Spectroscopic Investigation of Polycyclic Aromatic Hydrocarbons Trapped in Liquid Helium Clusters

Transcription

1 Spectroscopic Investigation of Polycyclic Aromatic Hydrocarbons Trapped in Liquid Helium Clusters Friedrich Huisken and Serge Krasnokutski Max-Planck-Institut für Strömungsforschung, Bunsenstr. 10, D 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 (C 14 H 10 ) and tetracene (C 18 H 12 ). Using both laser-induced fluorescence (LIF) and molecular beam depletion (MBD) spectroscopy, we have characterized the first electronic transition (S 1 S 0 ) of these species when they are solvated in liquid helium clusters. 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), C 14 H 10, and tetracene (TC), C 18 H 12. For both molecules, we present LIF and MBD spectra of their first electronic transition (S 1 S 0 ). While the LIF spectrum of TC@He N 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.

2 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 helium atoms are produced by supersonic expansion of helium gas at high pressure (p =20atm) through a cooled (T =11; 16 K) pinhole nozzle (d =5m). The larger clusters (N = 15000) are obtained at the lower temperature (T =11K). 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 (T AC =25 C; T TC =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 hni = The large peak at m =178amu results from the incorporation of anthracene molecules into the helium clusters. In contrast, the small peak at m =356amu is due to anthracene

3 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 S 1 S 0 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 =178amu. 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 =152amu can be assigned to C 12 H + and is obtained from C 8 14H + 10 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 (S 1 S 0 ) 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 =178amu) 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

4 FIGURE 4. Spectrum of the electronic S 1 S 0 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 S 1 S 0 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 (hni =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 =10and 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 S 0 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

5 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 =11K 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-

6 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. REFERENCES 1. Salama, F., Galazutdinov, G. A., Krelowski, J., Allamandola, L. J., and Musaev, F. A., Astrophys. J. 526, (1999). 2. Toennies, J. P., and Vilesov, A. F., Annu. Rev. Phys. Chem. 49, 1 41 (1998). 3. Lugovoj, E., Toennies, J. P., Grebenev, S., Portner, N., Vilesov, A. F., and Sartakov, B., in Atomic and Molecular Beams, The State of the Art 2000, edited by R. Campargue, Springer-Verlag, Berlin, 2000, pp Hartmann, M., Lindinger, A., Toennies, J. P., and Vilesov, A. F., J. Phys. Chem. A 105, (2001). 5. Even, U., Al-Hroub, I., and Jortner, J., J. Chem. Phys. 115, (2001). 6. Huisken, F., Werhahn, O., Ivanov, A. Y., and Krasnokutski, S. A., J. Chem. Phys. 111, (1999). 7. Krasnokutski, S., and Huisken, F. (to be published). 8. Hartmann, M., Lindinger, A., Toennies, J. P., and Vilesov, A. F., Phys. Chem. Chem. Phys. (2002, in press).