  The product formation channels of ground state carbon atoms, C(3P), reacting
with ammonia, NH3, have been investigated using two complementary experiments
and electronic structure calculations. Reaction products are detected in a gas
flow tube experiment (330 K, 4 Torr) using tunable VUV photoionization coupled
with time of flight mass spectrometry. Temporal profiles of the species formed
and photoionization spectra are used to identify primary products of the C +
NH3 reaction. In addition, H-atom formation is monitored by VUV laser induced
fluorescence from room temperature to 50 K in a supersonic gas flow generated
by the Laval nozzle technique. Electronic structure calculations are performed
to derive intermediates, transition states and complexes formed along the
reaction coordinate. The combination of photoionization and laser induced
fluorescence experiments supported by theoretical calculations indicate that in
the temperature and pressure range investigated, the H + H2CN production
channel represents 100% of the product yield for this reaction. Kinetics
measurements of the title reaction down to 50 K and the effect of the new rate
constants on interstellar nitrogen hydride abundances using a model of dense
interstellar clouds are reported in paper II.
