Excitation of a cadmium-hydrogen gas mixture with light from a hydrogen-cadmium electric discharge produces with great intensity a portion of the band system of the CdH molecule, while excitation of the same mixture with light from a helium-cadmium discharge does not produce these bands. The process consists, first, of a collision between a $2{}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$ cadmium atom and a ${\mathrm{H}}_2$ molecule to form an unexcited CdH molecule, which molecule is then brought to the excited levels by absorption from the exciting light of the band frequencies themselves, whence it emits the band radiation in returning to the normal state. A weak excitation of the entire ${}_{}{}^2{}_{}{}^{}\Pi -{}_{}{}^2{}_{}{}^{}\Sigma$ band system also occurs independently of the optical excitation as a result of collisions of the second kind with excited cadmium atoms.

ZnH bands are also produced as a true optical resonance phenomenon under exactly parallel conditions. The formation of the ZnH molecule results in a selective quenching by hydrogen of the triplet zinc spectrum, while the singlet spectrum is unaffected.

The experiment of Gaviola and Wood, in which HgH bands are excited through collisions of the second kind, is repeated and discussed in the light of the above results.

• Received 6 October 1930

DOI:https://doi.org/10.1103/PhysRev.36.1543