  The mixing of N2 with H2 leads to very different plasmas from pure N2 and H2
plasma discharges. Numerous issues are therefore raised involving the processes
leading to ammonia (NH3) formation. The aim of this work is to better
characterize capacitively-coupled radiofrequency plasma discharges in N2 with
few percents of H2 (up to 5 pct), at low pressure (0.3 to 1 mbar) and low
coupled power (3 to 13 W). Both experimental measurements and numerical
simulations are performed. For clarity, we separated the results in two
complementary parts. The actual one (first part), presents the details on the
experimental measurements, while the second focuses on the simulation, a hybrid
model combining a 2D fluid module and a 0D kinetic module. Electron density is
measured by a resonant cavity method. It varies from 0.4 to 5e9 cm-3,
corresponding to ionization degrees from 2e-8 to 4e-7. Ammonia density is
quantified by combining IR absorption and mass spectrometry. It increases
linearly with the amount of H2 (up to 3e13 cm-3 at 5 pct H2). On the contrary,
it is constant with pressure, which suggests the dominance of surface processes
on the formation of ammonia. Positive ions are measured by mass spectrometry.
Nitrogen-bearing ions are hydrogenated by the injection of H2, N2H+ being the
major ion as soon as the amount of H2 is larger than 1 pct. The increase of
pressure leads to an increase of secondary ions formed by ion (or radical) -
neutral collisions (ex: N2H+, NH4+, H3+), while an increase of the coupled
power favours ions formed by direct ionization (ex: N2+, NH3+, H2+).
