Abstract
We report on the effects of electron collision and indirect ionization processes, occurring at photoexcitation and electron kinetic energies well below 30 eV, on the photoemission spectra of liquid water. We show that the nascent photoelectron spectrum and, hence, the inferred electron binding energy can only be accurately determined if electron energies are large enough that cross sections for quasi-elastic scattering processes, such as vibrational excitation, are negligible. Otherwise, quasi-elastic scattering leads to strong, down-to-few-meV kinetic energy scattering losses from the direct photoelectron features, which manifest in severely distorted intrinsic photoelectron peak shapes. The associated cross-over point from predominant (known) electronically inelastic to quasi-elastic scattering seems to arise at surprisingly large electron kinetic energies, of approximately 10-14 eV. Concomitantly, we present evidence for the onset of indirect, autoionization phenomena (occurring via superexcited states) within a few eV of the primary and secondary ionization thresholds. These processes are inferred to compete with the direct ionization channels and primarily produce low-energy photoelectrons at photon and electron impact excitation energies below ∼15 eV. Our results highlight that vibrational inelastic electron scattering processes and neutral photoexcitation and autoionization channels become increasingly important when photon and electron kinetic energies are decreased towards the ionization threshold. Correspondingly, we show that for neat water and aqueous solutions, great care must be taken when quantitatively analyzing photoelectron spectra measured too close to the ionization threshold. Such care is essential for the accurate determination of solvent and solute ionization energies as well as photoelectron branching ratios and peak magnitudes.
Highlights
The development of liquid-jet photoelectron or more accurately photoemission‡ spectroscopy (LJ-PES) represents a milestone for research on the electronic structure of liquid water and aqueous solutions.[1,2,3] Among the quantities of prime interest are solvent and solute lowest vertical ionization energies (VIEs), which measure the energetic cost to detach an electron under equilibrium conditions and chart critical parts of the energy landscape that controls chemical reactivity.[4]
We show that the nascent photoelectron spectrum and, the inferred electron binding energy can only be accurately determined if electron energies are large enough that cross sections for quasi-elastic scattering processes, such as vibrational excitation, are negligible
Electron-scattering-induced changes in peak positions have been observed for water clusters as a function of their size[16] and for the solvated electron in liquid water,[17] the prevalent assumption made in condensed-phase PE spectroscopy is that PE peaks are associated with directly-produced photoelectrons that have escaped the sample entirely unscattered
Summary
The development of liquid-jet photoelectron or more accurately photoemission‡ spectroscopy (LJ-PES) represents a milestone for research on the electronic structure of liquid water and aqueous solutions.[1,2,3] Among the quantities of prime interest are solvent and solute lowest vertical ionization energies (VIEs), which measure the energetic cost to detach an electron under equilibrium conditions and chart critical parts of the energy landscape that controls chemical reactivity.[4]. The situation changes drastically when photon energies are significantly smaller, such that the primary photoelectrons have insufficient energy to excite/ionize water In such cases, in both the gas- and condensed phase, vibrational scattering pervades[25,26,27] and largely determines the scattering-induced changes to the nascent PE spectra. At specific excitation energies, we will present evidence for valence autoionization resonances in liquid water These metastable states appear to be accessed both via photon and electron impact excitation close to the primary and secondary ionization thresholds, respectively, competing with direct ionization processes and yielding low-KE primary and secondary electrons. These processes are found to occur below the expected energetic onsets of aqueous-phase non-local autoionization processes (at B30 eV), in particular Intermolecular Coulombic Decay (ICD).[85]
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