Long-Range Structures of Amorphous Solid Water

Abstract

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, I sd . 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.

Infrared Spectroscopy on Equilibrated High-Density Amorphous Ice

“…Most importantly, the peak position and fwhm of the derived LDA are identical to ASW, as also visible in Figure S4. This is consistent with X-ray and neutron data, demonstrating that ASW is a structural analogue to LDA. , This finding is in contrast to work by Kolesnikov et al, who observed considerable differences between the vapor deposits and the LDA obtained from HDA. The difference in their study might actually be due to the microporous nature of ASW, resulting in many molecules that are not tetrahedrally coordinated, as compared to the compact nature and perfect tetrahedral coordination in LDA.…”

“…How do the vibrational modes of the so-derived LDA and crystalline ice compare to vapor-deposited amorphous solid water (ASW) and other crystalline ices? A comparison of the different states and references ( 30 ), ( 33 ), ( 51 ), and ( 52 ) of ice Ih and amorphous solid water (ASW) is given in Table 1 . Most importantly, the peak position and fwhm of the derived LDA are identical to ASW, as also visible in Figure S4 .…”

“…That is to say that it needs to be clarified how porous or how compact the vapor deposit actually is�only well-annealed ASW samples (e.g., at 120 K) are similar to compact LDA. 53 We further discuss differently prepared crystalline ices, namely cubic ice crystallized here from LDA at 160 K and hexagonal ice obtained by directly freezing water in such a copper grid, freezing water between CaF 2 windows, and from crystallizing ASW at 160 K. 33 Both hexagonal ice samples, hence prepared directly from freezing liquid water, have peak maxima at 2422 cm −1 (see also Figure S5). The peak maximum of cubic ice obtained after the transition eHDA → LDA and annealing ASW is located at 2418 cm −1 .…”

“…At ambient pressure and low temperatures, the metastable amorphous states and their conversion have been intensively studied using different experimental methods such as X-ray ,, and neutron diffraction, , calorimetry, ,, broadband dielectric relaxation, , and deuteron and 17 O NMR , spectroscopy. The vibrational spectrum of amorphous ices was previously accessed using Raman spectroscopy , and incoherent inelastic neutron scattering. − Additionally, infrared spectroscopy allows studying amorphous ices at the molecular level by measuring vibrational states of hydrogen bonds. , This is of particular interest for a comparison with astrophysical data. , The low-density amorphous state of water grown by vapor deposition is studied intensively using infrared spectroscopy. − However, no IR data of the high-density forms obtained by pressure induced amorphization have so far been reported. Water unlike other liquids absorbs strongly in the mid-IR region.…”

Formation of Ethane Clathrate Hydrate in Ultrahigh Vacuum by Thermal Annealing

“…In the O−H stretching region (Figure S5a), no significant changes were noticed, but in the dangling O−H region, the peak at 3667 cm −1 disappeared upon the formation of CH (Figure S5b), which suggested the collapse of micropores in ASW. 45,46 To see the temperature dependency of CH Formation, we annealed the ethane/H 2 O (1:1) ice mixture at 55 K for 18 h. We did not observe any peak arising at 2982 cm −1 (Figure S6). Hence, we concluded that CH formation of ethane in UHV at cryogenic temperatures depends significantly on the temperature and annealing time.…”