Isomer-resolved IR–VUV–MATI spectroscopy of <i>cis</i> -3-penten-1-yne: Adiabatic ionization energy and vibronic structure

The formation of the first carbon rings represents an important step in hydrocarbon growth pathways relevant to Titan's atmosphere and related low-temperature environments. Among the proposed intermediates, cis-3-penten-1-yne (C5H6) links acyclic hydrocarbon isomers to cyclic species; however, its spectroscopic characterization has been hindered by isomeric congestion and the limited availability of high-resolution data, particularly for its cationic state. Herein, cis-3-penten-1-yne is characterized using infrared-resonant vacuum-ultraviolet mass-analyzed threshold ionization (IR-VUV-MATI) spectroscopy under jet-cooled conditions. Delayed VUV photoionization efficiency and high-resolution MATI measurements yield an adiabatic ionization energy (AIE) of 9.0902 ± 0.0005 eV (73 317 ± 4 cm-1). Isomer-specific IR-VUV double-resonance spectroscopy, supported by anharmonic calculations, shows that the neutral precursor adopts a single planar CS structure under isolated conditions. Franck-Condon (FC) simulations at the B3LYP and CCSD levels reproduce the observed vibronic structure of the cation and reveal substantial ionization-induced geometric relaxation dominated by low-frequency torsional and skeletal deformation modes. Mode-selective two-photon IR + VUV-MATI measurements further corroborate the vibrational assignments and clarify vibronic propensity effects. Quantitative analysis of the cis and trans 0-0 band intensities, corrected using calculated FC factors, provides an isomeric composition consistent with independent NMR measurements. The experimentally validated ordering of the cis and trans AIEs demonstrates the necessity of an isomer-resolved spectroscopic approach for reliable interpretation of the C5H6 system. These results provide quantitative molecular parameters and a detailed description of ionization-induced structural relaxation in a prototypical conjugated C5H6 isomer.