Pulsed field desorption

Block and co-workers [35] modified the atom probe to develop a method called pulsed-field desorption mass spectrometry (PFDMS), whereby a short high-voltage pulse desorbs all species present on the tip during a catalytic reaction. The repetition frequency of the field pulse controls the time for which the reaction is allowed to proceed. Hence, by varying the repetition frequency between desorption pulses in a systematic way, one can study the kinetics of a surface reaction [35], In fact, this type of experiment - where one focuses on a facet of desired structure, which may include steps and defects - comes close to one of the fundamental goals of catalyst characterization, namely studying a catalytic reaction on substrates of atomically resolved structure with high time resolution. 

Various surface sensitive techniques have been employed under ultrahigh vacuum conditions to study the interaction of NO with Pt and Ru single crystal surfaces (for a review see ref.l). Recently, the Pt(410) plane has been found to dissociate NO (ref.2), whereas the flat (111) plane is inactive which is in accord with theoretical considerations (ref.3). The plane to plane variations in the NO interaction with Pt have prompted us to use field emitter tips with their well defined crystallography as model catalysts and to perform face specific measurements in a time resolved manner by employing pulsed field desorption mass spectrometry (PFDMS). This method has been shown in a number of previous papers (see e.g. ref.4-7) to provide valuable kinetic data on surface processes. 

Fig. 2.5 An ion kinetic energy distribution of field desorbed He ions taken with a pulsed-laser time-of-flight atom-probe. In pulsed-laser stimulated field desorption of field adsorbed atoms, atoms are thermally desorbed from the surface by pulsed-laser heating. When they pass through the field ionization zone, they are field ionized. Therefore the ion energy distribution is in every respect the same as those in ordinary field ionization. Beside the sharp onset, there are also secondary peaks due to a resonance tunneling effect as discussed in the text. The onset flight time is indicated by to, and resonance peak positions are indicated by arrows. Resonance peaks are pronounced only if ions are collected from a flat area of the...

Two scientific aspects in cluster science have been investigated using pulsed-laser field desorption technique with a considerable degree of success.85 One is the critical numbers, which are the smallest numbers of atoms in multiply charged cluster ions when the ions can still resist being... 

Fig. 2.22 In [UP - 1 )/T] vs. 1 IT plot for field adsorption of He on the W (112) surface, obtained from a pulsed-laser stimulated field desorption experiment. In the high temperature region, the data points fit well into a straight line of H = 0.17 eV, as expected from a theoretical analysis. In the low temperature region, the data points deviate significantly from the linear plot of the solid line. This deviation indicates that there may be another binding state with a much smaller binding energy of 0.08 eV.

In pulsed-laser field desorption of helium from rhodium surfaces, if the... 

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