Photoelectron imaging of substituted benzenes in aqueous aerosol droplets

Abstract

Photochemical reactions can be orders of magnitude faster at the surface of water than in bulk solution, possibly due to changes in the stability of electronic ground and excited states. Yet, direct measurements of the interfacial electronic structure of aqueous reactants remain scarce, making it challenging to establish a clear connection between macroscopic photoreactivities and the underlying molecular-level electronic structure. Here, we employ surface-sensitive ultraviolet (UV) photoelectron velocity-map imaging to probe the valence electronic structure of 13 substituted benzenes at the interface of submicrometer-sized aqueous aerosol droplets. The droplet environment induces vertical binding energy (VBE) shifts of several electronvolts relative to the gas phase for aromatic anions, while neutral solutes show more modest gas-to-solution shifts. Increasing the solute concentration may shift the VBEs of some neutral, protic benzene derivatives, possibly due to increased solute-solute interactions such as hydrogen bonding or π-stacking. In contrast, their anionic conjugate bases show no such shift, likely due to electrostatic repulsion, preventing short-range solute-solute interactions. Changes in droplet surface tension and coverage were quantified through concentration-dependent photoelectron yields. The measured data reveal that 300-nm droplets require a 10 000-fold higher concentration of a proxy nonionic surfactant (Triton X-100) than macroscale solutions to achieve an equivalent surface tension. This observation exemplifies the altered surface partitioning behavior in submicron droplets. It underscores the necessity to account for significant solute depletion in the interior of droplets with considerable surface-to-volume ratios. Phenol-water clusters (170 water molecules) and dilute aqueous phenol droplets (50 mM) exhibit matching valence electronic structure, confirming surface selectivity in UV droplet photoelectron imaging and validating cluster studies as models for interfacial solvation.

Fig. 1. Structures (top) and UV-vis absorption spectra (bottom) of the neutral substituted benzenes (solid lines) and anionic conjugate bases (dashed lines) in aqueous solution recorded using a PerkinElmer LAMBDA 2 UV-vis spectrometer. The blue Gaussian marks the spectral profile of the 267-nm fs pulse used for one-color, two-photon photoelectron spectra, and the shaded region marks its full width at half maximum.

Fig. 2. Sketch of the photoelectron VMI spectrometer. Droplets are generated using an atomizer, optionally neutralized, transferred to vacuum and collimated by an ADL, and ionized by fs/ns 267-nm laser pulses. Photoelectrons are velocity-mapped onto a position-sensitive detector (MCP, phosphor screen, camera) using a three-plate electrostatic lens.

Fig. 3. Phenol: Photoelectron spectra and VMIs of phenol in aqueous droplets from single-pulse femtosecond 1 + 1 resonance-enhanced two-photon ionization at 267 nm. Full (a) and corresponding anisotropic VMI (b) from an 800-mM solution. Spectra (gray) with fit (red) as retrieved from the full (c and e) and anisotropic VMIs (d and f) at 50 and 800 mM. A rrows indicate laser propagation (k⃑) and polarization (E⃑) directions.

Fig. 4. Phenolate: Photoelectron spectra and VMIs of phenolate in aqueous droplets from single-pulse femtosecond 1 + 1 resonance-enhanced two-photon ionization at 267 nm. Full (a) and corresponding anisotropic VMI (b) from an 800-mM solution. Spectra (gray) with fit (red) as retrieved from the full (c and e) and anisotropic VMIs (d and f) at 50 and 800 mM. Arrows indicate laser propagation (k⃑) and polarization (E⃑) directions.

Fig. 5. Vertical electron binding energies (top row) and total photoelectron yield (bottom row) plotted as a function of concentration for phenol (left column) and phenolate (right column). Error bars represent ±0.1 eV uncertainty in vertical binding energy from sensitivity analysis and ±40% relative uncertainty in total photoelectron yield. The uncertainty value was chosen to conservatively account for systematic uncertainties (droplet-beam density fluctuations and laser-flux determination) while remaining sufficiently small to resolve the observed, reproducible increase in signal with increasing concentration.

Fig. 6. Total droplet photoelectron yield (circles) as a function of total solute concentration for aqueous phenol (top) and Triton X-100 (bottom). Error bars represent ±40% relative uncertainty in total photoelectron yield, chosen to conservatively account for systematic uncertainties (droplet-beam density fluctuations and laser-flux determination) while remaining sufficiently small to resolve the observed, reproducible increase in signal with increasing concentration. Model predictions for the concentration-dependent fractional surface coverage θ for a macroscale solution (Langmuir eqn (3), solid line) and submicrometer-sized droplets (finite-size model, eqn (5), dashed lines). Model predictions are based on values for K_ad and Γ_max obtained from fitting literature surface tension data to the Szyszkowski eqn (2): phenol:K_ad = 1.0 × 10⁴ cm³ mol⁻¹, Γ_max = 5.6 × 10⁻¹⁰ mol cm⁻²; Triton X-100:K_ad = 6.6 × 10⁸ cm³ mol⁻¹, Γ_max = 3.3 × 10⁻¹⁰ mol cm⁻².

Fig. 7. Photoelectron spectra of phenol–water clusters (blue) and phenol (50 mM) in aqueous droplets (orange) from single-pulse femtosecond 1 + 1 resonance-enhanced two-photon ionization at 267 nm.