the objects’ K band spectra. Section 5.2 describes how the strengths of the NaI lines and other spectral features were determined. Section 5.3 deals with the objects’ revised spectral types.

Section 6 relates our findings regarding collisionally induced absorption by H$_2$ in the K bandpass. Section 6.1 describes the derivation of a new index, the H$_{2}$(K) index, to measure this absorption and how the index is related to the objects’ ages. Section 6.2 uses models to show changes in the K band slope with surface gravity. Section 6.3 tests the H$_{2}$(K) index for the effects of hot dust, extinction, noise and metallicity, and examines whether the H$_{2}$(K) index varies as a function of spectral type.

Section 7 describes how the H$_{2}$(K) index was examined using an extended dataset of spectra from the literature.

Our results are discussed in Section 8. Section 8.1 contains a more detailed discussion of the results for our sample of brown dwarf calibrators. Section 8.2 discusses our observed spectrum of an ONC PMO. Section 8.3 describes a distinctive water absorption pattern in the K band which may be an additional diagnostic in classifying spectra.

Section 9 compares the strength of neutral alkali metal lines with the 