Valence Electronic\nStructure of Interfacial Phenol\nin Water Droplets

Biochemistry and a large part of atmospheric chemistry\noccur in\naqueous environments or at aqueous interfaces, where (photo)chemical\nreaction rates can be increased by up to several orders of magnitude.\nThe key to understanding the chemistry and photoresponse of molecules\nin and “on” water lies in their valence electronic structure,\nwith a sensitive probe being photoelectron spectroscopy. This work\nreports velocity-map photoelectron imaging of submicrometer-sized\naqueous phenol droplets in the valence region after nonresonant (288\nnm) and resonance-enhanced (274 nm) two-photon ionization with femtosecond\nultraviolet light, complementing previous liquid microjet studies.\nFor nonresonant photoionization, our concentration-dependent study\nreveals a systematic decrease in the vertical binding energy (VBE)\nof aqueous phenol from 8.0 ± 0.1 eV at low concentration (0.01\nM) to 7.6 ± 0.1 eV at high concentration (0.8 M). We attribute\nthis shift to a systematic lowering of the energy of the lowest cationic\nstate with increasing concentration caused by the phenol dimer and\naggregate formation at the droplet surface. Contrary to nonresonant\nphotoionization, no significant concentration dependence of the VBE\nwas observed for resonance-enhanced photoionization. We explain the\nconcentration-independent VBE of ∼8.1 eV observed upon resonant\nionization by ultrafast intermediate state relaxation and changes\nin the accessible Franck–Condon region as a consequence of\nthe lowering of the intermediate state potential energy due to the\nformation of phenol excimers and excited phenol aggregates. Correcting\nfor the influence of electron transport scattering in the droplets\nreduced the measured VBEs by 0.1–0.2 eV.