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

Abstract

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≈509 THz sodium laser beams of Paranal observatory, acquired with the ESPRESSO spectrograph at a resolution λ/Δλ≅140000≈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] Å≈ [+9540; –4315] cm^{−1}, with relative intensities spanning ∼5 orders of magnitudes. These lines are associated to the most abundant molecules of dry air, including their isotopes: ^{14}N^{14}N, ^{14}N^{15}N, ^{16}O^{16}O, ^{16}O^{17}O, ^{16}O^{18}O, and ^{12}C^{16}O^{16}O. 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 ≅−50∓10 fm with respect to λ_{*}=5891.59120 Å for LGSU1/2, and +94±10 MHz ≅−109∓10 fm for LGSU3/4. These offsets, including the difference of ∼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.