  We demonstrated the accurate prediction of a quasibound spectrum of a
negative ion using a novel high-precision theoretical approach. We used La$^-$
as a test case due to a recent experiment that measured energies of 11
resonances in its photodetachment spectrum attributed to transitions to
quasibound states [C. W. Walter et al., PRA, in press (2020);
arXiv:2010.01122]. We identified all of the observed resonances, and predicted
one more peak just outside the range of the prior experiment. Following the
theoretical prediction, the peak was observed at the predicted wavelength,
validating the identification. The same approach is applicable to a wide range
of negative ions. Moreover, theory advances reported in this work can be used
for massive generation of atomic transition properties for neutrals and
positive ions needed for a variety of applications.
