Balmer-series fluorescence from photodissociation fragments ofH2andD2

The photodissociation processes of ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ have been studied using photon wavelengths in the $\lambda \lambda 686-723$-Å region. The fluorescence radiation from the photofragments was dispersed in the $\lambda \lambda 3500-7000$-Å region and was identified as the Balmer series. The cross sections for the production of the Balmer series were measured and were used to infer the cross sections for the photodissociation of ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ into atomic products in the $n\ge 3$ levels. The dependence of the cross sections for the production of excited atoms in the $n$ levels on the excess kinetic energy is, in general, similar to that in the $n=2\mathrm{}$ level calculated by Allison and Dalgarno. The photodissociation process is therefore attributed mainly to the direct photoexcitation of molecules from the ground state into continua. This suggestion is further strengthened by the fact that the ratio of the cross sections for production of atoms formed in the $n=4\mathrm{} \mathrm{and} 5$ levels agrees with that of the square of their transition moments calculated by Rothenburg and Davidson. However, the ratio of the cross sections between the $n=3\mathrm{} \mathrm{and} 4$ levels does not agree well with the same calculation. It is therefore inferred that the cross section for the production of atoms excited in the $n=3\mathrm{}$ level may be partially through predissociation processes. This observation is consistent with the theoretical prediction of Berry and Nielsen that photodissociation of ${\mathrm{H}}_2$ into $H\left(1s\right)+H(nl,n\ge 3)$ through predissociation is only important in the $n=3\mathrm{}$ channel.