Fraction of amorphous H₂O vs. absorbed dose during irradiation with 10 keV electrons at 50 K determined from the peak-to-peak height of the derivative spectra (see M. J. Loeffler et al. 2020 for more details). For the 1.65 (open diamonds) and 4.14 μm (open circles) bands, the sample had a column density of 6.18 × 10¹⁸ H₂O cm⁻², and for the 3 μm (closed circles) band, the sample had a column density of 1.57 × 10¹⁷ H₂O cm⁻². Inset: corresponding derivative spectra in the HDO region compared to the spectrum of an amorphous sample of similar thickness; we used these spectra to estimate the amorphous fraction of the sample.