Absolute Photoionization Cross Section of the Ethyl Radical in the Range 8–11.5 eV: Synchrotron and Vacuum Ultraviolet Laser Measurements

The absolute photoionization cross section of C2H5 has been measured at 10.54 eV using vacuum ultraviolet (VUV) laser photoionization. The C2H5 radical was produced in situ using the rapid C2H6 + F → C2H5 + HF reaction. Its absolute photoionization cross section has been determined in two different ways: first using the C2H5 + NO2 → C2H5O + NO reaction in a fast flow reactor, and the known absolute photoionization cross section of NO. In a second experiment, it has been measured relative to the known absolute photoionization cross section of CH3 as a reference by using the CH4 + F → CH3 + HF and C2H6 + F → C2H5 + HF reactions successively. Both methods gave similar results, the second one being more precise and yielding the value: σC2H5 ion = (5.6 ± 1.4) Mb at 10.54 eV. This value is used to calibrate on an absolute scale the photoionization curve of C2H5 produced in a pyrolytic source from the C2H5NO2 precursor, and ionized by the VUV beam of the DESIRS beamline at SOLEIL synchrotron facility. In this latter experiment, a recently developed ion imaging technique is used to discriminate the direct photoionization process from dissociative ionization contributions to the C2H5 + signal. The imaging technique applied on the photoelectron signal also allows a slow photoelectron spectrum with a 40 meV resolution to be extracted, indicating that photoionization around the adiabatic ionization threshold involves a complex vibrational overlap between the neutral and cationic ground states, as was previously observed in the literature. Comparison with earlier photoionization studies, in particular with the photoionization yield recorded by Ruscic et al. is also discussed.