By using a two meter grazing incidence vacuum spectrograph, the extreme ultraviolet emission spectrum of a mixture of about 60 percent ordinary hydrogen and 40 percent "heavy" hydrogen has been photographed. All together twelve bands have been identified as belonging to the $2p{}_{}{}^1{}_{}{}^{}\Pi _{\mathrm{cd}}^{}{}_{}{}^{}-1s{}_{}{}^1{}_{}{}^{}\Sigma$ system of the ${\mathrm{H}}^1$${\mathrm{H}}^2$ molecule. Analysis of these bands indicates that the rotational and vibrational constants for the ${\mathrm{H}}^1$${\mathrm{H}}^2$ molecule are the same to within experimental error as those derived from the theory of the isotope effect. There is, however, an extremely large electronic isotope shift of 135 ${\mathrm{cm}}^{-1\mathrm{}}$ units. The apparent vibrational perturbation found previously in the $v=3\mathrm{}$ level of the $2p{}_{}{}^1{}_{}{}^{}\Pi _{\mathrm{cd}}^{}{}_{}{}^{}$ state of the ${\mathrm{H}}^1$${\mathrm{H}}^1$ molecule has its counterpart in the same level of the ${\mathrm{H}}^1$${\mathrm{H}}^2$ molecule where its magnitude is about 4.3 ${\mathrm{cm}}^{-1\mathrm{}}$ units, compared to 10.4 ${\mathrm{cm}}^{-1\mathrm{}}$ for ${\mathrm{H}}^1$${\mathrm{H}}^1$.

• Received 15 February 1934

DOI:https://doi.org/10.1103/PhysRev.45.480