  Energetic gamma rays (GeV to TeV photon energy) have been detected toward
several supernova remnants (SNR) associated with molecular clouds. If the gamma
rays are produced mainly by hadronic processes rather than leptonic processes
like bremsstrahlung, then the flux of energetic cosmic ray (CR) nuclei (>1 GeV)
required to produce the gamma rays can be inferred at the site where the
particles are accelerated in SNR shocks. It is of great interest to understand
the acceleration of the CR of lower energy (<1 GeV) accompanying the energetic
component. These particles of lower energy are most effective in ionizing
interstellar gas, leaving an observable imprint on the interstellar ion
chemistry. A correlation of energetic gamma radiation with enhanced
interstellar ionization can thus support the hadronic origin of the gamma rays
and constrain the acceleration of ionizing CR in SNR. We propose a method to
test the hadronic origin of GeV gamma rays from SNR associated with a molecular
cloud. We use observational gamma ray data for each of these SNR known,
modeling the observations to obtain the underlying proton spectrum assuming
that the gamma rays are produced by pion decay. Assuming that the acceleration
mechanism does not only produce high energy protons, but also low energy
protons, this proton spectrum at the source is then used to calculate the
ionization rate of the molecular cloud. Ionized molecular hydrogen triggers a
chemical network forming molecular ions. The relaxation of these ions results
in characteristic line emission, which can be predicted. We show that the
ionization rate for at least two objects is more than an order of magnitude
above Galactic average for molecular clouds, hinting at an enhanced formation
rate of molecular ions. There will be interesting opportunities to measure
crucial molecular ions in the infrared and submillimeter-wave parts of the
spectrum.
