Hyperfine structure and line intensities of selected (B1Π-c3Σ+)→ a3Σ+ fluorescence bands in KRb, NaK and NaRb

In a series of previous experiments laser induced fluorescence from the mixed $B^1\Pi$-$c^3{\Sigma }^{+}$ states in alkali metal dimers was used to study their lowest triplet state $a^3{\Sigma }^{+}$. The fluorescence lines show structure which was attributed mainly to the hyperfine splitting of the $a^3{\Sigma }^{+}$ levels. Only the recorded spectra of KRb (Pashov et al., 2007) demonstrated noticeable hyperfine structure of the $B^1\Pi$-$c^3{\Sigma }^{+}$ levels. In this study we used experimental data on the structure of $B^1\Pi$-$c^3{\Sigma }^{+}$ levels available in the literature for NaK, NaRb and KRb and performed calculations on the profiles of the spectral transitions to the $a^3{\Sigma }^{+}$ state. It was shown that it is not the hyperfine structure of $(B-c)$ levels which makes the KRb case different from the other alkali metal molecules in those studies, but it is a coincidence of molecular structure and Doppler broadening, which leads to nonuniform population of the hyperfine components of the $(B-c)$ levels. This effect becomes stronger when the hyperfine splitting approaches and even exceeds the Doppler width of the exciting $X\to (B-c)$ transitions. At lower instrumental resolution, however, the effect could be washed out and this may be the reason why it was not reported in all fluorescence studies of the $a^3{\Sigma }^{+}$ states through the complex of $B^1\Pi$-$c^3{\Sigma }^{+}$ states.