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Abstract: We measured the absolute ionic outgassing yield (AIOY) with a double-ion chamber method, characterized the outgassed ion species, derived the relative extent of outgassing F+, CH3 + and C2H5 + ions, determined the exposure rate constant for the formation of F+ and CH3 + with a quadrupole mass filter, and monitored the relative pressure rise with an ion gauge. These experiments were performed with radiation at wavelength 13.5nm from the synchrotron beamline 08A1BM-LSGM at National Synchrotron Radiation Research Center in Taiwan. Samples investigated included the round-robin resist, poly(methylmethacrylate), and twelve underlayer materials. The photochemistry in the extreme ultraviolet region of the fourteen samples is described according to the following results. The AIOY values, of order 10−3, are proportional to the absorption coefficients of the samples. The extent of F+ outgassing is related directly to the extent of F photoabsorption by the samples. Outgassing of CH3 + and C2H5 + ions depends strongly on the type of polymer; the CH3 + outgassing and relative pressure rise both depend on a parameter related to photoabsorption at 13.5nm and the polymeric structure of the sample. Dill''s C parameter for CH3 + outgassing is identified to depend on the polymer type, whereas that for F+ outgassing depends on the fluorine-containing moiety. Reaction mechanisms of EUV photochemistry are proposed according to the experimental results. [Copyright &y& Elsevier]