Fig. 2: Efficiency of ICD after photoionization of the 2a₁ band in liquid water with different XUV photon energies.

a Photoelectron energy spectrum of liquid H₂O in the region of 2a₁ binding energies. b Energy spectrum of ICD+photoelectron pairs obtained from the liquid H₂O coincidence map in the region of 2a₁ binding energies. a, c are derived from the same data set shown in Fig. 1. Here, a linear background was subtracted. Electron energy is represented as kinetic energy (bottom axis) or binding energy (top axis). b, d Same as (a, c) but for D₂O. e  Relative ICD efficiencies of H₂O and D₂O obtained by two different background subtraction methods (see section 2 in SI): a simple linear background (lin.BG) model, consistent with the analysis in ref. ⁵⁵ and a Shirley background (shirley.BG) model, which is widely used in x-ray photoelectron spectroscopy analysis⁵⁶. The absolute ICD efficiencies of H₂O and D₂O clusters (〈N〉 ~ 72) measured at a photon energy of hν = 60 eV are shown for comparison²⁴. Error bars are adapted from ref. ²⁴, which represent the standard deviation due to stochastic errors.