  This paper shows experimental results from the TCV tokamak that indicate
plasma-molecule interactions involving $D_2^+$ and possibly $D^-$ play an
important role as sinks of energy (through hydrogenic radiation as well as
dissociation) and particles during divertor detachment if low target
temperatures ($< 3$ eV) are achieved. Both molecular activated recombination
(MAR) and ion source reduction due to a power limitation effect are shown to be
important in reducing the ion target flux during a density ramp. In contrast,
the electron-ion recombination (EIR) ion sink is too small to play an important
role in reducing the ion target flux. MAR or EIR do not occur during $N_2$
seeding induced detachment as the target temperatures are not sufficiently low.
  The impact of $D_2^+$ is shown to be underestimated in present (vibrationally
unresolved) SOLPS-ITER simulations, which could result from an underestimated
$D_2 + D^+ \rightarrow D_2^+ + D$ rate. The converged SOLPS-ITER simulations
are post-processed with alternative reaction rates, resulting in considerable
contributions of $D_2^+$ to particle and power losses as well as dissociation
below the $D_2$ dissociation area. Those findings are in quantitative agreement
with the experimental results.
