Pure-rotational and rotational-vibrational Raman spectrum of the atmosphere at an altitude of 23 km

Ground-based optical astronomical observations supported by or in the vicinity of laser guide-star systems can be contaminated by Raman-scattered laser photons. Anticipating, alleviating, and correcting for the impact of this self-inflicted contamination requires a detailed knowledge of the pure-rotational and rotational-vibrational spectrum of the molecules in our atmosphere. We present a 15.3-hr-deep combined spectrum of the 4LGSF's 589 nm approximate to 509 THz sodium laser beams of Paranal observatory, acquired with the ESPRESSO spectrograph at a resolution A/4A similar to= 140000 approximate to 0.12 cm-1 and an altitude of 23 km above mean sea level. We identify 865 Raman lines over the spectral range of [3770; 7900] angstrom approximate to [+9540; -4315] cm-1, with relative intensities spanning similar to 5 orders of magnitudes. These lines are associated to the most abundant molecules of dry air, including their isotopes: 14N 14N, 14N 15N, 16O 16O, 16O 17O, 16O 18O, and 12C 16O 16O. The signal-to-noise of these observations implies that professional observatories can treat the resulting catalog of Raman lines as exhaustive (for the detected molecules, over the observed Raman shift range) for the purpose of predicting/correcting/exploiting Raman lines in astronomical data. Our observations also reveal that the four laser units of the 4LGSF do not all lase at the same central wavelength. We measure a blueshift of +43 +/- 10 MHz similar to= -50 -/+ 10 fm with respect to A* = 5891.59120 angstrom for LGSU1/2, and +94 +/- 10 MHz similar to= -109 -/+ 10 fm for LGSU3/4. These offsets, including the difference of similar to 50 MHz between LGSU1/2 and LGSU3/4, are larger than the observed 4LGSF spectral stability of +/- 3 MHz over hours. They remain well within the operational requirements for creating artificial laser guide-stars, but hinder the assessment of the radial velocity accuracy of ESPRESSO at the required level of 10 m s-1. Altogether, our observations demonstrate how Raman lines can be exploited by professional observatories as highly-accurate, on-sky wavelength references.

Automated summary

Ground-based optical astronomical observations can be contaminated by Raman-scattered laser photons, and a detailed understanding of atmospheric molecules' spectra is necessary to mitigate this contamination. We present a deep combined spectrum of the 4LGSF's sodium laser beams acquired with ESPRESSO spectrograph, identifying numerous Raman lines, which can be used as precise on-sky wavelength references by professional observatories.