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

Abstract

Time-resolved photoelectron spectroscopy (TRPES) enables realtime 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.

Ultrafast Geminate Recombination Facilitated by Hydrogen-Atom Transfer in Charge Transfer Reactions from Hydroxide and Methoxide Ions

“…When excess electrons are more energetically stabilized, their eBE shifts to a higher value. The eBE value after a long delay time is centered at 3.7 eV, which agrees with the previously determined vertical eBE (VBE) value for thermalized e aq – . , …”

“…In these measurements, 200 nm pump and 267 nm probe pulses were employed. The results have been corrected for spectral distortion and the energy dependence of the electron transmission efficiency through the gas–liquid interface using a spectral retrieval (SR) method . Also shown are the results for CTTS from I – to water photoexcited at 225 nm obtained using extreme ultraviolet (EUV) TRPES .…”

“…Figure a shows TRPE spectra measured for the CTTS reaction from OH – to water. As mentioned above, TRPE spectra obtained using UV probe pulses were corrected for electron inelastic scattering in the liquid , and the energy-dependent electron transmission efficiency at the gas–liquid interface . In Figure a, the vertical axis corresponds to the electron binding energy (eBE), which is given by the difference between the probe photon energy (4.6 eV) and the observed eKE.…”

Relation between the Hydrated Electron Solvation Structure and Its Partial Molar Volume

“…These challenges for both pseudopotential and AIMD simulation approaches have motivated a strong focus on testing how well different computational descriptions are able to reproduce experimental characterizations. ,,, Chief among these are the vertical detachment energy − and radius of gyration. ,− These properties probe the strength of the electron binding as well as the degree of delocalization of its wave function. They are, however, not sufficient to constrain the pseudopotential description; both the Turi–Borgis (TB) and optimized Turi–Borgis pseudopotentials adequately describe the hydrated electron radius of gyration and vertical detachment energy, but are inaccurate in predictions of other hydrated electron properties. , On the other hand, AIMD efforts often give smaller values for both quantities compared to experiment, ,,,, likely due primarily to the small system sizes …”

Empirically Optimized One-Electron Pseudopotential for the Hydrated Electron: A Proof-of-Concept Study

“…In this paper, we propose one such method based on the optimization of the pseudopotential parameters to match empirical data. We choose the average vertical detachment energy and radius of gyration as the two target properties because they have been accurately determined experimentally − and also provide some tension within the description, i.e., pseudopotentials can accurately predict one of these properties but often not both. , The present proof-of-concept study demonstrates that this approach can yield an improved pseudopotential, with respect to properties that are not directly optimized.…”