Stroboscopic Effect between Electronic and Nuclear Motion in Highly Excited Molecular Rydberg States

The ${\mathrm{Na}}_2$ Rydberg states have been observed for $15<~n<~90$ and $4<~J<~45$. The evolution of the spectra is studied as a function of the relative values of ${\omega }_{\mathrm{el}}$, the electronic rotational frequency, and ${\omega }_N$, the nuclear rotational frequency. The molecular eigenfunction is found to be pure Hund's case $a$ when $k{\omega }_{\mathrm{el}}=2\mathrm{}{\omega }_N$ ($k$ being an integer). This may be explained in terms of stroboscopic effects arising from the movement of the Rydberg electron relative to the ionic core.