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We report on the effects of electron collision and indirect ionizationprocesses, occurring at photoexcitation and electron kinetic energies wellbelow 30 eV on the photoemission spectra of liquid water. We show that thenascent photoelectron spectrum and, hence, the inferred electron binding energycan only be accurately determined if electron energies are large enough thatcross sections for quasi-elastic scattering processes, such as vibrationalexcitation, are negligible. Otherwise, quasi-elastic scattering leads tostrong, down-to-few-meV kinetic energy scattering losses from the directphotoelectron features, which manifest in severely distorted intrinsicphotoelectron peak shapes. The associated cross-over point from predominant(known) electronically inelastic to quasi-elastic scattering seems to arise atsurprisingly large electron kinetic energies, of approximately 10-14 eV.Concomitantly, we present evidence for the onset of indirect, autoionizationphenomena (occurring via superexcited states) within a few eV of the primaryand secondary ionization thresholds. These processes are inferred to competewith the direct ionization channels and primarily produce low-energyphotoelectrons at photon and electron impact excitation energies below ~15 eV.Our results highlight that vibrational inelastic electron scattering processesand neutral photoexcitation and autoionization channels become increasinglyimportant when photon and electron kinetic energies are decreased towards theionization threshold. Correspondingly, we show that for neat water and aqueoussolutions, great care must be taken when quantitatively analyzing photoelectronspectra measured too close to the ionization threshold. Such care is essentialfor both the accurate determination of solvent and solute ionization energiesas well as photoelectron branching ratios and peak magnitudes.

Low-energy constraints on photoelectron spectra measured from liquid water and aqueous solutions