Desorption signal

Although exit channel effects are capable of producing a range of non-Boltzmann population distributions, the wavelength dependence of the kinetic energy provides an indication that nonthermal activation is responsible for the fast component of the desorption signal. The activation mechanism responsible for this desorption process is not determined from these experiments, but will be re-addressed in section 4.6. 

In contrast to the detailed work on the Au(l 11) surface, desorption studies from the other low-index surfaces are scarce with, for example, MC9 and MC4/8 on Au(l 10) [45, 46] and MC4 [47] on Au(l 00). Compared to Au(l 11) thiols are more stable on Au(l 10) as reflected by a negative shift of the desorption peak by 200-300 mV, which was explained by the difference in the pzc for both surfaces [46]. N o obvious differences in the shape of the desorption peaks were found for Au( 1 0 0) compared to Au(l 11). Interestingly, for MC4 a higher thiol coverage compared to both MC4 on the Au(l 11) and MC2 Au(l 0 0) was concluded from the desorption studies. For polycrystalline surfaces the desorption signal is more complicated with additional features, possibly due to the presence of different crystallographic domains [94, 163, 164]. 

The shape of the curve shown in Fig. 15.6 is fortuitous in as far as the continuous flow method is concerned. For reasons to be discussed later, the desorption signal (see Fig. 15.3) is generally used to calculate the adsorbed volume. When, for example, 1.0 cm of nitrogen is desorbed into... 

The septum labelled in is used to inject known quantities of adsorbate into the flow to simulate a desorption signal for the purpose of calibration. 

To scan the hysteresis loop from the adsorption to the desorption isotherm, the sample, immersed in the coolant, is equilibrated with a gas mixture with a relative pressure corresponding to the start of the scan on the adsorption isotherm. The adsorbate concentration is then reduced to a value corresponding to a relative pressure between the adsorption and desorption isotherms. When equilibrium is reached, as indicated by a constant detector signal, the coolant is removed and the resulting desorption signal is calibrated. Repetition of this procedure, each time using a slightly different relative pressure between the adsorption and desorption isotherms, yields a hysteresis scan from the adsorption to the desorption isotherm. 

Lowell and Karp measured the effect of thermal diffusion on surface areas using the continuous flow method. Figure 15.14 illustrates a fully developed anomalous desorption signal caused by thermal diffusion. 

Attempts to increase the size of nitrogen adsorption or desorption signals, by using larger sample cells, results in enhanced thermal diffusion signals due to the increased void volume into which the helium can settle. However, when krypton is used, no thermal diffusion effect is detectable in any of the sample cells shown in Fig. 15.10. 

The adsorption signals using krypton-helium mixtures are broad and shallow because the adsorption rate is limited by the low vapor pressure of krypton. The desorption signals are sharp and comparable to nitrogen since the rate of desorption is governed by the rate of heat transfer into the powder bed. 

Column 7 is the volume required to calibrate the desorption signal and Column 8 is the corresponding weight of the calibration injection, calculated from the equation in the lower left side of the work sheet. The terms and A are the areas under the signal and calibration peaks, respectively. 

By calibrating the desorption signal with a known volume of nitrogen F, equation (15.16) can be rewritten as... 

Desorption is accomplished by incrementally decreasing the pressure and monitoring the nitrogen-rich desorption signals. 

The last important parameter to be determined is the desorption energy. It can be determined in an accurate way using a pulsed molecular beam [44]. At sufficiently low coverage to have a constant desorption energy, the desorption signal (in the second half period) decreases exponentially as a function of time, with a time constant that is the life time of the adsorbed molecule that depends on the temperature and on the desorption energy ... 

Table 19-4. Measured II, () and OH- LEE induced desorption signals (in arbitrary units) from thin films of the tetramer GCAT and its abasic forms. The percentage of the signal for each anion is given in the last column taking the yield from GCAT to be 100%...

Fig. 10.7. Thermal desorption signals as a function of temperature. The temperature ramping rate was 20 K/s. Temperature corrections regarding the speed of particles entering the line-of-sight QMS have not been applied, and thus relative magnitudes cannot be judged exactly from this figure [39]...

Pyridine will sorb on both Brpnsted and Lewis acid sites however the two adsorbed species can be readily differentiated using infiared (IR) detection. Using IR detection, Anderson and Klinowski observed a single desorption signal between 350 and 450 °C from H-Y zeolites corresponding to the Brpnsted acidity. The exact position of... 

Temperature Programmed Oxidation. These measurements characterize both the amount and chemical nature of the carbon on the surface. After a surface is exposed to ethylene and pretreated as desired, it receives a 6 L dose of O2 at 323 K. The TPO spectrum is the CO desorption signal at a 6 K/sec programming rate. CO2 accounts for less than 1% of the oxidation, so the CO signal accounts for essentially all of the carbon removed. O2 dosing is repeated until no further CO is evolved during heating. SIMS results show that all carbon has been removed from the surface at the TPO end point. 

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