Laboratory Investigations of the Formation of Superhydrogenated PAHs

Details Laboratory Investigations of the Formation of Superhydrogenated PAHs Laboratory Investigations of the Formation of Superhydrogenated PAHs

May 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011iaus..280e..63t&link_type=abstract

The Molecular Universe, Proceedings of the 280th Symposium of the International Astronomical Union held in Toledo, Spain, May 30

Astronomy and Astrophysics

Astronomy

Scientific paper

We present experimental investigations of the formation of superhydrogenated polycyclic aromatic hydrocarbons (PAHs) through exposure of neutral PAHs to atomic hydrogen. Post irradiation thermal desorption measurements demonstrate that it is possible to form superhydrogenated coronene molecules with surprisingly high numbers of additional H atoms. We observe species with in excess of 20 additional H atoms, approaching the fully hydrogenated limit, perhydrocoronene. Scanning tunneling microscopy (STM) measurements provide insight into the changes to the electronic structure of the coronene species as a result of the hydrogenation process. Submolecular structure observed in the STM images indicates that a range of different sites on the coronene molecule are hydrogenated. The formation of PAHs with such high degrees of superhydrogenation may have important implications for molecular hydrogen formation in the ISM. Recent density functional theory calculations1 have indicated the efficient formation of H2 through a combination of hydrogenation and abstraction reactions involving neutral PAHs and their superhydrogenated forms. Although previous measurements by Menella2 have demonstrated the efficiency of both H addition and subsequent abstraction reactions involving a complex aromatic carbon grain mimic, the measurements reported here provide the first direct observation of neutral PAH superhydrogenation by H atom addition. The role played by PAHs in H2 formation is suggested by observations of a correlation between H2 and PAHs abundances in photodissociation regions3. Routes to H2 involving PAH cations have been investigated by several groups4-6 and the role played by anions has also been considered7. The present work demonstrates that neutral PAHs may also provide a formation route for this most important of interstellar molecules.

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