Dissociative photoionization of 1,3-butadiene: Experimental and theoretical insights

The vacuum-ultraviolet photoionization and dissociative photoionization of 1,3-butadiene in a region ∼8.5-17 eV have been investigated with time-of-flight photoionization mass spectrometry using tunable synchrotron radiation. The adiabatic ionization energy of 1,3-butadiene and appearance energies for its fragment ions, C₄H₅⁺, C ₄H₄⁺, C₄H₃⁺, C₃H₃⁺, C₂H₄⁺, C₂H₃⁺, and C₂H₂ ⁺, are determined to be 9.09, 11.72, 13.11, 15.20, 11.50, 12.44, 15.15, and 15.14 eV, respectively, by measurements of photoionization efficiency spectra. Ab initio molecular orbital calculations have been performed to investigate the reaction mechanism of dissociative photoionization of 1,3-butadiene. On the basis of experimental and theoretical results, seven dissociative photoionization channels are proposed: C₄H ₅⁺ H, C₄H₄⁺ H ₂, C₄H₃⁺ H₂ H, C ₃H₃⁺ CH₃, C₂H ₄⁺ C₂H₂, C₂H ₃⁺ C₂H₂ H, and C₂H ₂⁺ C₂H₂ H₂. Channel C₃H₃⁺ CH₃ is found to be the dominant one, followed by C₄H₅⁺ H and C ₂H₄⁺ C₂H₂. The majority of these channels occur via isomerization prior to dissociation. Transition structures and intermediates for those isomerization processes were also determined.