NEUTRAL AND HYDROGENATED CARBON CLUSTERS : WHAT CAN WE LEARN WITH A REMPI EXPERIMENT ? Thomas Pino, Felix Güthe, Hongbin Ding and John P. Maier Institute of Physical Chemistry, University of Basel, Klingelbergstrasse 80, CH-4056 Basel, Switzerland. Introduction Mass spectrometry studies : the MPI diagnostic Experimental set-up R2PI Spectroscopy in the UV range In the Interstellar Medium (ISM), the carbonaceous matter includes a large range of different structures due to the ability of carbon to form complex structures [1]. On the molecular scale two types of structure have been detected, the PAH-like molecules and the carbon chains. Indeed most of the detected molecules in the ISM are carbon based. We present here our preliminary results on the study of the electronic spectray of C n H m neutral species. These hydrocarbons might provide an understanding of the connection between these two components of the carbonaceous matter in the ISM. For this purpose, a new experiment has been built. It combines a supersonic expansion coupled to a discharge and a linear time-of-flight mass-analyzer. The source is identical to the one used in our group for the study of the electronic spectroscopy of the negative carbon clusters [2]. Mass spectrometric studies have been carried out using different precursors (benzene, acetylene, and methane) diluted in argon, and at different wavelengths for the multiphoton ionization (MPI) process. The observed distributions appear to be similar to the ones observed in fuel-rich hydrocarbon flames [3]. We performed a spectroscopic study applying the resonance enhanced 2 photons 1 color ionization scheme in the UV, in a benzene discharge. Thus in addition to the mass distributions, the measured electronic spectra allow a clear identification of some clusters for the first time. They all appear to be aromatic compounds, some of them are known from literature and the electronic spectra of others species are new. The analysis is in progress. These preliminary results show two major points. First we are able to form cold C n H m clusters, which is of importance for a spectroscopic purposes. Second the mass selectivity and the optical selectivity allow for the first time a clear identification of the different isomers present in this kind of plasma, and the reality seems much more complicated than commonly assumed… References A new experiment has been built in the group to perform Resonance Enhanced MultiPhoton Ionization (REMPI) studies of the neutral C n H m clusters (figure 1). The apparatus is a molecular beam combined with a linear Time Of Flight (TOF) mass-analyser. The TOF is based on the Wiley-McLarren scheme and the mass resolution is 500 for the mass 166. The source is a pulsed valve (general valve) coupled to an electric discharge. The original configuration has been developed by Endo[5]. At the end of the discharge assembly, an expander shapes the expansion. For the data acquisition, the signal is fed into an ultrafast oscilloscope. Labview programs enable to monitor simultaneously the spectra of up to 24 different masses observed in the mass spectrum. [1] Th. Henning and F. Salama, Science 282, 2204 (1998). [2] M. Tulej, F. Güthe, M. Schnaiter, M.V. Pachkov, D.A. Kirkwood, J.P. Maier and G. Fisher, J.P.C. A 103, 9712 (1999). [3] J. Ahrens, A. Keller, R. Kovacs and K.-H. Homann, Ber. Bunsenges. Phys: Chem.102, 1823 (1998). [4] J.P. Maier, J. Phys. Chem. A 102, 3462 (1998). [5] Y. Ohshima and Y. Endo, J. Mol. Spec. 153, 627 (1992). Acknowledgment Because interstellar molecules are not the one stable on the earth, we have to produce them in the laboratory. Discharge source provide a powerful tool to produce radicals in sufficiently high concentration and can be easily coupled to a supersonic expansion. Then applying laser spectroscopic technique to measure electronic spectra of a large amount of radical species become possible with this type of source. On the other hand, carbon chemistry is so rich that a large quantity of clusters (anions, neutrals and cations) are systematically produced, making mass-selective methods necessary. Neutral species can be mass-selected using photodetachment of the related anion. This method has been applied in our group under rare gas matrix isolation conditions. Gas phase spectra are investigated using direct absorption measurement (Cavity Ring Down Spectroscopy…) and identification of the specie is based on the matrix spectra[4]. This method, although powerful, is limited to species with a rather high electron affinity. For that reason our purpose is to study the electronic spectroscopy of the neutral clusters directly produced. We carried out mass spectrometric studies to investigate the distribution of the formed clusters. We focused our attention only on the small species (number of carbons, Nc < 25). Results obtained with the F 2 laser (157 nm ; 7.59 eV) are presented. In the case of benzene as a precursor (figure 2), the distributions appear rather continuous in the mass spectrum. No alternation with the number of carbon atoms is seen. Also one can see that above Nc  8 no selectivity for the number of hydrogen (N H ) atoms is observed. Below that limit odd numbers of hydrogens are favored. In addition to these species, C n H 3 (n=4-16) clusters are present and quite abundant. In the case of acetylene as a precursor (figure 3) a clear alternation with the number of carbons is observed, the even ones being more abundant. This alternation becomes less pronounced