Atomic transitions for adaptive optics

Abstract

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, which probably precludes their use for ground-based adaptive optics.

Can amplified spontaneous emission produce intense laser guide stars for adaptive optics?

“…The optical depth depends only on the excitation fraction, the column density and the transition cross section for stimulated emission. The metallic species present in the mesosphere do not have sufficient column density to support ASE [19].…”

“…However, to support ASE, this population inversion must be maintained for a time comparable to the propagation time through the mesosphere, on the order of 10 −4 s. This puts an upper limit on the transition rate of 21 10 4 . That results in a very-low optical depth for all mesospheric metals [19]…”