  This paper reviews atoms and ions in the upper atmosphere, including the
mesospheric metals Na, Fe, Mg$^+$, Si$^+$, Ca$^+$, K and also non-metallic
species N, N$^+$, O, H, considering their potential for astronomical adaptive
optics. Na and Fe are the best candidates for the creation of polychromatic
laser guide stars, with the strongest returns coming from transitions that can
be reached by excitation at two wavelengths. Ca$^+$ and Si$^+$ have strong
visible-light transitions, but require short wavelengths, beyond the
atmospheric cutoff, for excitation from the ground state. Atomic O, N and N$^+$
have strong transitions and high abundances in the mesosphere. The product of
column density and cross section for these species can be as high as $10^5$ for
O and several hundred for N and N$^+$, making them potential candidates for
amplified spontaneous emission. However they require vacuum-ultraviolet
wavelengths for excitation.