with Nc  20. A selectivity on N H is observed for Nc < 13, the odd N H being favored. In this case also the C n H 3 (n = 4-16) clusters appeared to be quite abundant. Although the two mass spectra show a similar pattern, differences on the hydrogenation rate (Nc / N H ) are present. Below Nc = 8, the distribution appears to be rather identical. In the range Nc = 8-13, the rule (odd number being favored) is conserved only in the acetylene discharge. In the range Nc > 13, the distribution of the C n H m is centered around the masses of the classical PAH in the case of acetylene as a precursor. In the benzene discharge, the distribution is much broader. We attribute those differences to the aptitude of benzene to form complex aromatic structures (substituted benzene and so on) as shown below. Acetylene, benzene (and others) have been used as precursors in fuel-rich flames to study the formation of PAHs in combustion processes[3]. Although we use a discharge in benzene or acetylene diluted in a rare gas mixture as the plasma source, the distribution of species appears rather similar for Nc > 13. But in our case, the distribution is richer and the masses of the classical PAH are not the only dominant one. Figure 2 : Mass spectrum of the neutral clusters produced with a benzene discharge. The ionizing laser is a F 2 laser (157 nm ; 7.59 eV). A zoom shows details of the distribution of clusters for Nc = 8. Figure 3 : Mass spectrum of the neutral clusters produced with an acetylene discharge. The ionizing laser is a F 2 laser (157 nm ; 7.59 eV). A zoom shows details of the distribution of clusters for Nc = 8. Figure 1: Schematic representation of the experimental apparatus. The authors would like to thanks Georg Holderied and the workshop crew and especially Dieter Wild for their technical assistance. Tomasz Motylewski and Danielle Furio (LPPM, Orsay France) are also kindly thanked for their help in developing the programs of the experiment. This work is supported by the Swiss National Fundation (…). Figure 6 : R2PI spectrum monitored on the mass 128. No naphthalene were detected (through its S 1  S 0 transition). This isomer is a new and unexpected molecule as far as we know. Figure 7 : R2PI spectrum monitored on the mass 140. This rich spectrum could be the one of an isomer of the naphthylcarbene family. Either this molecule has been detected in mass spectrometry study, no spectroscopic analysis had been performed yet. Figure 4 : R2PI spectrum monitored on the mass 166. This molecule has been clearly identified and is the fluorene molecule. One other isomer has been detected through a weak transition at longer wavelength. Figure 5 : R2PI spectrum monitored on the mass 178. This molecule has been clearly identified and is the tolane molecule. Phenanthrene has not been detected, and the question of the detection of the other PAH isomer, the anthracene, is opened. In the case of the benzene discharge, we performed R2PI spectroscopy from 320 to 280 nm. A two color experiment has been realised only in trying to detect the naphthalene. Electronic spectra of about 20 molecules have been obtained. Four spectra are presented here showing three different situations. The spectra have not been corrected by the laser intensity variations, and several dyes have been used. As a remark, the one color process limits the observable species to the ones with an ionization potential (IP) lower than 2h (  eV). The first situation is illustrated by the fluorene (mass 166, figure 4) electronic spectrum. We identified this molecule in our discharge through its known spectrum. Indeed 6 species have been identified via their known gas phase electronic spectra (R2PI or LIF) : styrene, indene, tolane…All of the spectra obtained indicate that the vibrational temperature is rather cold since only very low intensity hot bands could be observed. The second example is given by the tolane (mass 178, figure 5) electronic spectrum. At this mass, we expected the well-known PAH isomers, phenanthrene and anthracene. To our surprise, these two isomers are by far less abundant than tolane in our discharge. The third electronic spectrum is that of the mass 128 (figure 6). In that case, transitions of the classical and expected PAH (the naphthalene) have not been observed. At present, we cannot identify this molecule. These last two examples clearly shows that mass spectrometric information is often not sufficient to identify species. The last situation is shown by the spectrum measured on the mass 140 (figure 7). This spectrum and a number of others have to be assigned with the aid of further experimental and theoretical work. Conclusion All the observed molecules are thought to be aromatic and stable compounds. Although the electronic origin is not observed for some species, analysis of all the spectra obtained is in progress and theoretical calculations are needed to identify the molecules. As a remark, those molecules are only a small part of the total composition. The resulting mass spectrum of the molecules absorbing in the range nm is clearly different from the MPI, the hydrogenation rate being higher in the “R2PI” one. These preliminary studies pointed out the difficulty of assigning a structure to an observed species, the most abundant isomer being not systematically the most stable. We want to emphasize the importance of spectroscopic information in assigning mass spectra. In the future, we would like to extend this experiments to different precursor gases.