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

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 wavelength-dependent photoionization quantum yields for the dark 1nπ* state of aqueous thymidine

Despite the important role of the dark 1nπ* state in the photostability of thymidine in aqueous solution, no detailed ultraviolet (UV) wavelength-dependent investigation of the 1nπ* quantum yield (QY) in aqueous thymidine has been experimentally performed. Here, we investigate the wavelength-dependent photoemission spectra of aqueous thymidine from 266.7 to 240 nm using liquid-microjet photoelectron spectroscopy. Two observed ionization channels are assigned to resonant ionizations from 1ππ* to the cationic ground state D0 (π-1) and 1nπ* to the cationic excited state D1 (n-1). The weak 1nπ* → D1 ionization channel appears due to ultrafast 1ππ* → 1nπ* internal conversion within the pulse duration of ∼180 fs. The obtained 1nπ* quantum yields exhibit a strong wavelength dependence, ranging from 0 to 0.27 ± 0.01, suggesting a hitherto uncharacterized 1nπ* feature. The corresponding vertical ionization energies (VIEs) of D0 and D1 of aqueous thymidine are experimentally determined to be 8.47 ± 0.12 eV and 9.22 ± 0.29 eV, respectively. Our UV wavelength-dependent QYs might indicate that different structural critical points to connect the multidimensional 1ππ*/1nπ* conical intersection seam onto the multidimensional potential energy surface of the 1ππ* state might exist and determine the relaxation processes of aqueous thymidine upon UV excitation.

Accurate Vertical Electron Detachment Energies and Multiphoton Resonant Photoelectron Spectra of Biochromophore Anions in Aqueous Solution

We introduce a new methodology for calculating vertical electron detachment energies (VDEs) of biologically relevant chromophores in their deprotonated anionic forms in aqueous solution. It combines a large-scale mixed DFT/EFP/MD approach with the high-level multireference perturbation theory XMCQDPT2 and the Effective Fragment Potential (EFP) method. The methodology includes a multiscale flexible treatment of inner (∼1000 water molecules) and outer (∼18000 water molecules) water shells around a charged solute, capturing both the effects of specific solvation and the properties of bulk water. VDEs are calculated as a function of system size for getting a converged value at the DFT/EFP level of theory. The XMCQDPT2/EFP approach, adapted for calculating VDEs, supports the DFT/EFP results. When corrected for a solvent polarization contribution, the XMCQDPT2/EFP method yields the most accurate estimate to date of the first VDE for aqueous phenolate (7.3 ± 0.1 eV), which agrees well with liquid-jet X-ray photoelectron spectroscopy data (7.1 ± 0.1 eV). We show that the geometry of the water shell and its size are essential for accurate VDE calculations of aqueous phenolate and its biologically relevant derivatives. By simulating photoelectron spectra of aqueous phenolate upon two-photon excitation at wavelengths resonant with the S₀ → S₁ transition, we also provide interpretation of recent multiphoton UV liquid-microjet photoelectron spectroscopy experiments. We show that its first VDE is consistent with our estimate of 7.3 eV, when experimental two-photon binding energies are corrected for the resonant contribution.

Distortion Correction of Low-Energy Photoelectron Spectra of Liquids Using Spectroscopic Data for Solvated Electrons

Time-resolved photoelectron spectroscopy (TRPES) enables real-time observation of ultrafast electronic dynamics in solutions. When extreme ultraviolet (EUV) probe pulses are employed, they can ionize solutes from all electronic states involved in the dynamics. However, EUV pulses also produce a strong ionization signal from a solvent that is typically 6 orders of magnitude greater than the pump-probe photoelectron signal of solutes. Alternatively, UV probe pulses enable highly sensitive and selective observation of photoexcited solutes because typical solvents such as water are transparent to UV radiation. An obstacle in such UV-TRPES measurements is spectral distortion caused by electron scattering and a yet to be identified mechanism in liquids. We have previously proposed the spectral retrieval (SR) method as an a posteriori approach to removing the distortion and overcoming this difficulty in UV-TRPES; however, its accuracy has not yet been verified by comparison with EUV-TRPES results. In the present study, we perform EUV-TRPES for charge transfer reactions in water, methanol, and ethanol, and verify SR analysis of UV-TRPES. We also estimate a previously undetermined energy-dependent intensity factor and expand the basis sets for SR analysis. The refined SR method is employed for reanalyzing the UV-TRPES data for the formation and relaxation dynamics of solvated electrons in various systems. The electron binding energy distributions for solvated electrons in liquid water, methanol, and ethanol are confirmed to be Gaussian centered at 3.78, 3.39, and 3.25 eV, respectively, in agreement with Nishitani et al. [ Sci. Adv. 2019, 5(8), eaaw6896]. An effective energy gap between the conduction band and the vacuum level at the gas-liquid interface is estimated to be 0.2 eV for liquid water and 0.1 eV for methanol and ethanol.

Charge Transfer Reactions from I- to Polar Protic Solvents Studied Using Ultrafast Extreme Ultraviolet Photoelectron Spectroscopy

Charge transfer reactions from I^- to solvent water, methanol, and ethanol were studied using extreme ultraviolet time-resolved photoelectron spectroscopy (EUV-TRPES). This technique eliminates spectral broadening, previously seen in UV-TRPES, caused by electron inelastic scattering in liquids, and enables clear observation of the temporal evolution of the spectral shape. The peak position, width, and intensity of the electron binding energy distribution indicate electron detachment and subsequent solvation and thermalization processes. Geminate recombination between detached electrons and iodine atoms is discussed using a diffusion equation and a global fitting analysis based on a kinetics model.

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.