Direct measurement of the potential-barrier height in theB1Πustate of the sodium dimer

We report a direct measurement of the barrier height of the ${\mathrm{Na}}_2$ $B$-state potential curve. The technique is a two-step spectroscopy—optical pumping followed by photodissociation—on a beam of rotationally cold ${\mathrm{Na}}_2$. Our measured value is 366 ± 8 ${\mathrm{cm}}^{-1\mathrm{}}$ above the energy of the separated atoms ($\mathrm{Na} 3s{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}+\mathrm{Na}+3p{}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}$). Based on observed structure due to tunneling, we propose a form of the potential valid near the top of the barrier.