The desorption kinetics of molecular beam deposited ultrathin films of CCl 4 and amorphous solid water (ASW) are studied. Overlayers of ASW impede CCl 4 desorption until the onset of crystallization, whereupon the CCl 4 desorbs abruptly. The abrupt desorption occurs through connected pathways that are formed in the water overlayer during the nucleation and growth of crystalline ice from ASW. The onset of the abrupt desorption corresponds to the threshold for dynamic percolation. As the crystallization proceeds, the number of connected pathways rapidly increases, giving rise to the episodic release of CCl 4 . [S0031-9007(97)03648-X] PACS numbers: 81.05. Kf, 61.43.Er, 68.35.Rh, Amorphous materials are important in a wide variety of scientific disciplines including physics, chemistry, materials science, and biology [1,2]. Amorphous solid water (ASW) is of special importance for many reasons including the open question over its applicability as a model for liquid water and fundamental interest in the properties of glassy materials [3][4][5]. Despite the considerable interest in ASW many questions remain about its physical properties, and whether amorphous solid water is a metastable extension of ordinary liquid water or a distinct thermodynamic phase [6][7][8][9][10][11][12][13][14][15][16]. In addition to the properties of ASW itself, understanding the intermolecular interaction between amorphous solid water and an adsorbate is important in such diverse areas as solvation in aqueous solutions, cryobiology, and desorption phenomena in cometary and interstellar ices [14,17,18].In this Letter we report the observation of abrupt CCl 4 desorption driven by the crystallization of ASW. We term this dramatic effect the "molecular volcano." The CCl 4 ͞ASW system was chosen for study because liquid water and CCl 4 are immiscible due to hydrophobic interactions. Water vapor deposited on low temperature substrates (,140 K) is known to form an amorphous phase termed ASW that is metastable with respect to crystalline ice [3,4]. Metastable ASW has a higher free energy than the crystalline ice, and as a result, the water desorption rate from ASW is greater. We have used the difference in the desorption rates to determine the relative free energy of the two phases [13] and to quantitatively study the desorption and crystallization kinetics of ultrathin films of ASW [19,20]. In the experiments described here the desorption kinetics of compositionally tailored, ultrathin films composed of ASW(H 2 O), ASW(D 2 O), and CCl 4 are examined using temperature programmed desorption (TPD).The experimental technique and apparatus have been described elsewhere and are only summarized here [21,22]. In brief, a quadruply differentially pumped effusive molecular beam of D 2 O, H 2 O, or CCl 4 was used to dose a 1 cm diameter single crystal metallic substrate [Au(111) or Ru (001)]. This highly collimated beam has a circular profile of ϳ0.35 cm diameter and enables precise and reproducible exposures to be attained without appreciable adsorption on s...

The condensation of H2O on ice multilayers on Ru(001) was studied using both molecular beam and optical interference techniques as a function of surface temperature. From the beam reflection technique, the H2O sticking coefficient, S, was determined to be S = 0.99 ± 0.03 at temperatures between 85 and 150 K and was independent of incident angle (0−70°) and beam energy (1−40 kcal/mol). The condensation coefficient, α, was dependent on both the incident H2O flux and the desorption H2O flux at the various surface temperatures. The magnitude of α varied continuously from unity at T < 130 K to zero at higher temperatures. The optical interference experiments yielded condensation coefficients and sticking coefficients of α = S = 0.97 ± 0.10 at temperatures from 97 to 145 K where the H2O desorption flux was negligible with respect to the incident flux. The optical interference measurements monitored the ice film thickness versus H2O exposure time and were dependent on the refractive index, n, and the density, ρ, of the vapor-deposited ice. Consequently, the combined molecular beam and optical interference measurements provided a means to evaluate the refractive index and density for vapor-deposited ice as a function of surface temperature. The values of the refractive index varied from n = 1.27 at 90 K to n = 1.31 at 130 K. The calculated densities varied from ρ = 0.82 g/cm3 at 90 K to ρ = 0.93 g/cm3 at 130 K. Previous optical interference data were also reanalyzed to yield refractive indices and ice densities for films grown at surface temperatures between 20 and 150 K. Both the refractive index and density increased monotonically with increasing growth temperature. The lower refractive index and density at lower temperatures indicate that microporous ice films are formed when H2O deposits on substrates at T < 120 K.

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