# Rotational and rotational-vibrational Raman spectroscopy of air to characterize astronomical spectrographs

## TL;DR

This paper demonstrates how Raman spectroscopy of laser guide-star beams can be used to calibrate and characterize high-resolution astronomical spectrographs, revealing detailed molecular rotational and vibrational lines.

## Contribution

It presents the first observation of Raman lines from laser guide-star beams with high-resolution spectrographs, showcasing a novel calibration method for astronomical instruments.

## Key findings

- Detected rotational Raman lines of key atmospheric molecules up to high quantum numbers
- Observed fine-structure lines induced by electronic spin interactions in oxygen
- Identified rotational-vibrational Q-branch features for oxygen and nitrogen

## Abstract

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 $\lambda/\Delta\lambda \sim 140'000$. In 900s on-source, we detect the pure rotational Raman lines of $^{16}$O$_2$, $^{14}$N$_2$, and $^{14}$N$^{15}$N (tentatively) up to rotational quantum numbers $J$ of 27, 24, and 9, respectively. We detect the $^{16}$O$_2$ fine-structure lines induced by the interaction of the electronic spin \textbf{S} and end-over-end rotational angular momentum \textbf{N} in the electronic ground state of this molecule up to $N=9$. The same spectrum also reveals the $\nu_{1\leftarrow0}$ rotational-vibrational Q-branch for $^{16}$O$_2$ and $^{14}$N$_2$. These observations demonstrate the potential of using laser guide-star systems as accurate calibration sources for characterizing new astronomical spectrographs.