  Ultra hot Jupiters orbit very close to their host star and consequently
receive strong irradiation that makes their atmospheric chemistry different
from the common gas giants. Here, we study the atmosphere of one of these
particular hot planets, MASCARA-2b/KELT-20b, using four transit observations
with high resolution spectroscopy facilities. Three of these observations were
performed with HARPS-N and one with CARMENES. We simultaneously observed one of
the transits with MuSCAT2 to monitor possible spots in the stellar surface. At
high resolution, the transmission residuals show the effects of
Rossiter-McLaughlin and center-to-limb variations from the stellar lines
profiles, which we correct to finally extract the transmission spectra of the
planet. We clearly observe absorption of CaII, FeII, NaI, H$\alpha$ and
H$\beta$ in the atmosphere of MASCARA-2b, and indications of H$\gamma$ and MgI
at low SNR. The results obtained with CARMENES and HARPS-N are consistent,
measuring an H$\alpha$ absorption depth of $0.68\pm0.05\%$ and $0.59\pm0.07\%$,
and NaI absorption of $0.11\pm0.04\%$ and $0.09\pm0.05\%$ ($0.75\r{A}$
passband), in both instruments respectively. For H$\beta$ only HARPS-N covers
this wavelength range, measuring $0.28\pm0.06\%$ of absorption. The CaII is
only covered with CARMENES, measuring an average absorption of $0.32\pm0.05\%$.
Three additional FeII lines are observed in the transmission spectrum by
HARPS-N, with mean absorption depth of $0.08\pm0.04\%$. The results presented
here are consistent with theoretical models of ultra hot Jupiters atmospheres,
suggesting the emergence of an ionised gas on the day-side of such planets. Ca
and Fe, with other elements, are expected to be singly ionised at these
temperatures and be more numerous than its neutral state. The CaII triplet
lines are detected here for the first time in transmission in an exoplanet
atmosphere.
