Table S1; Scheme S1; Figures S1-S7.

Supplementary material includes Table S1 containing additional computed structural parameters and relative energies; Schematics S1 illustrating the synthetic route to cis-3-penten-1-yne; Figures S1-S2 showing the 1H and 13C NMR spectra of cis-3-penten-1-yne; Figure S3 presenting the VUV photoionization mass spectrum; Figure S4 displaying VUV-MATI spectra recorded under different molecular-beam conditions; Figure S5 showing calculated harmonic and anharmonic vibrational spectra of the neutral species; Figure S6 comparing Franck-Condon simulations obtained at different electronic-structure levels; and Figure S7 presenting mode-selective IR + VUV-MATI spectra. The supplementary material is available online.