Dynamics of π∗-resonances in anionic clusters of para-toluquinone

Frequency-resolved photoelectron spectroscopy applied to mass-selected cluster anions is an insightful approach to characterise the dynamics of π∗-resonances with microsolvation. Here, the technique is demonstrated with monomer, dimer and trimer radical anions of para-toluquinone (pTQ) over a ∼1 eV excitation window above the detachment threshold. The pTQ^- spectra show similar resonances and dynamics to para-benzoquinone, a prototype electrophore. The dimer, (pTQ)₂ ^-, has a π-stacked geometry and shows a competition between photodissociation and prompt autodetachment. The trimer, (pTQ)₃ ^-, also has a π-stacked cluster geometry and shows vibrational autodetachment from a non-valence state up to ∼0.7 eV above-threshold, outcompeting dissociation. At higher photoexcitation energies, (pTQ)₃ ^- shows monomer-like dynamics, blue-shifted in photoexcitation energy by the cluster cohesion energy. Overall, the study highlights the variety of non-adiabatic dynamics available to π∗-resonances and the profound changes that occur through clusterization with one and two monomers.