  The interaction of exoplanets with their host stars causes a vast diversity
in bulk and atmospheric compositions, and physical and chemical conditions.
Stellar radiation, especially at the shorter wavelengths, drives the chemistry
in the upper atmospheric layers of close orbiting gaseous giants, providing
drastic departures from equilibrium. In this study, we aim at unfolding the
effects caused by photons in different spectral bands on the atmospheric
chemistry, with particular emphasis on the molecular synthesis induced by
X-rays. This task is particularly difficult because the characteristics of
chemical evolution emerge from many feedbacks on a wide range of time scales,
and because of the existing correlations among different portions of the
stellar spectrum.
  The weak X-ray photoabsorption cross-sections of the atmospheric constituents
boost the gas ionization to pressures inaccessible to vacuum and extreme
ultraviolet photons. Although X-rays interact preferentially with metals, they
produce a secondary electron cascade able to ionize efficiently hydrogen and
helium bearing species, giving rise to a distinctive chemistry.
