Investigation of nonadiabatic effects in molecular-hydrogen Rydberg states by electric field ionization

The technique of electric field ionization has been used to study molecular Rydberg states of ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ formed by electron capture of ${\mathrm{H}}_2^{+}$ or ${\mathrm{D}}_2^{+}$ beams in ${\mathrm{H}}_2$. The neutral molecules were passed through two intense electric field ionization regions in tandem, separated by an ∼ ${10}^{-7\mathrm{}}$-sec drift region. Up to 15% of the Rydberg population removed by the first ionization region was repopulated during transit through the drift region. This result has been interpreted in terms of nonadiabatic state mixing of nearly degenerate adiabatic molecular states. A difference in the state repopulation between Rydberg states of ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ was observed, indicating the importance of the rovibrational spectrum on the repopulation mechanism. By comparison, similar studies on atomic Rydberg states showed no difference between H and D. Moreover, the atoms exhibited only a small repopulation effect, consistent with the contribution due to $\overrightarrow{1}·\overrightarrow{\mathrm{s}}$ coupling.