The photoelectron spectroscopic spectra (PES) of two important atmospheric pollutants, peroxyacetyl nitrate (PAN) and peroxypropionyl nitrate (PPN), are reported. To assign the PES of the molecules both HF and OVGF (outer valence green function) methods have been performed. Moreover, the molecular geometries of the two compounds have been optimized. There are two separate peaks in the low energy region of PES (11.42 eV and 12.07 eV for PAN, 11.08 eV and 11.79 eV for PPN) resulted from respective ionizations of the electrons of the HOMO and the SHOMO, which have dominant contributions of the NO2 group in the molecules. The first ionization energy (11.08 eV) of PPN is lower than that (11.42 eV) of PAN, which shows that PPN possesses more potent bioactivity than PAN, which is attributed to the electron donating action of the increasing "CH_2" group.

Key words: AB INITIO CALCULATION, PAN, PPN, TOXICITY, IONIZATION ENERGY