Vibronic Spectroscopy and Cationic Features of p-Diethynylbenzene: Insights into Electronic Conjugation and Accidental Resonances

We report a comprehensive spectroscopic investigation of p-diethynylbenzene (pDEB) using two-color resonance-enhanced multiphoton ionization (REMPI) and mass-analyzed threshold ionization (MATI) spectroscopy, complemented by density functional theory (DFT) calculations. The S1 ← S0 electronic origin is observed at 34,255 ± 2 cm−1. The adiabatic ionization energy (IE) is determined to be 69,095 ± 5 cm−1 from two-color MATI spectra recorded via the S1 origin. Notably, a narrow peak is observed at 32 cm−1 above the IE in the two-color MATI spectrum, which is assigned to a one-color, two-photon accidental resonance arising from the near-resonant condition where the S1 ← S0 transition energy (34,255 cm−1) is close to half of the IE (69,099/2 = 34,549.5 cm−1). This observation is consistent with similar reports for p-chlorofluorobenzene and p-difluorobenzene. Franck–Condon simulations show good agreement with the experimental spectra. The present results provide a spectroscopic basis for identifying pDEB and distinguishing it from its ortho and meta isomers.