Valence Electronic Structure of Interfacial Phenol in Water Droplets

Abstract

Biochemistry and a large part of atmospheric chemistry occur in aqueous environments or at aqueous interfaces, where (photo)chemical reaction rates can be increased by up to several orders of magnitude. The key to understanding the chemistry and photoresponse of molecules in and "on" water lies in their valence electronic structure, with a sensitive probe being photoelectron spectroscopy. This work reports velocity-map photoelectron imaging of submicrometer-sized aqueous phenol droplets in the valence region after nonresonant (288 nm) and resonance-enhanced (274 nm) two-photon ionization with femtosecond ultraviolet light, complementing previous liquid microjet studies. For nonresonant photoionization, our concentration-dependent study reveals a systematic decrease in the vertical binding energy (VBE) of aqueous phenol from 8.0 ± 0.1 eV at low concentration (0.01 M) to 7.6 ± 0.1 eV at high concentration (0.8 M). We attribute this shift to a systematic lowering of the energy of the lowest cationic state with increasing concentration caused by the phenol dimer and aggregate formation at the droplet surface. Contrary to nonresonant photoionization, no significant concentration dependence of the VBE was observed for resonance-enhanced photoionization. We explain the concentration-independent VBE of ∼8.1 eV observed upon resonant ionization by ultrafast intermediate state relaxation and changes in the accessible Franck-Condon region as a consequence of the lowering of the intermediate state potential energy due to the formation of phenol excimers and excited phenol aggregates. Correcting for the influence of electron transport scattering in the droplets reduced the measured VBEs by 0.1-0.2 eV.

Figure 1

Schematic illustration of the photoionization of phenol molecules at the vacuum–water interface, photoelectron transport scattering and escape (left), and the energy-level diagram of aqueous phenol (right). The vertical arrows represent the employed nonresonant (blue) and 1 + 1 resonance-enhanced (purple) two-photon ionization schemes.

Figure 2

Photoelectron VMIs (A,B) and corresponding photoelectron spectra (C) recorded after two-photon ionization at 274 nm for aqueous phenol droplets [(A), blue spectrum in (C)] and neat liquid phenol droplets [(B), green spectrum in (C)] generated by drying the aqueous droplets. The 1 + 1 REMPI photoelectron spectrum of gas-phase phenol at 275 nm from Riley et al. (orange) is shown as a reference. (Adapted with permission from ref (37). Copyright 2018 American Chemical Society.) The arrows indicate laser propagation and polarization direction.

Figure 3

Experimental photoelectron spectra of aqueous phenol in submicrometer-sized water droplets recorded after nonresonant two-photon ionization (N2PI) at 288 nm (top panel) and 1 + 1 resonance-enhanced two-photon ionization (R2PI) at 274 nm (bottom panel). The concentrations of the atomized bulk aqueous phenol solutions are 0.01 M (blue) and 0.8 M (green). Literature photoelectron spectra of phenol in liquid-water microjets are shown for comparison.^,, To enhance clarity, a binomial smoothing routine was applied to the droplet spectra and the LJ spectra from Scholz et al. The LJ spectra from refs ( and 38) are digitized versions from the publications. Adapted with permission from ref (38). Copyright 2020 Royal Society, licensed under a Creative Commons Attribution 3.0 Unported License. Adapted with permission from refs ( and 35). Copyright 2012 and 2022 American Chemical Society.

Figure 4

(A) Experimental N2PI droplet photoelectron spectra as a function of the phenol concentration (legend). For clarity, a binomial smoothing routine was applied to the raw spectra. (B) Concentration dependence of VBE₁. The error bars indicate the estimated overall uncertainty of the determination of VBE₁.

Figure 5

Left: Simulated photoelectron VMIs. Arrows indicate laser propagation and polarization direction . Right: Experimental (gray), simulated (blue), and genuine (red) photoelectron spectra. Top row: N2PI of droplets at 0.01 M phenol concentration. Second row: N2PI of droplets at 0.8 M phenol concentration. Third row: R2PI of droplets at 0.01 M phenol concentration. Bottom row: R2PI of droplets at 0.8 M phenol concentration. The lower abscissa in black shows the eKEs and the upper abscissa in red shows the eBEs. The vertical dotted lines indicate the genuine VBE₁ (red number) and the corresponding genuine eKE (black number) in eV. See text and Figure S4 for more information.

Figure 6

Schematic illustration of the droplet–vacuum interface (top row) and energy-level diagrams and ionization schemes (bottom row) at low (left) and high (right) phenol concentrations. The formation of phenol excimers and excited phenol aggregates at high concentration results in a stabilization of the S₁ intermediate state and the D₀ cationic ground state.