Critical ion energy deficit

An attempt has also been made to derive the binding energy of atoms in clusters from a measurement of the critical energy deficit of cluster ions. For n+ cluster ions of m atom size, from consideration of Born-Haber energy cycle, the critical ion energy deficit can be easily shown to be given by100... 

Fig. 3.19 Mass separated energy distributions of pulsed-laser field desorbed 4He+ and ions obtained with the Penn State pulsed-laser ToF atom-probe when the flight path length was 778 cm. Their onset flight times are separated by 34 ns, exactly that calculated from the system constants and their critical ion energy deficits at 5.5 kV.

In a preliminary pulsed-laser atom-probe measurement,158 the critical ion energy deficits of Rh and W atoms, field evaporated from kink sites, are measured at a rate of 1010 layers per second at a temperature around 100 K. Using the system constants, i.e. the flight path constant and the... 

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+. 

Table 3.3 Ionic masses and critical energy deficits of gas ions... 

Table 4.3 Critical energy deficits for various metal ions...

Fig. 5.22 Critical energy deficits of various ion species in a liquid Ga ion source as functions of the ion currents (solid lines and left and lower scales), from Swanson.144 Dashed lines are the peak positions of the ion energy distributions (upper and right scales), from Culbertson et al.145...

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