The high vibrational levels of the $B''\bar{B}\; {}^1\Sigma_u^+$ state of H$_2$

The \emph{ungerade} $B''\bar{B}\; {}^1\Sigma_u^+$ state of H$_2$ is of interest in molecular physics due to its double-well shape that supports extremely long range, high vibrational levels converging to the third dissociation limit. The highest vibrational levels are expected to have ion-pair character and therefore follow an energy pattern that smoothly connects to the ion-pair series previously observed above the third dissociation limit. Resonantly enhanced multiphoton ionization (REMPI) spectroscopy through the $E,F$, $v = 6$ state and time of flight ion detection was used to acquire the vibrational spectrum of the $B''\bar{B}$ state over an energy range of 2000 cm$^{-1}$ leading up to the third dissociation limit. Vibrational energies inclusive of $v=51$ to $v=69$ were measured to a precision of $\sim 0.5$ cm$^{-1}$. Assignments were aided by previous measurements and theoretical predictions from the literature. The results are used to explore the ion-pair nature of these high vibrational states.