Hyperfine structure in photoassociative spectra ofLi26andLi27

We present spectra of hyperfine resolved vibrational levels of the ${\mathit{A}}^1$ ${\mathrm{\Sigma }}_{\mathit{u}}^{+}$and 1${\mathrm{}}^3$ ${\mathrm{\Sigma }}_{\mathit{g}}^{+}$ states of ${}_{}{}^6{}_{}{}^{}\mathrm{Li}_2^{}{}_{}{}^{}$ and ${}_{}{}^7{}_{}{}^{}\mathrm{Li}_2^{}{}_{}{}^{}$ obtained via photoassociation of colliding ultracold atoms in a magneto-optical trap. A simple first-order perturbation theory analysis accurately accounts for the frequency splittings and relative transition strengths of all observed hyperfine features. Assignment of the hyperfine structure allows accurate determination of a vibrational level center of gravity, which significantly decreases the experimental uncertainty of vibrational energies. Differences in the spectra of ${}_{}{}^6{}_{}{}^{}\mathrm{Li}_2^{}{}_{}{}^{}$ and ${}_{}{}^7{}_{}{}^{}\mathrm{Li}_2^{}{}_{}{}^{}$ are attributed to quantum statistics. The 1${\mathrm{}}^3$ ${\mathrm{\Sigma }}_{\mathit{g}}^{+}$ series obeys Hund’s case ${\mathit{b}}_{\mathrm{\beta }\mathit{S}}$ coupling and the hyperfine constant is extracted for both isotopes. © 1996 The American Physical Society.