Photolysis of allene and propyne in the $7–30eV$ region probed by the visible fluorescence of their fragments

The photolysis of allene and propyne, two isomers of $C3H4$⁠, has been investigated in the excitation energy range of $7–30eV$ using vacuum ultraviolet synchrotron radiation. The visible fluorescence excitation spectra of the excited neutral photofragments of both isomers were recorded within the same experimental conditions. Below the first ionization potential (IP), this fluorescence was too weak to be dispersed and possibly originated from $C2H$ or $CH2$ radicals. Above IP, three excited photofragments have been characterized by their dispersed emission spectra: the CH radical $(AΔ2-XΠ2)$⁠, the $C2$ radical (⁠$dΠg3–aΠu3$⁠, “Swan’s bands”), and the H atom (4–2 and 3–2 Balmer lines). A detailed analysis of the integrated emission intensities allowed us to determine several apparition thresholds for these fragments, all of them being interpreted as rapid and barrierless dissociation processes on the excited potential energy surfaces. In the low energy range explored in this work, both isomers exhibit different intensity distributions in their fragment emission as a function of the photolysis energy, indicating that mutual $allene↔propyne$ isomerization is not fully completed before dissociation occurs. The effect of isomerization on the dissociation into excited fragments is present in the whole excitation energy range albeit less important in the $7–16eV$ region; it gradually increases with increasing excitation energy. Above $19eV$⁠, the fragment distribution is very similar for the two isomers.