Knowledge Bank

Predissociation of two quasi-bound vibrational levels of the $O+f4\pi g$ state is observed in transitions from $\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}\pi u(v\prime \prime =5,6)$ using high resolution photofragment $spectroscopy.1$ This is the first experimental observation of bound levels in this state and confirms the existence of the barrier in its potential at large internuclear distances that had been predicted by $thery.2$ Isotope shifts in the spectra suggest vibrational numbering of the observed levels $v\prime =1,2$ with $\omega e\prime =597,\omega exe\prime =104$ and $De=476$ $cm-1$. Preliminary rotational analysis of the bands gives fine structure and effective rotational constants for the two vibrational levels $A1\prime =-53,B1\prime =0.485,A2\prime =-43$, and $B2\prime =0.420$ $cm-1$. Bound levels are observed up to $N\prime =15$ in $v\prime =2$ which sets a lower a limit on the barrier height of $>440$ $cm-1$. Kinetic energy analysis of the photofragment $O+$ ions allows a determination of the branching ratio among the $O(3P)$ products produced in the predissociation of the $f4\pi g$ state. For dissociation of the $v\prime =2$ levels, which lie in energy above the separated atom limits for all three O atom spin-orbit components, $\Omega \prime =3/2$ levels are found to predominantly form $O(3P0)$ whereas $\Omega \prime =5/2$ levels appear to dissociate to all three limits. The dissociation lifetimes given by the linewidths of the observed transitions, are markedly dependent on the vibrational, rotational, and fine structure quantum numbers of the predissociated levels. These lifetimes are inconsistent with dissociation of the $f4\pi g$ levels by tunneling through the potential barrier. Rather, the observed lifetimes and product branching ratios suggest allowed couplings to other gerade state correlating to the $O+(4S)+O(3P)$ separated atom limit are responsible for the predissociation of the quasi-bound levels in this state.