Knowledge Bank

Laser absorption spectrum of the LaF $B1\Pi -X1\Sigma +$ (2,0), (1,0), (2,1), and (3,2) bands was recorded. Gas phase LaF was obtained by heating a mixture of a 0.2 g of La metal and 0.3 g of $laF3$ powder to 1900K. In order to modulate the absorption signal, the concentration of LaF was varied by passing a 25 kHz discharge (80-100W, 0.5A peak current) through the vapor. Under these conditions the Tantalum heating tube used to vaporize the sample also acted as a hollow cathode. The B-X band system was observed previously In analyzing the spectrum, we have combined absorption data with that from the Reference. Accurate molecular constants (in $cm-1$) and internuclear distance $\tau e$ (in nm) for the $X1\Sigma +$ state are:$$\omega_{e}=573.318, \omega_{e}x_{e}=2.18, B_{e}=0.246411(15), \alpha_{e}=1.251(15)\times10^{-3}, \tau_{e}=0.20232.$$ Rotational perturbations were evident in the levels. The pattern of the state perturbations was shown to be a consequence of the interaction with the state by using the experimental value of $\omega e=450$ $cm-1$ (see Reference) and estimated value of $\omega exe$ $cm-1$ for the triplet state. The principal new results to emerge from this investigation are the equilbrium constants for the $B1\Pi$ and $X1\Sigma +$ states of LaF.