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Hydronium ion motion in nanometer 3-methyl-pentane films
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Image of FIG. 1.
FIG. 1.

Experimental procedure. (1) Fabricate the film with a molecular beam at low temperatures, (2) produce and soft-land the ions on the film surface at a kinetic energy less than , and (3) measure the temperature evolution of the film voltage and TPD spectra.

Image of FIG. 2.
FIG. 2.

Isothermal desorption of a 40 ML 3MP film at . Half the area of the shaded region with respect to the total area represents the desorption contribution due to nonuniformity of the film.

Image of FIG. 3.
FIG. 3.

The film voltage and TPD for a 100 ML thick 3MP film. The film is grown at and annealed at before ions are deposited on top of the film at . The film voltage (left axis) is directly proportional to the average position (right axis) of the ions within the film. The film begins to evaporate near as shown in the TPD spectrum. The shaded area in the TPD spectrum is due to the partial blocking effect of the Kelvin probe.

Image of FIG. 4.
FIG. 4.

Comparison of (solid curve) and (dashed curve) ion transports through 200 ML thick 3MP films with similar electric fields of .

Image of FIG. 5.
FIG. 5.

Freezing the ions in place: for moving in a 119 ML 3MP film. The heating ramp is interrupted twice with cooling, “freezing” the ions in place (the arrows indicate the temperature loops). Also shown is the TPD spectrum (dotted line).

Image of FIG. 6.
FIG. 6.

Thermal desorption spectra for 0.5, 1, 1.6, 2, 2.4, 3, and 3.2 ML of 3MP on Pt(111). The films were deposited at and a TPD ramping rate was used.

Image of FIG. 7.
FIG. 7.

Absolute film thickness calibration using reflectometry. (a) The reflectance (solid line) and desorption TPD (dashed line) of a 3MP film. (b) The solid curve is the reflected light intensity vs the instantaneous surface coverage calculated from the integrated TPD spectrum normalized to the dosing time plotted on the left and bottom axes, respectively. Using the thickness calibration (see text) the dotted curve is the simulated reflectance calculated via the optical constants. The dashed curve is the simulated reflectance rescaled to match the experimental data.

Image of FIG. 8.
FIG. 8.

Spontaneous partial alignment of 3MP molecules in the bulk film grown at on Pt(111). Results for a 166 ML (solid line) and for a 83 ML (dashed line) film are shown. The initial negative voltage disappears upon annealing to . The temperature is ramped from at , then dropped to , and then the ramp is resumed upward from . During the temperature loops of (indicated by the arrow) the film voltage does not change.

Image of FIG. 9.
FIG. 9.

Film-growing temperature effect on subsequent motion in 200 ML films. The three curves for films grown at [solid line— ], [dashed line— ], and [dotted line— ], then annealed at , and ion dosed at , are nearly identical.

Image of FIG. 10.
FIG. 10.

electric field intensity effect on ion motion. (a) Four 167 ML thick 3MP films are shown, with the electric field strengths as indicated. (b) Rescaled to show average ion position.

Image of FIG. 11.
FIG. 11.

Ion motion through 3MP films of varying thickness at shown for (a) the absolute film voltage and (b) the offset film voltage vs temperature curves for 25, 50, 100, 200, 300, and 500 ML films. An expanded view (c) of the offset data demonstrates that the initial ion motion is independent of the film thickness.

Image of FIG. 12.
FIG. 12.

Viscosity of 3MP. The solid curve is the fit of the viscosity of 3MP to the literature data of Ling and Willard (Ref. 29) (circles), Von Salis and Labhart (Ref. 45) (triangles), Berberian and co-workers (Ref. 46) (squares), and Ruth (Ref. 32) (diamonds). The dashed curve represents an inversion of the experimental ion mobilities via the Stokes law, to get an “effective viscosity.” Fit parameters are in Table II. The dotted curve is extrapolated.

Image of FIG. 13.
FIG. 13.

The experimentally determined ion mobility is shown as the solid curve. The dashed curve represents the experimental mobility extrapolated to higher and lower temperatures. For comparison, the mobility estimated via Stoke’s law and the literature viscosity (with ion radius) is plotted (dotted curve).

Image of FIG. 14.
FIG. 14.

Experimental (solid) and Stokes-law-estimated (dotted) , for a 550 ML 3MP film with the ions initially starting 500 ML above the Pt substrate at an electric field of . Note the departure from Stokes law behavior.

Image of FIG. 15.
FIG. 15.

Short-range interfacial perturbations. Experimental (solid) and simulated film voltage curves for a 25 ML 3MP film at an electric field of . Ions initially placed on top of the film. Simulated curve (dashed) using the best-fit bulk mobility.

Image of FIG. 16.
FIG. 16.

Comparison of normalized film voltage curves of experimental data (solid) vs that simulated (dashed) using the best-fit mobility , for ion motion in 200 ML thick 3MP films. The curves are for heating rates of (a) 0.02, (b) 0.2, and (c) with electric field strengths of .

Image of FIG. 17.
FIG. 17.

Electric field effect on ionic motion in 200 ML thick 3MP films. The electric field strengths are (a) , (b) , and (c) comparing the experimental results (solid curves) to that simulated using the best-fit mobility (dashed curves). The normalized film voltage curves are shown.

Image of FIG. 18.
FIG. 18.

The effect of gross water contamination on the mobility of through a 200 ML 3MP film at an electric field of . Curves are for 0% (solid), 1.8% (dotted), 2.9% (dashed), and 5.2% (dash-dotted) mole percent in 3MP.


Generic image for table
Table I.

Fitting parameters for the interfacial position dependent shift in effective [Eq. (8)].

Generic image for table
Table II.

Parameters of the temperature dependence of the viscosity and mobility.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Hydronium ion motion in nanometer 3-methyl-pentane films