Valence Threshold Photoionization Spectra of C 5 $_5$ H and C 5 $_5$ H 2 $_2$ Isomers

$C_5{H}_x$ species are important intermediates in the formation of large carbonaceous molecules. Mass-selected threshold photoelectron spectra (TPES) of l-C ${}_5$ H, HC ${}_5$ H, and c-C ${}_3$ H-CCH were recorded using double-imaging photoelectron-photoion coincidence spectroscopy. The TPES of l-C ${}_5$ H and c-C ${}_3$ H-CCH are reported for the first time. These transient species were generated in situ by hydrogen abstraction from cyclopropylacetylene and 1,3-pentadiyne by fluorine atoms in a discharge flow-tube reactor. Supported by ab initio calculations and Franck-Condon simulations, unambiguous assignments were achieved, enabling accurate determination of adiabatic ionization energies. The C ${}_5$ H ${}_2$ results show that hydrogen abstraction from structurally distinct precursors leads to similar isomeric distributions. More generally, the high-resolution TPES fingerprints reported here provide reliable discrimination between linear and cyclic C ${}_5$ H ${}_x$ isomers, which is not possible using only photoion yields, offering robust benchmarks for identifying reactive hydrocarbon intermediates in combustion and astrochemical environments.