Knowledge Bank

Recent optical-optical double resonance experiments on BaO employing two 1 MHz line width cw dye lasers have demonstrated several advantages over broadband (30GHz) OODR experiments; a fifty-fold increase in signal to noise; resolution for excitation spectroscopy typically 20% of the Doppler width-high precision and accuracy ($0.003cm-1).1,2$ The rotationally linked BaO $C1\Sigma +\leftarrow A1\Sigma +$ and $A1\Sigma +\leftarrow X1\Sigma +$ transitions excite a single velocity group. Fluorescence is observed from the extensively perturbed $C1\Sigma +$ state into the previously unobserved $b3\Sigma +$ state as well as into $a3\Pi (\Omega =0 and 1,v=0-11)$ and $A\prime \mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Pi (v=0-11)$. Four vibrational levels of $b3\Sigma +$ are observed, the lowest is situated at $\sim 19637 cm-1(B\sim 0.237 cm-1)$ above$X1\Sigma +(v\prime \prime =0,J\prime \prime =0)$. $b3\Sigma +$ is significantly perturbed by both $a3\Pi$ and $A\prime \mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Pi$. Because the molecules available for $C\leftarrow A$ excitation by che second laser are velocity as well as state selected, OODR excitation line-widths are limited only by pressure and power broadening. In addition to permitting a precise rotational analysis of $C1\Sigma +$, it has been possible to detect extra lines due to perturbations. Extra lines are recognizable by their narrower line widths. They are partially forbidden transitions and ate therefore less susceptible than main lines to power broadening.