Electronic Spectroscopy of long Carbon Chains HC 2n H (n= 8-13) in the Gas Phase

1. + - HC C C CH - C HC C C HC C C CH - C C C HC - + CH HC C C Electronic Spectroscopy of long Carbon Chains HC2nH (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 CnHm from discharge source to record REMPI spectra Mass spectum of diacetylene discharge recorded at 157 nm A new apparatus for the investigation of neutral CnHm 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 Argon1,2 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 (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 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 F2 excimer laser operating in the VUV (157nm – 7.9 eV) is used. The CnHm 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. U0=500-1000V Width=10-200ms I=0.2-5 A Bond length Alternation and electron delocalisation Electronis spectroscopy and intramolecular dynamics Gas phase electronic spectra of the HC2nH 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 HC2nH. 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. Nonlinear l/N-plot For the neutral polyacetylenes! HC2nH HC2n- Resonance structures 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. Future work 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. • Spectra of the much weaker A-system • Work in the visible