  The dissociative chemisorption of molecular nitrogen on clean lanthanide
surfaces at ambient temperature and low pressure is explored. In-situ
conductance measurements track the conversion from the lanthanide metals to the
insulating lanthanide nitrides. A small partial pressure of oxygen ($\sim
10^{-8}$ mbar) is shown to inhibit the nitridation of lanthanides at $10^{-4}$
mbar of N$_2$. The rate of nitridation as a function of nitrogen pressure is
measured at low pressure for a series of lanthanide elements, gadolinium,
terbium, dysprosium, ytterbium and praseodymium. Exposure of the lanthanide
surfaces to both N$_2$ and H$_2$ results in the formation of NH$_3$.
