Desorption adiabatic

Fig. 3. Vibrational population distributions of N2 formed in associative desorption of N-atoms from ruthenium, (a) Predictions of a classical trajectory based theory adhering to the Born-Oppenheimer approximation, (b) Predictions of a molecular dynamics with electron friction theory taking into account interactions of the reacting molecule with the electron bath, (c) Born—Oppenheimer potential energy surface, (d) Experimentally-observed distribution. The qualitative failure of the electronically adiabatic approach provides some of the best available evidence that chemical reactions at metal surfaces are subject to strong electronically nonadiabatic influences. (See Refs. 44 and 45.)...

The desorption of ions and neutrals into the vacuum upon irradiation of a laser pulse onto a surface proceeds as a jet-like supersonic expansion. [38] a small, but initially hot and very rapidly expanding plume is generated. [49] As the expansion is adiabatic, the process is accompanied by fast cooling of the plume. [38]... 

A CH4 pyrolysis mechanism appears to be consistent with our observation that preheating improves partial oxidation selectivity. First, higher feed temperatures increase the adiabatic surface temperature and consequently decrease the surface coverage of O adatoms, thus decreasing reactions lOa-d. Second, high surface temperatures also increase the rate of H atom recombination and desorption of H2, reaction 9b. Third, methane adsorption on Pt and Rh is known to be an activated process. From molecular beam experiments which examined methane chemisorption on Pt and Rh (79-27), it is known that CH4 must overcome an activation energy barrier for chemisorption to occur. Thus, the rate of reaction 9a is accelerated exponentially by hi er temperatures, which is consistent with the data in Figure 1. 

Figure 3.3. Schematic of direct and precursor-mediated dissociation processes on a typical adiabatic PES (given by the solid line). Solid arrow labeled S represents direct dissociation and that labeled a represents trapping into a molecular adsorption well. Dashed arrows represent competing thermal (Arrhenius) rates for desorption (kd) and dissociation (kc) from the molecular well.

Figure 3.7. Schematic showing that associative desorption induced by thermal excitation of the lattice can create e-h pairs (hot electrons) via non-adiabatic damping of nuclear coordinates (described here...

Figure 3.25. Probability of a given energy loss into e-h pairs of magnitude eh vs. A/icM occurring in associative desorption of a diatomic from a metal surface from 3D non-adiabatic dynamics, (a) is for H2 associative desorption from Cu(lll), with ( ) 0.02 eV and (b) is N2 associative desorption from Ru(0001), with (A/i ch) 0.5 eV. From Ref. [68].

Figure 3.28. N2 vibrational state distribution in associative desorption from Ru(0001). (a) Observed in experiment. From Ref. [126]. (b) From 3D (Z, R, q) first principles quasi-classical dynamics, with the solid triangles pointing upward being adiabatic dynamics and the squares from molecular dynamics with electronic frictions also from DFT. Based on the PES and frictions of Ref. [68]. The open triangles pointing downward are the results of 6D first principles adiabatic quasi-classical dynamics from Ref. [253].

Recently, new 2D-methods for the analysis of complex mixtures have been developed for time-of-flight mass spectrometry (22), which could also be utilized in external ionization FTMS. Specifically, the combination of IR-laser desorption of nonvolatile neutrals, followed by adiabatic cooling to 2°K in a supersonic jet, and subsequent compound-selective Resonance-Enhanced Multiphoton Ionization (REMPI) could increase the role of FTMS in the analysis of biological mixtures. The coupling of supersonic jets to the external ion source would also be of interest in ion- and neutral cluster experiments. 

Adsorption from the gas or vapor phase is usually associated with significant heat release upon uptake, or cooling upon desorption. Three modes of operation are possible isothermal, adiabatic, and intermediate (not quite either extreme). Isothermal operation can often be assumed for liquid-phase adsorption but generally not for gas- or vapor-phase systems. Temperature shifts affect adsorption capacity strongly but diffusivity to a lesser extent. 

MASS TRANSFER IN DRYERS. In all dryers in which a gas is passed over or through the solids, mass must be transferred from the surface of the solid to the gas and sometimes through interior channels of the solid. The resistance to mass transfer, not heat transfer, may control the drying rate. This is most often true in cross-circulation drying of slabs, sheets, or beds of solids. From the standpoint of the gas, this kind of drying is much like adiabatic humidification from that of the solid it is like evaporation when the solid is very wet and like solvent desorption from an adsorbent when the solid is nearly dry. 

Inoue et al. [150] demonstrated that large amounts of carbon tetrachloride can be absorbed into 1-dimensional tunnels in copper(II) trans-1,4-cyclohexane dicarboxylate (Figure 14) under the saturated vapour pressure at room temperature, and the desorption can be performed easily by evacuation above room temperature. It was also confirmed that the absorption/desorption is reversible. The thermodynamic and structural properties were studied for the empty (non-absorbed) sample and partially-filled (10, 22 and 31% of the fiill carbon tetrachloride-absorbed) samples, using adiabatic calorimetry between 13 and 300 K and by powder XRD with high-energy synchrotron radiation. The heat- capacity anomaly due to the first-order phase transition observed in the empty sample was not observed in the fully-absorbed sample. However, the partially absorbed samples showed smaller heat-capacity anomalies at lower temperatures than the empty sample. Such phenomena were compared with the previous results for toluene-absorbed samples [151-153] and tfie differences were discussed. 

FIGURE 2 A potential energy eurve for adiabatic reaction = Pf + ne - activation energy of platinum atom departure into solution - energy of desorption of hydrated platinum ion, yielding during anodic process. 

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