Knowledge Bank

Doppler-limited laser excitation spectroscopy using narrow band fluorescence detection has been used to obtain a rotaional analysis of the $B2\Sigma +-X2\Sigma +$ system of Cal. The extreme spectral congestion $(>20$ lines per $cm-1$) necessitates selective detection of fluorescence produced in the excitation spectrum using a monochromato at a spectral slit width of about $2cm-1$. The 0-1, 1-2, 2-3, and 0-0 bands have been examined using a single mode tunable laser with Rhodamine 101 dye. Constants for the 0-0 band $(3\sigma$ uncertainties in parentheses), $\nu 00=15716.751(3)$, are presented below: The spin-rotation constant, $\gamma$, which is typically a small parameter, is particularly large in Cal $B2\Sigma +$ due to the proximity of the $A2\pi$ state.