Under irradiation of a pure solid CO sample at low temperature by a CW CO laser beam,
13C
16O,
12C
18O and
13C
18O molecules are excited to high vibrational levels, up to

, and a strong population inversion is observed. The
12C
16O molecules are pumped to the

level by laser line absorption in the low frequency phonon sideband, and the vibrational energy is subsequently transferred to the other isotopes. Quantum yield, efficiency, and heat release are discussed to clarify the mechanism.