Rotational and Rotational-Vibrational Raman Spectroscopy of Air to Characterize Astronomical Spectrographs

Raman scattering enables unforeseen uses for the laser guide-star system of the Very Large Telescope. Here, we present the observation of one up-link sodium laser beam acquired with the ESPRESSO spectrograph at a resolution λ /Δ λ ∼140 000 . In 900 s on source, we detect the pure rotational Raman lines of ¹⁶O₂, ¹⁴N₂, and ¹⁴N ¹⁵N (tentatively) up to rotational quantum numbers J of 27, 24, and 9, respectively. We detect the ¹⁶O₂ fine-structure lines induced by the interaction of the electronic spin S and end-over-end rotational angular momentum N in the electronic ground state of this molecule up to N =9 . The same spectrum also reveals the ν_{1 ←0} rotational-vibrational Q-branch for ¹⁶O₂ and ¹⁴N₂ 14. These observations demonstrate the potential of using laser guide-star systems as accurate calibration sources for characterizing new astronomical spectrographs.