  (Abr.) Laser guide stars employed at astronomical observatories provide
artificial wavefront reference sources to help correct (in part) the impact of
atmospheric turbulence on astrophysical observations. Following the recent
commissioning of the 4 Laser Guide Star Facility (4LGSF) on UT4 at the VLT, we
characterize the spectral signature of the uplink beams from the 22W lasers to
assess the impact of laser scattering from the 4LGSF on science observations.
We use the MUSE optical integral field spectrograph to acquire spectra at a
resolution of R~3000 of the uplink laser beams over the wavelength range of
4750\AA\ to 9350\AA. We report the first detection of laser-induced Raman
scattering by N2, O2, CO2, H2O and (tentatively) CH4 molecules in the
atmosphere above the astronomical observatory of Cerro Paranal. In particular,
our observations reveal the characteristic spectral signature of laser photons
-- but 480\AA\ to 2210\AA\ redder than the original laser wavelength of
5889.959\AA\ -- landing on the 8.2m primary mirror of UT4 after being
Raman-scattered on their way up to the sodium layer. Laser-induced Raman
scattering is not unique to the observatory of Cerro Paranal, but common to any
astronomical telescope employing a laser-guide-star (LGS) system. It is thus
essential for any optical spectrograph coupled to a LGS system to handle
thoroughly the possibility of a Raman spectral contamination via a proper
baffling of the instrument and suitable calibrations procedures. These
considerations are particularly applicable for the HARMONI optical spectrograph
on the upcoming Extremely Large Telescope. At sites hosting multiple
telescopes, laser collision prediction tools also ought to account for the
presence of Raman emission from the uplink laser beam(s) to avoid the
unintentional contamination of observations acquired with telescopes in the
vicinity of a LGS system.
