We studied the mutual neutralization of Li+ and H- at effective collision energies of a few
hundred meV, which corresponds to temperatures of around 2000 K, in the double ion storage
ring DESIREE. We present a new approach to match beam velocities and a new general
analysis method for non-fragmenting mutual neutralization at DESIREE. Our results show
two features, which we could clearly assign to the product channel into the electronically
excited 3s state of neutral lithium and an unresolved combination of 3p and 3d final state
contributions. Branching fractions into 3s are extracted for ten different collision energies via
spectral binning and compared to several theoretical investigations and two previous
measurements, which focused on the heavier isotope deuterium. We find a significant isotope
effect, as theoretically predicted, but in contrast to previous experimental results. The
branching fractions agree well with different theoretical approaches using non-empirical
couplings and best with a combination of ab initio potentials combined with a combination of
ab initio potentials and Landau-Zener transition probabilities.