Accurate Determination of Dissociation Energy Using UV‐UV Double Resonance Coupled Resonant Two‐Photon Ionization Spectroscopy

The excited‐state dissociation energy (D 0 (S 1 )) of the 2‐(2′‐pyridyl)benzimidazole·Ar (PBI·Ar) complex has been precisely determined using a combination of resonant two‐photon ionization (R2PI) and UV‐UV double resonance (UV–UV DR) spectroscopy. The R2PI spectrum of the complex exhibits vibronic transitions similar to the PBI monomer up to 400 cm −1 above the origin, while bands between 400–550 cm −1 show decreased intensity and those above 590 cm −1 disappear entirely, suggesting the onset of predissociation. The UV–UV DR spectroscopy confirmed the presence of Franck–Condon activity beyond the observed R2PI range, demonstrating that the missing bands arise from rapid dissociation rather than the absence of optical transitions. By correlating the first missing R2PI band (562 cm −1 ) and the last Franck‐Condon (FC)‐active band detected in the UV–UV DR spectrum (570 cm −1 ), the S 1 ‐state dissociation energy was accurately bracketed at 566 ± 4 cm −1 . This study highlights the effectiveness of UV–UV DR spectroscopy as a precise tool for probing nonradiative decay and binding strength in weakly bound aromatic molecular complexes in the higher electronic state.