Valence Electronic Structure of Interfacial Phenol in Water Droplets

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.


S3. MODELLING
Table S1.Complex refractive indices,  =  + , of aqueous phenol solution employed in this work for different wavelengths and concentrations of phenol.We assumed  to be equal to that of water in the UV 3,4 and calculated  from the molar extinction coefficient of aqueous phenol solution (S2).To record these spectra of pure water, the laser powers were substantially increased compared with the power used for the spectra of the aqueous phenol droplets.We note here that the water background spectra that were subtracted from the aqueous phenol droplet spectra have even lower signal-to-noise than the gray 288 nm water spectrum shown in this figure because of the lower laser powers.

S5
Version of 12 August 2024  and polarization direction ì .Middle row: Corresponding simulated photoelectron VMIs for 274 nm two-photon ionization of aqueous phenol droplets.The phenol surface layer was modeled by a Gaussian radial concentration profile with a FWHM of 6 nm (Fig. S4).Bottom row: Forward-backward (with respect to ì ) asymmetry parameter  of the simulated photoelectron VMIs as a function of droplet radius (Eq.2, main manuscript).The simulated  values agree well with the measured asymmetry of  ∼ 0.8 for droplet radii of ∼250 nm.

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Version of 12 August 2024

S5. EFFECT OF ELECTRIC CHARGES AND ADDED ALKALI HALIDES ON THE PE SPECTRA S5.1 Effect of neutralizer
. Effect of a soft X-ray bipolar diffusion charger ("neutralizer"): Photoelectron spectra (smoothed) following resonance-enhanced twophoton ionization at 274 nm of 0.1 M PhOH(aq) in water droplets with an aerosol neutralizer in path turned on (blue) vs off (orange).Imposing a net neutral charge distribution on the aqueous phenol particles has only a very minor effect on the photoelectron spectrum.The effect is too small to explain the difference between R2PI droplet and LJ spectra.

Figure S1 .Figure S2 .Figure S3 .
Figure S1.Schematic sketch of the conical intersection between S 1 and S 2 and the potential energy curves along the photodissociation reaction coordinate.The potential energy curve of the S 2 state in bulk aqueous solution (orange, solid) and the lowered curve at the air-water interface (orange, dashed) are shown.The relative potential of the S 1 minimum and the S 1 /S 2 conical intersections (CIs) are quantitative quantum chemical calculation results from Ishiyama et al.1 Note that experiments suggest that at the air-water interface, the CI might already be accessible at 4.64 eV.2

Figure S4 . 8 S4
Figure S4.Simulated photoelectron VMIs (first column) and corresponding simulated and measured photoelectron spectra (second column) recorded after two-photon ionization (2PI) of aqueous phenol droplets with a radius of 250 nm.The simulations were performed with our electron scattering program.6The arrows in the first column indicate the laser propagation direction ì  and the laser polarization direction ì .The displayed eKE distributions in the second column correspond to the measured (gray), the simulated (blue), and the genuine (red) droplet photoelectron spectra.The centers of the Gaussian-shaped genuine spectra are marked by the vertical dotted lines.A secondary abscissa in red at the top shows the corresponding two-photon electron binding energies (eBE = 2ℎ − eKE).Top row: Nonresonant 2PI (N2PI) at 288 nm at low (0.01 M) phenol concentrations.Second row: N2PI at 288 nm at high (0.8 M) phenol concentrations.Third row: Resonance-enhanced 2PI (R2PI) at 274 nm at low (0.01 M) phenol concentrations.Bottom row: R2PI at 274 nm at high (0.8 M) phenol concentrations.The origins of the detected photoelectron as a function of the radial distance  from the droplet's center and the eKE are plotted as a heat map (third column; black: high electron yield, yellow: low electron yield, white: zero electron yield).The phenol surface layer for low and high concentration was modeled by Gaussian-shaped radial concentration profiles of 1 and 6 nm FWHM, respectively (fourth column) in accordance with neutron reflectivity experiments and recent MD simulations.1,7,8

Figure S6 .Figure S7 .
Figure S6.Cation (top) and anion (bottom) time-of-flight mass spectra of the ions ejected after disintegrating/ablating water droplets with intense, focused 800-nm femtosecond laser pulses.The TOF spectra show water clusters up to  = 8.Such clusters are detectable only when water droplets are present in the ionization region.