Desorption velocities

The activation energy for desorption comprises the heat of adsorption and the activation energy of adsorption, (see Fig. 1), but, as the adsorption of alkali metals and most gases on clean metal surfaces is non-activated, the activation energy of desorption is, in fact, equal to that of adsorption. Two classes of measurements have been made (1) those in which desorption occurred without subsequent readsorption, and (2) those where equilibrium conditions were approached during the desorption process. A true desorption velocity is observed in the first case only. 

The selectivity to nitrile is higher than the comparable selectivity to ben-zaldehyde. This is probably due to the greater stability of the nitrile or a difference in desorption velocity of the imine compared with benzalde-hyde. Nitriles are only weakly adsorbed. 

Figure 18 Angular width b of the Gaussian angular distribution for desorbed NO (v = 0, Q = 1/2) of hep hollow species from Pt(l 1 1) in different rotational states J = 6.5 (open circle) and 15.5 (filled square) as a function of desorption velocity vd, presented in a b2 vs. 1/v diagram [61].

The Gaussian width b of the angular distributions of NO detected in different rotational quantum states (J = 6.5 and 15.5) exhibits comparable values and the same dependence on the desorption velocity,... 

Figure 26 Quadrupole moment Aq2) for the desorption of CO (v = 0) from 0 203(0 001) as a function of J. Filled circles show experimental data, solid lines show calculated Aq2) averaged with respect to the desorption velocity and other marks connected by line are calculated by various imaginary flat PESs. The excited state resonance lifetime tr = 10 fs [79].

The distinction between the direct inelastic and trapping—desorption channels is most clearly demonstrated for a beam temperature of 3400 K at Ts = 400 K, where a lobular distribution is found for the former and a cosine distribution for the latter. However, Alnot and King [432] note several unresolved contradictions in the analysis of their data in particular, their assumption that the trapping—desorption velocity distribution is Boltzmann cannot be justified. 

In a recent work, improved calibration of M ALDI-TOF mass spectra was obtained by optimization of the electrostatic field generated by the ions. In contrast to conventional methods, where the relationship between ion flight time and mass is an arbitrary polynomial equation, this method is based on the physics of ion motion. Parameters needed to describe the physics are numerically optimized using a simple algorithm. Once these parameters are established, unknown masses can be determined from their times-of-flight. This calibration method gives well-behaved results, since non-linear parameters (due to extraction delay, desorption velocity, and other events) are properly taken into account in the time-of-flight calculation. ... 

Tl e method outlined is inapplicable when the desorption velocity depends on the square of the concentration at the surface. 

Because dC/dh, and therefore d increase rapidly at high humidity, sorption must proceed more and more slowly as humidity increases, or saturation is approached. In desorption, as humidity decreases, the converse is true, i.e. the desorption velocity steadily increases. The first prediction was fulfilled when water was sorbed by vulcanised and unvulcanised rubber(54,8), gutta-percha, and paragutta(54) and the second... 

Its main features are given by the use of a stream of inert carrier gas which percolates through a bed of an adsorbent covered with adsorbate and heated in a defined way. The desorbed gas is carried off to a detector under conditions of no appreciable back-diffusion. This means that the actual concentration of the desorbed species in the bed is reproduced in the detector after a time lag which depends on the flow velocity and the distance. The theory of this method has been developed for a linear heating schedule, first-order desorption kinetics, no adsorbable component in the entering carrier gas (Pa = 0), and the Langmuir concept, and has already been reviewed (48, 49) so that it will not be dealt with here. An analysis of how closely the actual experimental conditions meet the idealized model is not available. 

Beavis, R. C. Chait, B. T. Velocity distributions of intact high mass polypeptide molecule ions produced by matrix-assisted laser-desorption. Chem. Phys. Lett. 1991,181, 479 184. 

Chan, T.-W. D. Thomas, I. Colburn, A. W. Derrick, P. J. Initial velocities of positive and negative protein molecule-ions produced in matrix-assisted ultraviolet laser desorption using a liquid matrix. Chem. Phys. Lett. 1994,222,579-585. 

Colby, S. M. King, T. B. Reilly, J. P. Improving the resolution of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry by exploiting the correlation between ion position and velocity. Rapid Comm. Mass Spectrom. 1994,8, 865-868. 

The velocity profile is uniform across the entire width of the channel if the channel is open at the electrodes, as is most often the case. However, if the electric field is applied across a closed channel (or a backpressure exists that just counters that produced by the pump), a recirculation pattern forms in which fluid along the center of the channel moves in a direction opposite to that at the walls further, the velocity along the centerline of the channel is 50% of that at the walls (Fig. 11.32a, see Plate 12 for color version). Figure 11.32b (see Plate 12 for color version) illustrates an electric field generating a net force on the fluid near the interface of the fluid/solid boundary, where a small separation of charge occurs due to the equilibrium between adsorption and desorption of ions. The charge region from excess cations localized near the interface by coulombic... 

Examination of Figures 2 to 4 clearly indicates flow enhancement of the desorption as seen by the substantial decreases in adsorbance once flow is applied. The overall trend of the desorption curves for the three molecular weight samples is similar and is characterized by a fairly rapid initial desorption followed by an approach to steady state. The desorption rate increases with the velocity gradient whereas the steady state adsorbance decreases as the flow is increased. 

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