Absolute photoionization cross-section of the propargyl radicala)

Using synchrotron-generated vacuum-ultraviolet radiation and multiplexed time-resolved photoionization mass spectrometry we have measured the absolute photoionization cross-section for the propargyl (C₃H₃) radical, σ _{propargyl}^{ion} (E), relative to the known absolute cross-section of the methyl (CH₃) radical. We generated a stoichiometric 1:1 ratio of C₃H₃ : CH₃ from 193 nm photolysis of two different C₄H₆ isomers (1-butyne and 1,3-butadiene). Photolysis of 1-butyne yielded values of σ _{propargyl}^{ion} (10{.213 eV}) = (26.1 ± 4.2) Mb and σ _{propargyl}^{ion} (10{.413 eV}) = (23.4 ± 3.2) Mb, whereas photolysis of 1,3-butadiene yielded values of σ _{propargyl}^{ion} (10{.213 eV}) = (23.6 ± 3.6) Mb and σ _{propargyl}^{ion} (10{.413 eV}) = (25.1 ± 3.5) Mb. These measurements place our relative photoionization cross-section spectrum for propargyl on an absolute scale between 8.6 and 10.5 eV. The cross-section derived from our results is approximately a factor of three larger than previous determinations.