SPECTRA OF JET-COOLED ALL- BENZENOID PAHS - TRIPHENYLENE (C 18 H 12 ) AND HEXA-PERI- HEXABENZOCORONENE(C 42 H 18 ) DAMIAN L. KOKKIN, TYLER P. TROY, NEIL J. REILLY, MASAKAZU NAKAJIMA, KLAAS NAUTA, NIGEL LUCAS and TIMOTHY W. SCHMIDT, School of Chemistry, University of Sydney, NSW 2006, Australia; THOMAS D. VARBERG, Department of Chemistry, Macalester College, St. Paul, MN USA; GREGORY F. METHA, School of Chemistry & Physics, The University of Adelaide, SA 5005 Australia.

Orion Nebula The Diffuse Interstellar Bands. Diffuse absorption bands observed in the spectra of reddened stars. Some 300 such absorption features exist spanning across the visible and into the near IR. The nature of the carriers remain elusive despite exhaustive investigations of the DIBs since their discovery in HD

Interstellar carbon Sun’s carbon abundance : 370 C atoms per 10 6 H atoms Recommended galactic value : 225±50 C atoms per 10 6 H atoms In the lines of sight towards many stars, carbon is depleted:  Sco – C atoms per 10 6 H atoms  Oph – C per 10 6 H atoms …available for molecules and dust grains!

Postulates Upon the Mystery Dust Grains Carbon Chains HC n H n > 26 Polycyclic Aromatic Hydrocarbons (PAHs) Fullerene

The Unidentified Infra Red (UIR) bands Infrared emission features that match the vibrational modes of the polycyclic aromatic hydrocarbons (PAHs) Carriers must be carbon rich Must survive harsh conditions and  must be extremely stable

= 20600!!!

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All Benzenoid PAHs & the Clar rule of the Aromatic Sextet A class of PAHs which contain only aromatic sextets. i.e p-electrons in groups of six. Lower chemical reactivityHigher thermodynamic stability Largest HOMO-LUMO gapHighest first ionisation energy Number of carbon atoms necessary is an integral multiple of six. Of the possible PAHs with 4 to 10 fused benzene rings only 17 are all benzoid

Triphenylene The Experiment Heat the sample and nozzle to 150 o C Probed via LIF and DF.

D 3h symmetry distorted into C 2v symmetry by e’ modes. Electronic transition moment Strength of a vibronic transition Oscillator Strength Triphenylene The Calculations

Triphenylene The Results

Hexa-peri- hexabenzocoronene (HBC): A Candidate for the DIBs? A fully benzenoid Clar structure,  highly stable A large species and a quintessential all-benzenoid “ Its melting point could not be determined because the melting-point tube melted long before the hydrocarbon.”

HBC the Experiment Solid sample ablated with 532nm. Carrier gas: Ar Probed via 2 colour REMPI- TOFMS

HBC The Results D 6h Symmetry –Forbidden Origin B 2u  A 1g –Modes induced by e 2g vibrations a and b false origins f = 1.4 x –Upper limit of 4x10 12 cm -2 2x10 -4 fraction of carbon

Benzene-like orbitals HOMOs LUMOs Is the transition B 2u – A 1g, like benzene?

Future Directions C 78 H 26 Even bigger systems

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