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.

UV Photoelectron Spectroscopy of Aqueous Solutions.

Knowledge of the electronic structure of an aqueous solution is a prerequisite to understanding its chemical and biological reactivity and its response to light. One of the most direct ways of determining electronic structure is to use photoelectron spectroscopy to measure electron binding energies. Initially, photoelectron spectroscopy was restricted to the gas or solid phases due to the requirement for high vacuum to minimize inelastic scattering of the emitted electrons. The introduction of liquid-jets and their combination with intense X-ray sources at synchrotrons in the late 1990s expanded the scope of photoelectron spectroscopy to include liquids. Liquid-jet photoelectron spectroscopy is now an active research field involving a growing number of research groups. A limitation of X-ray photoelectron spectroscopy of aqueous solutions is the requirement to use solutes with reasonably high concentrations in order to obtain photoelectron spectra with adequate signal-to-noise after subtracting the spectrum of water. This has excluded most studies of organic molecules, which tend to be only weakly soluble. A solution to this problem is to use resonance-enhanced photoelectron spectroscopy with ultraviolet (UV) light pulses (hν ≲ 6 eV). However, the development of UV liquid-jet photoelectron spectroscopy has been hampered by a lack of quantitative understanding of inelastic scattering of low kinetic energy electrons (≲5 eV) and the impact on spectral lineshapes and positions.In this Account, we describe the key steps involved in the measurement of UV photoelectron spectra of aqueous solutions: photoionization/detachment, electron transport of low kinetic energy electrons through the conduction band, transmission through the water-vacuum interface, and transport through the spectrometer. We also explain the steps we take to record accurate UV photoelectron spectra of liquids with excellent signal-to-noise. We then describe how we have combined Monte Carlo simulations of electron scattering and spectral inversion with molecular dynamics simulations of depth profiles of organic solutes in aqueous solution to develop an efficient and widely applicable method for retrieving true UV photoelectron spectra of aqueous solutions. The huge potential of our experimental and spectral retrieval methods is illustrated using three examples. The first is a measurement of the vertical detachment energy of the green fluorescent protein chromophore, a sparingly soluble organic anion whose electronic structure underpins its fluorescence and photooxidation properties. The second is a measurement of the vertical ionization energy of liquid water, which has been the subject of discussion since the first X-ray photoelectron spectroscopy measurement in 1997. The third is a UV photoelectron spectroscopy study of the vertical ionization energy of aqueous phenol which demonstrates the possibility of retrieving true photoelectron spectra from measurements with contributions from components with different concentration profiles.

Ionisation of PF3: absolute partial electron ionisation cross sections and the formation and reactivity of dication states.

Absolute partial electron ionisation cross sections, and precursor-specific partial electron ionisation cross sections, for the formation of cations from phosphorus trifluoride (PF3) are reported over the electron energy range 50-200 eV. The absolute values are determined by the measurement of cross sections relative to the formation of PF3+ using 2D ion-ion coincidence time-of-flight mass spectrometry and subsequent scaling using binary encounter-Bethe calculations of the total ionisation cross section. This new dataset significantly augments the partial ionisation cross sections for electron ionization of PF3 found in literature, addressing previous discrepancies in the branching ratios of product ions, and provides the first values for the precursor-specific cross sections. Comparisons to calculated cross sections from the literature are encouraging, although there are discrepancies for individual ions. The coincidence experiments indicate that double and triple ionisation generate approximately 20% of the cationic ionisation products at 200 eV electron energy. One dissociative dication state, dissociating to PF2+ + F+, is clearly identified as the lowest triplet state of PF32+ and five different dications (PF32+, PF22+, PF2+, P2+ and F2+) are detected in the mass spectra. The dication energetics revealed by the experiments are supported by a computational investigation of the dication's electronic structure. The cross sections reported will allow more accurate modelling of the role of the ionization of PF3 in energetic environments. A first investigation of the bimolecular reactivity of metastable states of PF32+ is also reported. In collisions with Ar, O2 and CO dissociative single electron transfer dominates the product ion yield, whereas collision-induced dissociation of the dication is important following collisions with Ne. Consideration of the energetics of these processes indicates that the reactant dication beam contains ions in both the ground singlet state and the first excited triplet state. The deduction regarding the longevity of the triplet state is supported by metastable signals in the coincidence spectra. Weak signals corresponding to the formation of ArF+ are detected following PF32+ collisions with Ar, and experimental and computational considerations indicate this new chemical bond is formed via a collision complex.

Wavelength dependent mechanism of phenolate photooxidation in aqueous solution.

Phenolate photooxidation is integral to a range of biological processes, yet the mechanism of electron ejection has been disputed. Here, we combine femtosecond transient absorption spectroscopy, liquid-microjet photoelectron spectroscopy and high-level quantum chemistry calculations to investigate the photooxidation dynamics of aqueous phenolate following excitation at a range of wavelengths, from the onset of the S0-S1 absorption band to the peak of the S0-S2 band. We find that for λ ≥ 266 nm, electron ejection occurs from the S1 state into the continuum associated with the contact pair in which the PhO˙ radical is in its ground electronic state. In contrast, we find that for λ ≤ 257 nm, electron ejection also occurs into continua associated with contact pairs containing electronically excited PhO˙ radicals and that these contact pairs have faster recombination times than those containing PhO˙ radicals in their ground electronic state.

Accurate Vertical Ionization Energy of Water and Retrieval of True Ultraviolet Photoelectron Spectra of Aqueous Solutions.

Ultraviolet (UV) photoelectron spectroscopy provides a direct way of measuring valence electronic structure; however, its application to aqueous solutions has been hampered by a lack of quantitative understanding of how inelastic scattering of low-energy (<5 eV) electrons in liquid water distorts the measured electron kinetic energy distributions. Here, we present an efficient and widely applicable method for retrieving true UV photoelectron spectra of aqueous solutions. Our method combines Monte Carlo simulations of electron scattering and spectral inversion, with molecular dynamics simulations of depth profiles of organic solutes in aqueous solution. Its application is demonstrated for both liquid water, and aqueous solutions of phenol and phenolate, which are ubiquitous biologically relevant structural motifs.