TPD spectra for 5 ML of Ar (bottom trace) underneath 300 ML of ASW (upper trace) at a heating rate of 1 K/s. The Ar spectrum has three desorption features labeled “burst,” “volcano,” and “trapped.” The Ar “burst” peak is coincident with a sharp desorption peak in the ASW spectrum which is accounts for the desorption of ∼20% of the ASW overlayer.
TPD spectra for (a) 5 ML and (b) 50 ML of Ar (gray line), Kr (red line), and Xe (green line) beneath 300 ML ASW heated at a rate of 1 K/s. (c) The TPD spectra from (b) plotted versus the reduced temperature, T/T c , which was obtained by dividing the temperature for each gas by its respective T c (150.87 K for Ar, 209.41 K for Kr, and 289.77 K for Xe). Rescaling aligns the most prominent peaks for Ar and Kr to the same reduced temperature (vertical dashed line) corresponding to a pressure of ∼2.5 atm.
The trapped fraction of Ar (○), Kr (▵) and Xe (⋄) versus underlayer thickness for a fixed 300 ML ASW overlayer. (Inset) An expanded view of the same data from 0 to 2 ML which shows that between 0.5 and 1.5 ML the trapped fraction is relatively insensitive to the underlayer thickness.
TPD spectra (a) Ar, (b) Kr, (c) Xe, (d) CH4, (e) N2, (f) O2 and (g) CO for underlayer thicknesses of 1, 5, 10, 20, 30, 40 and 50 ML and a 300 ML ASW overlayer. The heating rate was 1 K/s. The spectra for atomic underlayers display primarily narrow burst peaks that increase in number and intensity with underlayer thickness. The molecular species have a single broad low temperature desorption feature that shifts to low temperature with underlayer thickness.
Comparison of two TPD experiments performed at a heating rate of β = 1 K/s. The upper trace is offset by 1.5 ML/s and the peak at 130 K has been multiplied by 0.5. The upper spectrum is for 5 ML of Ar is deposited on graphene and covered by 340 ML of ASW. This spectrum displays a characteristic “burst” peak at 130 K. The lower spectra are from an experiment where 5 ML of Ar is co-dosed with 3 ML of O2 and covered by 340 ML of ASW. In these spectra no “burst” peak is observed, but rather both O2 and Ar desorb in a broad low temperature feature centered around 68 K.
(a) Plot of the low temperature region of a series of TPD spectra for 50 ML O2 beneath thicknesses of ASW from 50 to 2000 ML. The heating rate was 1 K/s. (b) Arrhenius plot of the quantity, βL 2/T 2 Peak (symbols), where β is the heating, L is the ASW overlayer thickness, and T Peak is the peak temperature of the low temperature desorption feature for the TPD spectra in (a) (1 K/s) and analogous sets of TPD spectra with heating rates of 0.1, 0.2, 0.5, and 2 K/s. The quantity βL 2/T 2 Peak is proportional to the diffusivity of the gas through the overlayer (see Eq. (1) in the text). A fit to the β = 1 K/s data set for ASW thicknesses up to 300 ML is given by the solid line and yields Arrhenius diffusion parameters of ν = 2.5 × 10−8 cm2/s and E = 3.7 kJ/mol. (c) Simulated TPD spectra at β = 1 K/s assuming a diffusion model for a variety of overlayer thicknesses using the Arrhenius parameters obtained in (b). The “stars” demark the experimental peak temperatures for the corresponding overlayer thicknesses from (a).
(a) Plot of the desorption peak temperature versus ASW overlayer thickness for 50 ML of CO, O2, N2 and CH4 on graphene and heated at a rate of 1 K/s. The dashed lines are diffusion model predictions using parameters fit to the overlayer data up to 300 ML for each gas. (b) Plot of the desorption peak temperature data in (a) rescaled by the critical temperature, T Peak /T c . The horizontal dashed lines demark values of T Peak /T c where the CO and CH4 data and the O2 and N2 data appear to converge at large ASW overlayer thicknesses. These reduced temperatures correspond to pressures of 0.4 and 0.004 atm.
TPD spectra for 1, 20, and 50 ML of Ar ((a), (c), and (e), respectively) and O2 ((b), (d), and (f), respectively) deposited on graphene (solid lines) and 2 ML of ASW on top of graphene (dashed lines), and covered by 300 ML of ASW. The heating rate was 1 K/s. The TPD spectra for both gases are dependent on the underlying substrate.
Desorption of (a) 2 ML Ar, (b) 2 ML O2, (c) 20 ML Ar and (d) 20 ML of O2 from graphene (solid) or ASW (dashed) deposited without an ASW overlayer. The heating rate was 1 K/s. Very different desorption behavior is observed for thin Ar and O2 layers on graphene versus ASW while the multilayer desorption temperature is substrate independent.
TPD spectra of 50 ML Ar deposited on graphene with 500 ML ASW deposited above the Ar layer at incident deposition angles of 0° (top), 5°, 10°, 15° and 20° (bottom). The Y-axis scale for the first three spectra is identical to emphasize the decrease in magnitude of the sharp desorption features with ASW deposition angle.
Desorption of 50 ML of (a) Ar and (b) O2 from graphene after being covered by 150 ML of ASW deposited at various angles of incidence. The low temperature desorption peak for 50 ML of the respective gases from graphene is marked by a vertical dashed line. (c) Comparison of the low temperature peaks in (a) and (b) for Ar (squares) and O2 (circles).
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