Spectroscopy of $^{39}$K$^{85}$Rb electronic states in the predicted region of resonantly coupled excited states for the direct formation of the $X$(0,0) state*

The 2$^{1}\Pi $(v$\prime $=17) and 1$^{1}\Pi $(v$\prime $=60) vibrational levels of KRb are predicted to be resonantly coupled, based on extrapolation from high rotational levels reported in [1] to \textit{J$\prime $ }=1 [2], and on tentative assignments of photoassociation spectra in the region near 12535cm$^{-1}$ [3]. Access to the 2$^{1}\Pi $ levels is desirable for formation of ultracold KRb molecules in their ground $X^{1}\Sigma ^{+}$, v=0, J=0 level because of strong Franck-Condon overlap [3]. The 1$^{1}\Pi $ component of these two mixed states provides PA access to short range region where 2$^{1}\Pi $ emission to the $X$(0,0) level can occur. Experiments are being carried out to better understand the spectra and perturbations in the region near 12535cm$^{-1}$, which should include 3(0$^{+})$, 2(1), 4(1), 5(1) and possibly other states. The 4(1) and 5(1) long-range states correlate with 1$^{1}\Pi $ and 2$^{1}\Pi $ short-range states. These studies have also produced new information on efficient formation of specific vibrational levels in a$^{3}\Sigma ^{+}$ state and new data on 3$^{3}\Sigma ^{+ }$and 3$^{3}\Pi $ states. \\[4pt] [1] Kasahara et al., JCP 111, 19 (1999)\\[0pt] [2] Wang et al, EPJD 31, 165 (2004)\\[0pt] [3] Stwalley et al., JPCA 114, 81 (2010)