  High-resolution X-ray spectroscopy has advanced our understanding of the hot
Universe by revealing physical properties like kinematics, temperature, and
abundances of the astrophysical plasmas. Despite the technical and scientific
achievements, the lack of scientific products at a level higher than count
spectra is hampering full scientific exploitation of high-quality data. This
paper introduces the Catalogue of Ionized Emission Lines Observed by the
Reflection Grating Spectrometer (CIELO-RGS) onboard the XMM-Newton space
observatory. The CIELO-RGS catalogue aims to facilitate the exploitation of
emission features in the public RGS spectra archive, in particular, to perform
the correlation between X-ray spectral diagnostics parameters with measurements
at other wavelengths. This paper focuses on the methodology of catalogue
generation, describing the automated line detection algorithm. A moderate
sample (~2400 observations) of high-quality RGS spectra available at XMM-Newton
Science Archive is used as our starting point. A list of potential emission
lines is selected based on a multi-scale peak detection algorithm in a uniform
and automated way without prior assumption on the underlying astrophysical
model. The candidate line list is validated via spectral fitting with simple
continuum and line profile models. We also compare the catalogue content with
published literature results on a small number of exemplary sources. We
generate a catalogue of emission lines ~12000 detected in ~1600 observations
toward stars, X-ray binaries, supernovae remnants, active galactic nuclei, and
groups and clusters of galaxies. For each line, we report the observed
wavelength, broadening, energy and photon flux, and equivalent width, etc.
