Chemical evolution of cold dark clouds in the vicinity of supernova
  remnants
  The supernova explosion increases cosmic ray and x-ray fluxes in the
surrounding interstellar medium. Cosmic ray particles and x-ray radiation
penetrate nearby molecular clouds and affect the chemical and thermal evolution
of the gas. Here we study chemical changes in the dense molecular gas
influenced by a sudden increase of the ionization rate that may be caused by
the supernova explosion. At the cloud core density $2 \times 10^4$ cm$^{-3}$,
the H atom abundance reaches the equilibrium value at about $3 \times 10^4$ yr
after the change in irradiation conditions. The response time of abundances of
icy mantle species is $10^4-10^5$ yr. The abundances of neutral and grain
mantle species may not reach their equilibrium values in molecular clouds in
the vicinity of 'middle-aged' supernova remnants.
