  Context. Investigating the diagnostic power of H$\alpha$/H$\beta$
Charge-Exchange (CE) emission as Low-Energy Galactic Cosmic Rays(LECRs) tracer
in diffuse regions. Aims. In this work, we define and test a spectroscopic
indicator of CE reactions between LECRs protons and neutral hydrogen atoms of
the diffuse medium. This indicator can be used for mapping LECRs density in
diffuse clouds and can lead to the identification of new LECRs sources as we
expect density variations caused by the distance between an observed cloud and
the nearest site of particle acceleration. We also lay the foundations for the
definition of a photometric indicator to be used in the next full-sky
photometric surveys such as the Vera Rubin 10-year Legacy Survey of Space and
Time (LSST). Methods. Based on literature cross-sections, we calculate
H$\alpha$/H$\beta$ line profile ratio in the case of CE and compare it with the
recombination ratio. We then test our results on the Balmer-dominated filaments
of the SNR RCW 86 and we explore how the spectroscopic constraints can turn
into a photometric indicator based on colour indices. Results. We find that, in
shocked environments, CE between LECRS and neutral hydrogen become the dominant
process for Balmer lines emission. The hydrogen spectroscopic emission is
expected to be modified, with respect to the recombination Balmer decrement,to
result in double the H$\alpha$/H$\beta$ with respect to a similar but quiescent
region. The test on the known Balmer-dominated filaments of the SNR RCW 86
confirm the efficiency of our spectroscopic indicator. Therefore we explore
possible conversions of the spectroscopic indicator into colour indices
combinations. This is the first step toward the definition and test of a
photometric indicator for tracing LECRs to be applied in the LSST pipelines to
photometrically identify new LECRs accelerators in the whole Galaxy.
