Physisorbed region

Figure Al.7.14. 3.4 mn x 3.4 mn STM images of 1-docosanol physisorbed onto a graphite surface in solution. This image reveals the hydrogen-bonding alcohol molecules assembled in lamellar fashion at the liquid-solid interface. Each bright circular region is attributed to the location of an individual hydrogen...

Representative results are given in Table 5.4. From column 7, it is seen that the ratio iV,/ Afj - - N/,) is in the region of 1 2 (in contrast to the 1 1 found with silica) suggesting that each molecule of water in the physisorbed monolayer is bonded to two surface hydroxyl groups. 

In contact with both propane and oxygen, physisorbed propane is the main adsorbed species, but a new adsorbed species characterized by a band centred at 1623 cm may be observed (Fig. 6B). Simultaneously a stronger perturbation in the Vqh region is noted (Fig. 6A). The band at 3728 cm attributed to free silanols (7), disappears leaving a less intense band at higher frequency (3742 cm ). At the same time a broader band centred at about 3664 cm appears. By evacuation at room temperature, all the adsorbed species disappear (only a weak band centred at 1623 cm remains) and the original spectrum in the vqh region is restored (Fig. 6, spectrum c). 

Ex situ measurements in the presence of dissolved oxygen have proved that the mixed monolayer was stable in the solution free of 6TG and guanine. Madueno etal. [Ill] have also studied adsorption and phase formation of 6TG on mercury electrode. At high potentials, the molecules were chemisorbed and were able to form a self-assembled monolayer. When the potential was scanned to more negative values, reductive desorption of the monolayer was observed. Cathodic voltam-metric peaks, which are typical of a 2D condensed phase transition, divided the potential window into two regions one, in which self-assembled monolayer was stable, and the second, in which a physisorbed state existed. 

The observed spectra in the C = N stretching v region were carefully deconvoluted in a similar way to the method published earlier[ 15]. The one example of the deconvolution for the band at P/P = 0.70 is shown in Fig. 3. Here we designate the v2 band of hydrogen-bonded acetonitrile at 2265 cm 1 as the v,a band and that of physisorbed acetonitrile 2254 cm 1 as the v2p band. Also we corrected the contribution by the hot band transitions of v2hl... 

The vibrational spectrum of benzene around 1000 cnf has also been measured. IQ. Benzene was physisorbed on a cooled copper substrate in the vacuum chamber. Figure 19 shows the transmission for several thicknesses of benzene and a prism separation of 3 cm. The thickness was determined from the measured transmission in transparent regions using Eg. (7). The solid curves were calculated from Eqs. (5) and (6) using optical constants for benzene obtained from an ordinary transmission experiment.il The benzene film was assumed to be isotropic. Of the two absorption lines seen, one belongs to an in-plane vibrational mode, and one to an out-of-plane vibration. Since the electric field of the SEW is primarily perpendicular to the surface, the benzene molecules are clearly not all parallel or all perpendicular to the copper surface. Also it should be noted that the frequencies are the same (within the experimental resolution) as those of solid benzene22 and of nearly the same width. These features indicate that the benzene interacts only weakly with the copper surface, as would be expected for physisorbed molecules. 

The identification of species adsorbed on surfaces has preoccupied chemists and physicists for many years. Of all the techniques used to determine the structure of molecules, interpretation of the vibrational spectrum probably occupies first place. This is also true for adsorbed molecules, and identification of the vibrational modes of chemisorbed and physisorbed species has contributed greatly to our understanding of both the underlying surface and the adsorbed molecules. The most common method for determining the vibrational modes of a molecule is by direct observation of adsorptions in the infrared region of the spectrum. Surface spectroscopy is no exception and by far the largest number of publications in the literature refer to the infrared spectroscopy of adsorbed molecules. Up to this time, the main approach has been the use of conventional transmission IR and work in this area up to 1967 has been summarized in three books. The first chapter in this volume, by Hair, presents a necessarily brief overview of this work with emphasis upon some of the developments that have occurred since 1967. 

A few words on the form of P, (p) are in order. Our opacity is very different from that often encountered in textbooks of gas phase reactions, where P, is assumed to be constant up to some value of p, and zero beyond that. Similar holes (regions of low reactivity at low p) in the opacity function have been computed for the ER reactions of H(g) with Cl adsorbed onto Au( 1 1 1) [91,92] and with H physisorbed onto graphite [85]. For H atoms on a corrugated Cu(l 1 1) surface we find smaller holes than in Fig. 4, but the reactivity still becomes small near zero impact [38]. Note that the reaction cross section, defined as... 

In Fig. 14 we show HREEL spectra of ethylene adsorbed at Ag(4 1 0) and at Ag(2 1 0) at T = 105 K and compare them with the spectra recorded for Ag(l 00). On stepped surfaces (upper two spectra) C2H4 was dosed with a pure beam. Non-activated adsorption is witnessed by the loss in the 121-125 meY region. No adsorption takes place, on the other hand, on the extended (100) terraces of Ag(l 00) [90] up to much higher energies (see spectrum recorded at Ex = 0.31 eV in Fig. 14). Chemisorption on Ag(l 0 0) is observed when the ethylene exposure is performed with Ex — 0.35 eV. Adsorption on the flat surface is therefore translationally activated for extended (100) terraces and non activated for stepped surfaces. Physisorbed molecules do not contribute to the HREEL spectra since desorption takes place within a few seconds after the end of the dose at 105 K (as evident from Fig. 2) and recording a spectrum requires many minutes. 

Fig, 3. illustrates the influence of CO adsorption on the spectra of samples pretreated at 673 and 973 K. For both the pretreatment temperatures in the region of physisorbed CO instead of a single band, a doublet appears with an extra band at 2146 cm. Strong CO bands occur at 2160 and 2180 cm with a shoulder at about 2190 cm and one more band at 2232 cm. After pretreatment at 973 K the shoulder is more pronounced and the intensity of the 2232 cm band is higher. No new bands indicative for the appearance of any other surface sites were detected. 

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