Determination of absolute photoionization cross-sections of some aromatic hydrocarbons.

Aromatic hydrocarbons play an important role in the formation and growth of polycyclic aromatic hydrocarbon (PAH) and soot particles. Measurements of their absolute photoionization cross-sections (PICSs), that benefit the quantitative investigation of mass spectrometry, are still lacking, however. PICSs of some aromatic hydrocarbons were measured with tunable synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). Nitric oxide and benzene were chosen as standard references for PICS calibration, since their photoionization cross-sections are well documented in the literature. Binary liquid mixtures of the investigated molecules and their specific solvents were used in the measurements. The investigated aromatics include naphthalene, phenanthrene, 1-methylnaphthalene, indene, 2-/3-/4-methylphenylacetylene, 2-methylindene, diphenylacetylene, 1-/2-ethynylnaphthalene and acenaphthylene. Photo-induced fragments from the molecules were also observed with increasing photon energy. Based on our measurements and literature data, PICSs of most aromatic molecules have very similar values beyond their ionization energies. However, molecules that contain the phenylacetylene structure have PICSs higher than other aromatics.