A quantitative study of proton irradiation and UV photolysis of benzene in interstellar environments

¹ Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands e-mail: ruiterka@strw.leidenuniv.nl
² Astrobiology Laboratory, Leiden Institute of Chemistry, PO Box 9502, 2300 RA Leiden, The Netherlands
³ NASA/Goddard Space Flight Center, Code 691, Greenbelt, Maryland 20771, USA
⁴ Department of Chemistry, Eckerd College, St. Petersburg, Florida 33733, USA

Received: 29 September 2004
Accepted: 13 May 2005

Abstract

Benzene is an essential intermediate in the formation pathways of polycyclic aromatic hydrocarbons (PAHs) and carbon dust. Therefore, it is important to understand the interplay of formation and destruction in order to assess the lifetime of benzene in space. We performed UV photolysis and proton (0.8 MeV) bombardment experiments on benzene (C₆H₆) isolated in inert argon matrices and in oxygen-rich solid mixtures in the laboratory. The destruction of benzene in different chemical environments was measured for both methods of energetic processing. Additionally, we quantitatively determined the absorbed photon fraction in the sample layers when exposed to our UV lamp with actinometry. This enabled us to derive destruction cross sections for benzene for both UV photolysis and proton bombardment allowing us to compare these two ways of energetic processing. The laboratory data were extrapolated to different interstellar environments and we found that benzene is efficiently destroyed in diffuse interstellar clouds, but could survive dense cloud environments longer than the average lifetime of the cloud. Benzene is likely to survive in the dense parts of circumstellar envelopes around carbon-rich AGB stars but only in a very finite region where UV photons are attenuated.