Knowledge Bank

The $F(4)1\Sigma +$ state of $Li2$ is known to have an unusual shape because of two (or three) avoided crossings with other states of the same symmetry. One of these avoided crossings gives rise to a well defined shelf'' in the effective adiabatic potential curve, which makes analysis of data for this state by traditional methods quite difficult, since conventional molecular constant expressions cannot describe the resulting level energy patterns., Recent experimental $results^{a}$ have yielded precise measurements of selected vibration-rotation levels of this state spanning almost the entire interval from the potential minimum to dissociation. The present paper describes our use of direct-potential-fit'' (DPF) methods to quantitatively reanalyse those data determine an accurate analytic potential energy function for this shelf-state system.