  Low-energy (~MeV) cosmic rays (CRs) ionize molecular clouds and create the
neutral iron line (Fe I K\alpha) at 6.4 keV. On the other hand, high-energy (>~
GeV) CRs interact with the dense cloud gas and produce gamma rays. Based on a
one-dimensional model, we study the spatial correlation among ionization rates
of gas, 6.4 keV line fluxes, and gamma-ray emissions from a molecular cloud
illuminated by CRs accelerated at an adjacent supernova remnant. We find that
the spatial distributions of these three observables depend on how CRs intrude
the cloud and on the internal structure of the cloud. If the intrusion is
represented by slow diffusion, the 6.4 keV line should be detected around the
cloud edge where ionization rates are high. On the other hand, if CRs freely
stream in the cloud, the 6.4 keV line should be observed where gamma rays are
emitted. In the former, the cooling time of the CRs responsible for the 6.4 keV
line is shorter than their cloud crossing time, and it is opposite in the
latter. Although we compare the results with observations, we cannot conclude
whether the diffusion or the free-streaming is dominantly realized. Our
predictions can be checked in more detail with future X-ray missions such as
XRISM and Athena and by observations of ionization rates that cover wider
fields.
