Long-Range Structures of Amorphous Solid Water

High-energy X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) of amorphous solid water (ASW) were studied during vapor deposition and the heating process. From the diffraction patterns, the oxygen–oxygen pair distribution functions (PDFs) were calculated up to the eighth coordination shell and an r = 23 Å. The PDF of ASW obtained both during vapor deposition at 80 K as well as the subsequent heating are consistent with that of low-density amorphous ice. The formation and temperature-induced collapse of micropores were observed in the XRD data and in the FTIR measurements, more specifically, in the OH stretch and the dangling mode. Above 140 K, ASW crystallizes into a stacking disordered ice, Isd. It is observed that the fourth, fifth, and sixth peaks in the PDF, corresponding to structural arrangements between 8 and 12 Å, are the most sensitive to the onset of crystallization.

(50 m). Kapton and Diamond was chosen for the X-ray measurements and has been used for 18 the FTIR measurements for comparison. p-ASW vapor deposited on Kapton 20 As in the X-ray experiments, we also performed FTIR measurements using Kapton as substrate. 21 Figure S1a shows the growth of p-ASW within 120 min, and Figure S1b shows the subsequent 22 heating. As for the X-ray data (Figure 3), crystallization is observed at a cryostat temperature 23 Tm of 130 -135 K. The crystallization at the CaF 2 window instead takes place at Tm = 145 K. 24 This confirms the estimated temperature offset between cryostat temperature and sample 25 temperature when using a Kapton window, due to bad thermal contact. This is also consistent 26 with our previous calibration on powder samples using the same cryostat (see supplementary 27 material of Perakis et al. 1 ). We would like to point out, that the peak at around 3600 cm -1

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showing up at high temperatures in Figure S1b is not real, and most likely caused by the poor S2 29 reference subtraction when using a Kapton window. The Background reference was measured temperature was measured at the cold finger, crystallization temperature confirms the sample 36 temperature to be warmer by 10 K ± 2 K when using Kapton.  according to the literature. 3 The calculated ASW thickness is presented in Figure  × cm -1

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S3b. It is observed that the thickness increases much faster in the first 10 min deposition, then 60 it increases linearly with a rate of around 0.016 µm/min, resulting in a total thickness of 3 µm 61 after 2h deposition time.

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Alternatively to the normalization to the OH stretch band at 3250 , shown in the inset of cm -1 63 Figure 1b, we here normalized the spectra to the OD-stretch band (2440 ) instead. This is cm -1 64 presented in Figure S3c. The ratio between the main peak at 3250 and the shoulder at cm -1 65 3400 cm -1 changes continuously. This is consistent with our interpretation discussed in the main 66 manuscript that the shoulder at 3400 cm -1 is indicative for the growing porosity of the p-ASW 67 sample during the deposition process. In an independent measurement, the vapor deposition on 68 CaF 2 window was monitored for 260 min ( Figure S4), similar to the X-ray experiment. The structure factor of p-ASW at 80 K is compared with equilibrated high-density amorphous 87 ice (eHDA), which was prepared through decompression of very high-density amorphous ice 88 to 0.07 GPa at 140 K, then quenched to 80 K. The presented eHDA-data is replotted from our 89 previous publication. 4 It has been measured by our group under the same conditions at the same 90 beamline. The calculated S OO (Q), g OO (r), and r 2 (g OO (r)-1) of ASW on Diamond are summarized in Figure   116 S7. Surprisingly, the freshly formed p-ASW begins to crystallize after 30 minutes of vapor Bragg peaks, respectively. *Note: the temperature was measured at the cold finger (T m ), we 140 assume the sample temperature (T s ) to be warmer by 10 K ± 2 K (T s  T m + 10 K).

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Finally, we would like to add a comparison of our X-ray data with neutron scattering results 143 from Playford et al. 12 The g OO (r) curves for the formed crystalline structure (ice I sd ) at T s = 160 144 K (green) and 260 K (red) are plotted in Figure S9 in the range of 7-15 Å. They are compared 145 with the differential correlation functions D(r) calculated from neutron data for I sd samples