High-resolution threshold photoelectron study of the propargyl radical by the vacuum ultraviolet laser velocity-map imaging method

By employing the vacuum ultraviolet (VUV) laser velocity-map imaging (VMI) photoelectron scheme to discriminate energetic photoelectrons, we have measured the VUV-VMI-threshold photoelectrons (VUV-VMI-TPE) spectra of propargyl radical C ₃H ₃(X̃2B ₁) near its ionization threshold at photoelectron energy bandwidths of 3 and 7 cm ^-1 (full-width at half-maximum, FWHM). The simulation of the VUV-VMI-TPE spectra thus obtained, along with the Stark shift correction, has allowed the determination of a precise value 70 156 ± 4 cm ^-1 (8.6982 ± 0.0005 eV) for the ionization energy (IE) of C ₃H ₃. In the present VMI-TPE experiment, the Stark shift correction is determined by comparing the VUV-VMI-TPE and VUV laser pulsed field ionization-photoelectron (VUV-PFI-PE) spectra for the origin band of the photoelectron spectrum of the X̃+-X̃ transition of chlorobenzene. The fact that the FWHMs for this origin band observed using the VUV-VMI-TPE and VUV-PFI-PE methods are nearly the same indicates that the energy resolutions achieved in the VUV-VMI-TPE and VUV-PFI-PE measurements are comparable. The IE(C ₃H ₃) value obtained based on the VUV-VMI-TPE measurement is consistent with the value determined by the VUV laser PIE spectrum of supersonically cooled C ₃H ₃(X̃2B ₁) radicals, which is also reported in this article.