Observation of Bound-Free-Bound Triplet Absorption Bands inLi2

Multiphoton ionization spectroscopy has yielded the first evidence for triplet absorption bands in ${\mathrm{Li}}_2$ in the spectral region from 6215 to 5875 Å. This spectrum is the first of a new class of spectra resulting from a two-photon transition from a bound (${{}_{}{}^3{}_{}{}^{}\Sigma _{\mathrm{u}}^{}{}_{}{}^{}}^{+}$) van der Waals state to a previously unknown bound ${}_{}{}^3{}_{}{}^{}\Lambda _{\mathrm{u}}^{}{}_{}{}^{}$ state (where $\Lambda =0, 1, or 2$) through an intermediate free state (${}_{}{}^3{}_{}{}^{}\Pi _{\mathrm{g}}^{}{}_{}{}^{}$). The final state either autoionizes or collisionally forms the ${{}_{}{}^2{}_{}{}^{}\Sigma _{\mathrm{g}}^{}{}_{}{}^{}}^{+}$ ground state of ${\mathrm{Li}}^{2+}$.