Measurement of Threshold Electrons in the Photoionization of H2 and D2*

Measurements of electrons produced in the photoionization of H₂ and D₂ are reported. The experimental technique employs a differential electron‐energy analyzer to detect photoelectrons produced with nearly zero initial kinetic energy (≲0.1 eV). By varying the photon energy a spectrum of “threshold” photoelectrons is recorded exhibiting the vibrational structure of the H₂⁺ and D₂⁺ ions. From these spectra values of $\mathrm{I(}{\mathrm{H}}_2)$ , $\mathrm{I(}{\mathrm{D}}_2)$ , ${\omega }_e({\mathrm{H}}_2^{+})$ , ${\omega }_e({\mathrm{D}}_2^{+})$ , ${\omega }_e{x}_e({\mathrm{H}}_2^{+})$ , and ${\omega }_e{x}_e({\mathrm{D}}_2^{+})$ are derived. In favorable cases the relative vibrational transition probabilities are also reported and compared with theoretical predictions as well as with other experiments. Some disagreements are noted in comparing the results obtained by this method with theoretical Franck–Condon factors based on potential curves appropriate to nonrotating molecules. It is suggested that the vibration–rotation interaction should be taken into account in calculations pertaining to electronic transitions in light molecules.