1. Electronic Spectroscopy of long Carbon Chains HC 2n H (n= 8-13) in the Gas Phase Felix Güthe*, Hongbin Ding, Thomas Pino and John P. Maier Institut für Physikalische Chemie, Universität Basel, Klingelbergstrasse 80, CH-4056 Basel, Schweiz *email: felix.guethe@unibas.ch Mass spectum of diacetylene discharge recorded at 157 nm C n H m from discharge source to record REMPI spectra A new apparatus for the investigation of neutral C n H m molecules has been built in to perform resonance enhanced multi photon ionization (REMPI) studies. They are produced in an pulsed electrical discharge source from a mixture of Diacetylene (0,2 – 0,5 %) in Argon 1,2 and are cooled by supersonic expansion. The apparatus couples the discharge source with a REMPI detection in a linear time of flight (TOF) mass-analyzer (R 50% = ~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 formed in the source the neutral molecules are ionized by lasers. For the ionization either two photons of the same wavelength (R2PI) or two photons of different wavelength (R2C2PI) can be used. For the second colour an F 2 excimer laser operating in the VUV (157nm – 7.9 eV) is used. References 1) F. Güthe; H. Ding; T. Pino; J. P. Maier, Chemical Physics, 269, 347 (2001). 2) T. Pino; H. Ding; F. Güthe; J. P. Maier, Journal of Chemical Physics, 2208, 114, (2001). 3) Kirkwood, D. A.; Tulej, M.; Pachkov, M. V.; Schnaiter, M.; Güthe, F.; Grutter, M.; Wyss, M.; Maier, J. P.; Fischer, G. J. Chem. Phys.1999, 111, 9280. Acknowledgment The authors would like to thank Georg Holderied and Dieter Wild and the mechanical workshop for their technical assistance. Tomasz Motylewski and Danielle Furio are also kindly thanked for their help with software development. for the experiment. Gas phase electronic spectra of the HC 2n H species for n=8-13 in the UV obtained using a R2C2PI scheme.They are assigned to the strong electronic transition known for the polyacetylenic chains HC 2n H. All bands are lifetime broadened (independent of the laser pulse energy) due to internal conversion to lower electronic states. The VUV ionisation from the vibrationally excited ground state is favoured due to because of good Franck-Condon factors. A very large red-shift relative to the gas phase position, of -3700 cm -1 in methanol and -1280 cm -1 in the neon matrix, is observed for n=8-12 without a significant dependence on the size of the chain. U 0 =500-1000V Width=10-200  s I=0.2-5 A Spectra of the much weaker A-system Work in the visible Future work The C n H m species are important intermediates in combustion processes and interstellar chemistry. Although these species are known from mass spectra their electronic absorption spectra are not well known. Direct absorption methods yield spectra, but the assignment to their carriers is difficult without information about their mass. Their electronic spectra are important for their identification and could be related to the diffuse interstellar band (interstellar absorptions in the visible range) problem. Bond length Alternation and electron delocalisation HC 2n H HC 2n - CHCCCH + CHCCCH - + CHCC CH - C HC CC - C CC - Electronis spectroscopy and intramolecular dynamics Resonance structures Nonlinear /N-plot For the neutral polyacetylenes!