  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 both the accurate determination of solvent and solute ionization energies
as well as photoelectron branching ratios and peak magnitudes.
