Surface reactivity in the formation of

In a time-of-flight atom-probe, if the vacuum condition is not good enough, many hydride ions of the tip material can be found in the mass 

In field ionization, hydrogen molecules near the tip region are attracted to the tip surface. They either hop around the tip surface or are field adsorbed on it. As the hopping motion and the field adsorption are dynamical phenomena, some of the ionic species detected may also come from field adsorbed states, not necessarily just from the gas phase. On the other hand, in pulsed-laser stimulated field desorption, where gas pressure is very low, of only 1 X 10-8 Torr, gas molecules are thermally desorbed by laser pulses from their field adsorbed and chemisorbed states. When they pass across the field ionization zone some of them are field ionized. The critical ion energy deficit in pulsed-laser stimulated field desorption of a gas is therefore found to be identical to that found in field ionization. In both pulsed-laser stimulated field desorption and field ionization of hydrogen, the majority of ions detected are H3 and H+. 

An interesting question is how H3 is formed on the emitter surface and whether H3 molecules can exist on the surface. This question can be investigated with a measurement of the appearance energy of Hj ions. Jason etal.264 find Hj in field ionization of condensed layers of hydrogen, and measure the appearance energy to be 12.7 eV. This value is 2.9 eV smaller than that of H2. Ernst Block conclude265 from a similar measurement in field ionization mass spectrometry of hydrogen that an H 3 ion is formed at the moment when a chemisorbed H atom combines 

Another question is how H3 molecules are formed on the emitter surface. At least two distinctive mechanisms can be envisioned. They are dissociative and associative mechanisms as represented by  

An asterisk ( ) here represents an adsorption site. There are several observations which provide strong support for the association mechanism.266 First, Hf is observed from some metals such as Au, which is known not to show dissociative chemisorption of H2 at low temperatures. Second, for some metals such as Ir, even though chemisorption of H2 is dissociative, few Hj ions are observed. Third, when a H2-D2 mixed gas is used, atomic exchanges always occur regardless of whether hydro- 

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