Diffusion and Crystallization of Dichloromethane within the Pores of Amorphous Solid Water

Abstract

Dichloromethane (CH2Cl2) thin films deposited on Ru(0001) at low temperatures (∼80 K or lower) undergo a phase transition at ∼95 K, manifested by the splitting of its wagging mode at 1265 cm–1, due to factor group splitting. This splitting occurs at relatively higher temperatures (∼100 K) when amorphous solid water (ASW) is deposited over it, with a significant reduction in intensity of the high-wavenumber component (of the split peaks). Control experiments showed that the intensity of the higher wavenumber peak is dependent on the thickness of the water overlayer. It is proposed that diffusion of CH2Cl2 into ASW occurs and it crystallizes within the pores of ASW, which increases the transition temperature. However, the dimensions of the CH2Cl2 crystallites get smaller with increasing thickness of ASW with concomitant change in the intensity of the factor group split peak. Control experiments support this suggestion. We propose that the peak intensities can be correlated with the porosity of the ice film. Diffusion of CH2Cl2 has been supported by low-energy Cs+ scattering and temperature-programmed desorption spectroscopies.

Effect of Coadsorbed Oxygen on the Photochemistry of Methane Embedded in Amorphous Solid Water

“…Evidently here, owing to the preparation mode as a sandwich and methane’s inert characteristics (negligible interaction with D 2 O molecules), the molecules do not form a well-confined sandwich layer, but rather they diffuse throughout the ASW film, as can be seen from Figure . A similar case was recently reported in the sandwich preparation of dichloromethane in ASW …”

“…The low-temperature peak at 40 K is associated with the desorption of CH 4 molecules adsorbed on the top surface layers of ASW and are not entrapped inside the pores. The 146 K peak is associated with caged methane molecules that are released and desorbed at the ASW to crystalline ice-phase transition, the so-called “volcano” desorption. , …”

“…The 146 K peak is associated with caged methane molecules that are released and desorbed at the ASW to crystalline ice-phase transition, the socalled "volcano" desorption. 53,54 To ensure that the pure methane molecules were kept where we deposited them, namely, between the two 50 MLs of ASW, we performed a mild sputter experiment of the sandwich structure, by employing a 300 eV Ne + ion beam and subsequently, monitoring the remaining content of both the methane and the water molecules. These results are summarized in Figure 2.…”

“…A similar case was recently reported in the sandwich preparation of dichloromethane in ASW. 53 3.2. Photochemistry of the ASW|CH 4 |ASW System.…”

Spontaneous Formation of Tetrahydrofuran Hydrate in Ultrahigh Vacuum

“…This again supports the formation of irregular THF crystallites within the ASW pores, which influences the ASW network. This type of diffusion of dichloromethane followed by crystallization has been reported previously …”