If a sodium chloride (NaCl) molecule could undergo an n → n - 1 vibrational transition with no change in rotational quantum number, a photon with wavelength 20.0 mm would be emitted. The mass of a sodium atom is 3.82 × 10^-26 kg, and the mass of a chlorine atom is 5.81 × 10^-26 kg. Calculate the force constant k' for the interatomic force in NaCl.