Study of field adsorption using pulsed-laser atom-probe field-ion microscopy

The high resolution pulsed-laser time-of-flight atom-probe, and the pulsed-laser imaging atom-probe are discussed. Under good operating conditions its mass resolution can be as high as 4000, or better. It is capable of separating for example (4)He(+) from D2(+), (3)He(+) from H3(+), and (56)Fe2(+) from N2(+) down to 5 or 10% of the peak height. This system is also a high resolution ion energy analyzer. The resolution, E/delta E, can be as high as 20,000 when heavy ions such as Au(+) and AuH2(+) are analyzed. Formation of H3(+) and NH3(+) was studied. Pulsed-laser field desorption of hydrogen results in formation of H(+), H2(+), and H3(+). The relative abundances of H(+) and H2(+) are found to be independent of the emitter materials or surface planes. They are dependent only on the applied field. Thus these ions, though formed right at or near the surface, are not surface catalyzed reaction products. They are formed by the high applied electric field. From the field dependence of their relative abundances, and the widening ion energy distribution width of the H(+) at higher field, it is concluded that H(+) is formed by field dissociation-ionization of H2(+).