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 $\lambda /\mathrm{\Delta }\lambda \sim 140000$. In 900 s on source, we detect the pure rotational Raman lines of ${}^{16}{\mathrm{O}}_2$, ${}^{14}{\mathrm{N}}_2$, and ${}^{14}\mathrm{N}{}^{15}\mathrm{N}$ (tentatively) up to rotational quantum numbers $J$ of 27, 24, and 9, respectively. We detect the ${}^{16}{\mathrm{O}}_2$ fine-structure lines induced by the interaction of the electronic spin $\mathbf{S}$ and end-over-end rotational angular momentum $\mathbf{N}$ in the electronic ground state of this molecule up to $N=9$. The same spectrum also reveals the ${\nu }_{1\leftarrow 0}$ rotational-vibrational Q-branch for ${}^{16}{\mathrm{O}}_2$ and ${}^{14}{\mathrm{N}}_2$. These observations demonstrate the potential of using laser guide-star systems as accurate calibration sources for characterizing new astronomical spectrographs.

Figure 1

Diagram (not to scale) of the manual acquisition steps for the ESPRESSO observations described in this article. (a) All 4LGSF lasers are propagating. The four laser guide-stars (orange dots) are positioned to a nominal square asterism using the LPC. From the instrument perspective, the up-link laser beams (visible up to $\sim 35\mathrm{km}$ of altitude) are located on the outside of the laser guide stars (located at $\sim 90\mathrm{km}$ of altitude). (b) Using the LPC, a single laser guide star is placed in centerfield. The propagation is stopped for all other laser guide stars. (c) The up-link beam is brought over the ESPRESSO fiber (black circle) using manual offsets applied to the jitter-loop mirror of the laser launch telescope. The focus of the telescope is adjusted to bring the beam better into focus at the location of the fiber.

Figure 2

ESPRESSO view of the pure rotational Raman lines (top), and $Q$ branch of the ro-vibrational Raman lines of ${}^{16}{\mathrm{O}}_2$ (bottom left) and ${}^{14}{\mathrm{N}}_2$ (bottom right), associated with one 4LGSF up-link laser beam. Orange (for ${}^{16}{\mathrm{O}}_2$) and purple (for ${}^{14}{\mathrm{N}}_2$) lines mark the theoretical position of each Raman line derived with the model of nonrigid singlet diatomic rotators (see the Supplemental Material [31] for details). Colored integers denote the rotational quantum number $J$ associated with the different analytical wavelength predictions. The CCD did saturate within $\pm 0.2\AA$ of the main laser line.

Figure 3

ESPRESSO spectrum of one 4LGSF up-link laser beam, focusing on the spectral regions containing pure rotational Raman lines. Each panel is 8 Å wide. For a given row, the left and right panels are equidistant from the main laser line at 5891.5912 Å (which is saturated). Pure rotational Raman lines, associated with the Stokes (right column) and anti-Stokes (left column) branches, are detected up to $J=27$ for ${}^{16}{\mathrm{O}}_2$, $J=24$ for ${}^{14}{\mathrm{N}}_2$, and $J=9$ for ${}^{14}\mathrm{N}{}^{15}\mathrm{N}$. Solid orange, solid purple, and dash-dotted blue vertical lines mark the theoretical position of the pure rotational Raman lines of ${}^{16}{\mathrm{O}}_2$, ${}^{14}{\mathrm{N}}_2$, and ${}^{14}\mathrm{N}{}^{15}\mathrm{N}$ (respectively), when treated as nonrigid singlet diatomic rotators. The molecular rotational quantum number $J$ associated with each of these lines is indicated above them (only the ${}^{14}{\mathrm{N}}_2$ $J=11$ anti-Stokes and $J=9$, 19 Stokes lines are not tagged because of crowding). Dashed orange vertical lines, derived using a numerical model of ${}^{16}{\mathrm{O}}_2$ accounting for the nonzero electronic spin of its ground state, highlight the fine-splitting of the pure rotational lines for this molecule. The nature of six lines (tagged with dotted grey vertical lines and the first two decimals of their wavelength in the top panels) remains unknown at this time.