Diagnosis of a benzene discharge with a mass-selective spectroscopic technique

1. Diagnosis of a benzene discharge with a mass-selective spectroscopic technique Felix Güthe, Hongbin Ding, Thomas Pino and John P. Maier Institute of Physical Chemistry, University of Basel, Klingelbergstrasse 80, CH-4056 Basel, Switzerland. Diagnosis of a plasma with REMPI detection Mass spectrometry with different laser excitation schemes A new experiment has been built in the group to perform resonance enhanced multi photon ionization (REMPI) studies of the neutral CnHm clusters. They are produced in a plasma formed by a pulsed discharge source1,2 and are quenched by cooling in a supersonic expansion. The apparatus couples the discharge source with a REMPI detection in a linear time of flight (TOF) mass-analyzer (R50%= ~900 ), enabling to combine the information on the masses and the electronic spectra. 200 or more species can be recorded simultaneously. After deflection of charged species the neutral molecules are ionized by lasers. For the ionization either one two photons of the same wavelength (R2PI) or two photons of different wavelength (R2C2PI) can be used. Picture of the plasma from a slit nozzle discharge on C2H2 taken from H. Linnartz3. In this work a nozzle of circular geometry with benzene as precursor gas was used but could not be photographed. The produced CnHm species are important intermediates in combustion processes and interstellar chemistry. Their electronic spectra are important for their identification and could be related to the diffuse interstellar band problem. Mass spectra of the neutral clusters produced with an benzene discharge recorded with different ionization wavelengths: A VUV-laser in one case (F2-excimer at 157 nm; 7.59 eV) and a spectrum taken during a scan from 278-290 nm (~4.5 eV). The arrows point to the masses 128 and 178, which are coinciding with the masses of the first members of the polycyclic aromatic hydrocarbons (PAH). But only the electronic spectra can reveal the identity of the carriers of this mass peaks ! U0=500-1000V Width=10-200ms I=0.2-5 A Identification of molecules by R2PI spectroscopy in the UV range Chemistry in the discharge plasma: The species identified in this work are known as intermediates in several chemical models for combustion. The ethynyl-PAHs (E-PAH) and cyclopentafused PAH (CP-PAH) have been proposed to be intermediates in fullerene formation and in the build up of larger carbon species. Thus the direct sampling of the benzene discharge in the molecular beam reflects 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 temperature4. 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 sources5: Conclusion • Combinationofa plasma discharge source with spectroscopic characterisation by REMPI. • -A model system for hydrocarbon flames?? Future work For the benzene discharge we recorded R2PI spectra between 320 and 280 nm. Electronic spectra of more 30 different molecules have been obtained. In the graphs the spectra of phenylacetylene(C8H6), styrene(C8H8), indene(C9H8), methylstyrene(C9H10), fluorene(C13H10), tolane (C14H10) as well as the of molecule C10H8are shown. From these 6 molecules could be identified by there spectra from literature. Note that the spectrum of . C14H10 is not that of anthracene or the phenanthrene, the compact all 6-ring PAHs, but that of the tolane molecule. The strong S0-S2 phenantrene transition is clearly absent. The spectrum of the C10H8 molecule is not that of the bicyclic naphthalene, but probably that of a monocycclic substituted benzene. • -Identification of other species (C6D6 as precursor ...) to gain deeper understanding of the chemistry in plasmas • -Characterisation of other mixtures • -Work in the visible. Acknowledgment References 1) F. Güthe; H. Ding; T. Pino; J. P. Maier, Chemical Physics, accepted. 2) T. Pino; H. Ding; F. Güthe; J. P. Maier, Journal of Chemical Physics, 2208, 114, (2001). 3) http://www.chemie.unibas.ch/~linnartz/ 4) W. Jenneskens, M. Sarobe, Polycyclic Aromatic Compounds,. 14/15, 169 (1999). 5) C. Lifshitz, G. Reuben, J. Chem. Phys. 50, 951 (1969). The authors would like to thanks Georg Holderied and Dieter Wild and the mechanical workshop for their technical assistance. Tomasz Motylewski and Danielle Furio (LPPM, Orsay France) are also kindly thanked for their help in developing the software of the experiment.