Molecules desorbing

However, the temperature at which a molecule desorbs also reflects how strongly it is bound to the surface [Eq. (12)]. The activation energy in Eq. (12) equals the heat of adsorption provided the adsorption of the molecule occurred without an activation barrier. This condition is usually fulfilled. 

Desorption Rates. Using the above model for the temperature jump associated with pulsed laser heating, the rate of desorption versus time and the total number of molecules desorbed from a finite surface area heated by the laser can be calculated. For the particular case of first-order desorption kinetics, the desorption rate is ... 

Figure 2. Plot of the desorption rate, molecules/sec, (solid circles) and the Integrated number of molecules desorbed (solid line) for an adsorbate with a desorption activation energy of 20Kcal/mole and a preexponentlal of 10 sec-. The temperature jump shown In Figure 1 was used for this calculation.

Figure 6 shows the sequence of events in a laser desorption FTMS experiment. First, a focused laser beam traverses the analyzer cell and strikes the crystal normal to the surface. Molecules desorbed by the thermal spike rapidly move away from the crystal and are ionized by an electron beam which passes through the cell parallel to the magnetic field and 3 cm in front of the crystal. 

Laser desorption FTMS is fundamentally different from SIMS because the desorption and ionization steps are separate. With FTMS, neutral atoms and molecules desorbed by the laser are ionized by the electron beam after they have moved about 3 cm away from the surface. As a result, complications Introduced into SIMS spectra by gas-phase reactions above the surface are minimized because neutral-neutral reactions are typically two-orders of magnitude slower than ion-molecule reactions. We believe, therefore, that laser desorption FTMS spectra are representative of the species actually present on the surface. 

Permeation of small molecules through polymers takes place in four steps. In the first stage, the permeating molecules, know as the diffusants, wet or adsorb onto the polymer s surface. Secondly, the diffusant molecules dissolve in the polymer. In the third step, the molecules diffuse down a concentration gradient towards the opposing surface. Finally, the diffusant molecules desorb or evaporate from the surface, or are absorbed into another material. 

Figure 1.1 Schematic representation of a well known catalytic reaction, the oxidation of carbon monoxide on noble metal catalysts CO + Vi 02 —> C02. The catalytic cycle begins with the associative adsorption of CO and the dissociative adsorption of 02 on the surface. As adsorption is always exothermic, the potential energy decreases. Next CO and O combine to form an adsorbed C02 molecule, which represents the rate-determining step in the catalytic sequence. The adsorbed C02 molecule desorbs almost instantaneously, thereby liberating adsorption sites that are available for the following reaction cycle. This regeneration of sites distinguishes catalytic from stoichiometric reactions.

The rotational population distributions were Boltzmann in nature, characterized by 7Ji = 640 35 K. This seems substantially lower than yet somewhat larger than the temperature associated with the translational degree of freedom. The lambda doublet species were statistically populated. The population ratio of i =l/t =0 was roughly 0.09, consistent with a vibrational temperature Ty— 1120 35K. The same rotational and spin-orbit distributions were obtained for molecules desorbed in t = 1 as for f = 0 levels. Finally, there was no dependence in the J-state distributions on desorption angle. 

A typical N2 adsorption measurement versus relative pressure over a solid that has both micropores and mesopores first involves essentially a mono-layer coverage of the surface up to point B shown in isotherm IV (lUPAC classification) in Figure 13.1. Up to and near point B the isotherm is similar to a Langmuir isotherm for which equilibrium is established between molecules adsorbing from the gas phase onto the bare surface and molecules desorbing from the adsorbed layer. The volume of adsorbed N2 that covers a monolayer volume, hence the surface area of N2 can then be determined from the slope of the linearized Langmuir plot when P/V is plotted against P ... 

The studies of Au(l 10) [230] have revealed that Py molecules adsorb vertically also on this surface and are attached to the metal via the nonbonding orbital of the nitrogen atom. For Au(lll), the dependence of the orientation of Py molecules on the surface charge has been found [231]. Several modem spectroscopies, for example, SHG [235,236], DEG on Au(lll) [237] and SERS on Au(210) [238], have been employed to determine the orientation of Py molecules at Au surfaces. At Au(210) and Au(311), the adsorbed Py molecules attain vertical N-bonded orientation. In general, the studies on different types of Au surfaces have shown that it is not always clear whether Py molecules desorb from the electrode or only change their orientation [11]. 

The VEEL spectra of the species formed from cyclohexane on Pt(lll) show that at least two intermediate species occur along the decomposition pathway to benzene. These spectra are discussed in Sections VI.A and VI.C, in the context of spectra of species formed from adsorbed cyclohexene (239) and cyclo-l,3-hexadiene (240) on the same surface. On Pt(100) hex, in contrast to Pt(lll), most of the cyclohexane molecules desorb before conversion to benzene, but the latter was formed after adsorption at 300 K. An intermediate in the conversion of cyclohexane into benzene on Pt(100) (1 X 1), stable between ca. 200 and 300 K, was recognized spectroscopically, but not structurally identified, by RAIRS (230) and by VEELS (224). It seems that there is a smooth transition from the spectrum of adsorbed cyclohexane on Pd(100) to that of benzene at temperatures exceeding 250 K without the detection of intermediate spectra (220). 

From the function of a channeltron/channelplate detector it is obvious that high gains are desirable. However, ion feedback and space charge effects limit the gain with increasing charge of the electron avalanche, electron impact ionization with molecules of the residual gas or molecules desorbed under electron bombardment from the channel surface occurs more frequently. The ions produced are then accelerated towards the channel input. If such an ion hits the surface at the channel entrance, it may release an electron which again can start an avalanche of practically the same size, i.e., it causes after-pulses. 

Apart from this hydroxyl-specific interaction, also non-hydroxyl specific adsorption was demonstrated to occur at high concentration and long reaction times. This includes physical interactions with the surface or other adsorbed molecules. Only the molecules, adsorbed specifically in the first monolayer were shown to remain chemisorbed on the surface after curing. Other molecules desorb upon curing. 

A high vacuum chamber contains a liquid N2-cooled surface (A= 400 cm2) to provide a high, in situ, pumping speed for water molecules desorbing from its surface. Calculate the pumping speed for water vapour at 50 °C if all the impinging molecules are trapped. 

The rotational energy distributions by the REMPI spectra of desorbed NO from the hep hollow species on Pt(l 1 1), which is induced by 2.3-6.4 e V laser irradiation, are represented by a Boltzmann distribution such as in Fig. 16a. It is impossible to understand that molecules desorbed by a non-thermal process reach thermal equilibrium in the rotational energy distribution during the very short residence time in the excited state for chemisorbed species on metal and semiconductor surfaces (lifetime = 10 16-10-14 s [62, 72, 73]). Besides, no rotational freedom exists in the chemisorbed state and the desorption is... 

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