ELECTRONIC TRANSITIONS AND SPIN-ORBIT SPLITTING OF LANTHANUM DIMER

Author Institution: Department of Chemistry, University of Kentucky, Lexington, KY 40506-0055Lanthanum dimer (La$_2$) was studied by mass-analyzed threshold ionization (MATI) spectroscopy and a series of high-level multi-configuration ab initio calculations (CASSCF, CASPT2, and MRCI). The MATI spectrum exhibits three band systems originating at 39044, 40312, and 40862 cm$^{-1}$, respectively. Above the band origin, the first band system displays a vibrational progression of $\sim$232 cm$^{-1}$, and the other two show vibrational progression with the same interval of $\sim$240 cm$^{-1}$. Below the band origin, the three systems exhibit the same vibrational interval of $\sim$207 cm$^{-1}$. These band systems are assigned to three electronic transitions from the ground state of La$_2$ to the low-lying electronic states of La$_2$$^+$: $^2\Sigma^+_g$ $\leftarrow$ $^1\Sigma^+_g$, $^2\Pi_{u,1/2}$ $\leftarrow$ $^1\Sigma^+_g$, and $^2\Pi_{u,3/2}$ $\leftarrow$ $^1\Sigma^+_g$. The spin-orbit splitting in the $^2\Pi_{u}$ ion state is 550 cm$^{-1}$. In addition, the electronic states and bonding of La$_2$ will be compared with those of Sc$_2$ and Y$_2$