An atom-probe field-ion microscope study of 200-eV ¹H ⁺₂ ions implanted in tungsten at 29 K

The atom-probe field-ion microscope (FIM) technique was used to study the low-temperature diffusive behavior of hydrogen (¹H) in tungsten. Tungsten FIM specimens were implanted in situ with 200-eV ¹H ⁺₂ ions, at a specimen temperature of 29 K. The specimens were analyzed chemically on an atomic scale, via the atom-probe FIM technique, during the controlled pulse field evaporation of the (110) planes. No hydrogen events were detected at depths below the surface which corresponded to the calculated mean projected range of either ¹H or ¹H ₂. The mean projected ranges were calculated employing a modified version of Biersack's and Haggmark's Monte Carlo program entitled trim (Transport of Ions in Matter). The experimental results set a lower bound of (1-10)×10^-18 cm² s ^-1 on the diffusivity of hydrogen (¹H) in tungsten at 29 K. This extremely high diffusivity of hydrogen in tungsten at 29 K, when compared with the value extrapolated from ∼1100 K on an Arrhenius plot, suggests strongly that the diffusion of hydrogen in tungsten should be described by a nonclassical model. In addition, experiments were performed which demonstrated the adsorption of hydrogen on the surface of tungsten FIM tips (initially cleaned atomically by field evaporation) from the ambient pressure in the FIM.