Partial cross sections for positive and negative ion formation following electron impact on uracil

We report absolute partial cross sections for the formation of selected positive and negative ions resulting from electron interactions with uracil. Absolute calibration of the measured partial cross sections for the formation of the three most intense positive ions, the parent C4H4N2O2+ ion and the C3H3NO+ and OCN+ fragment ions, was achieved by normalization of the total single uracil ionization cross section (obtained as the sum of all measured partial single ionization cross sections) to a calculated cross section based on the semi-classical Deutsch-Mark formalism at 100 eV. Subsequently, we used the OCN+ cross section in conjunction with the known sensitivity ratio for positive and negative ion detection in our apparatus (obtained from the well-known cross sections for SF4+ and SF4- formation from SF6) to determine the dissociative attachment cross section for OCN- formation from uracil. This cross section was found to be roughly an order of magnitude smaller, about 5 x 10(-22) m(2) at 6.5 eV, compared to our previously reported preliminary value. We attribute this discrepancy to the difficult determination of the uracil target density in the earlier work. Using a reliably calculated cross section for normalization purposes avoids this complication.