Knowledge Bank

The Van der Waals molecule NaHe produced in a supersonic expansion has been studied with a single-mode dye laser. Excitation spectra of $A2\Pi r\leftarrow X2\Sigma +$ and $B2\Sigma +\leftarrow X2\Sigma +$ transitions were obtained by monitoring the total laser-induced fluorescence. The analysis of the spectra was performed using the classical, technique of combination differences, and the vibrational numbering of the $A2\Pi r$ state was determined from isotope shifts. The $v\Pi =3,4,5$, and 6 vibrational levels of the $A2\Pi r$ state have been observed. The spin orbit constant $A\Pi$ varies from $12.109cm-1,11.968cm-1$ for $v\Pi =2,3,4$ to $15,379cm-1$ for $v\Pi =6$. The lower vibrational levels are close to the theoretical Hund’s case a” value $11.464 cm^{-1}$ and are well described by a Hand’s case a” basic set. Higher vibrational levels tend to approach Hund’s case c” because of strong interaction with the $B^{2} \Sigma^{+}$ state. The observed $B^{2} \Sigma^{+}$ level is best described by a core c” basic set; however, it is not known if the single, observed $B2\Sigma +$ level is the lowest vibrational level. An upper limit on the repulsive part of the $B2\Sigma +$ potential can be obtained from the observed $\Lambda$ doubling of the $A\Pi 1/2,v\Pi =3,4$ levels. The long range analysis of the $A2\Pi$ potential is complicated by the $A2\Pi -B2\Sigma +$ interactions for high $v\Pi$ levels. An extended analysis of higher $v\Pi$ levels, where a Hund’s case ``c” basic set is applicable, is now in progress.