The technique of modulated gain spectroscopy has provided detailed information about the shape of the Na2B1Πu state potential barrier to dissociation. By measuring the rotation–vibration energies (to ±0.006 cm1) of all (v′=27–33) quasibound vibrational levels and the rotation‐dependent tunneling rates (as obtained from linewidth measurements) of the highest quasibound vibrational level, v′=33, we have been able to characterize this barrier. Our studies show that the barrier height is U(rmax,J=0)=375.2±3.9 cm1, relative to the center of gravity of the Na(3s)+Na(3p) atomic limit, and the barrier maximum is located at rmax=6.85±0.02 Å.

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