  We used residual gas analysis (RGA) to identify the species desorbed during
field emission (FE) from a carbon nanotube (CNT) fiber. The RGA data show a
sharp threshold for H2 desorption at an external field strength that coincides
with a breakpoint in the FE data. A comprehensive model for the gradual
transition of FE from adsorbate-enhanced CNTs at low bias to FE from CNTs with
reduced H2 adsorbate coverage at high bias is developed which accounts for the
gradual desorption of the H2 adsorbates, alignment of the CNTs at the fiber
tip, and importance of self-heating effects with applied bias.
