  We report multiconfiguration Dirac-Hartree-Fock and relativistic
configuration interaction calculations on the Thallium (Tl) electron affinity,
as well as on the excited energy levels arising from the ground configuration
of Tl$^-$. The results are compared with the available experimental values and
further validated by extending the study to its homologous, lighter element,
Indium (In), belonging to Group 13 (III.A) of the periodic table. The
calculated electron affinities of In and Tl, 383.4 and 322.8 meV, agree with
the latest measurements by within 1\%. Three bound states $^3P_{0,1,2}$ are
confirmed in the $5s^25p^2$ configuration of In$^-$ while only the ground state
$^3P_{0}$ is bound in the $6s^26p^2$ configuration of Tl$^-$. The isotope
shifts on the In and Tl electron affinities are also estimated. The E2/M1
intraconfiguration radiative transition rates within $5s^25p^2 \; ^3P_{0,1,2}$
of In$^-$ are used to calculate the radiative lifetimes of the metastable
$^3P_{1,2}$ levels.
