire region studied here.
There are some pronounced and reproducible spectroscopic features in the phonon wing which have not yet been
discussed. They are clearly seen in the upper spectrum of Fig. 4 and they are also observed by Hartmann et al. [4].
According to a new comparative study involving various chromophores by Hartmann et al. [8], these features are
attributed to the excitation of vibrational modes of the helium atoms located in the shells around the chromophore.
Our MBD spectrum provides further support for this interpretation. The peak labelled by the small arrow in Fig. 4
appears to be much more pronounced in the depletion spectrum. This points to the excitation of a mode with a strong
coupling to the phonon bath and, thus, it is very likely that vibrational excitation is involved.
In Fig. 5, we compare the spectra of anthracene and tetracene that we have measured with the LIF technique. There
are some similarities and some differences. At first we note that, in both cases, we observe a splitting of the zero-
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phonon line with approximately the same separation. On the other hand, the spectroscopic features for anthracene are
much broader so that the splitting is barely resolved. Moreover, the gap between zero-phonon line and the phonon wing,
which is clearly visible for tetracene, is completely smeared out for anthracene. At present, we have no explanation for
this behavior. On the other hand, we would like to draw the reader’s attention to the fact that the two spectra become
again very similar for blueshifts larger than 5 cm ;1 .
OUTLOOK
One final comment should be devoted to the astrophysical issue. For experimental reasons, the present studies have
been carried out for rather small PAH molecules. Their absorption bands lie in a spectroscopic range where the density
of DIBs is rather low, and no coincidences are found. The wealth of DIBs are observed at larger wavelengths (higher
energies). This wavelength range can only be accessed with either larger PAHs or PAH cations. Therefore, in the
experiments to follow, we will concentrate on larger molecules and try to obtain spectra of positively charged PAHs.
ACKNOWLEDGMENTS
The authors are grateful to the Deutsche Forschungsgemeinschaft for financial support.
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