  We theoretically analyse the process of charge recombination in the planar
Mott-Hubbard insulators with the aim to explain short picosecond-range lifetime
of photoexcited carriers, experimentally studied via pump-probe experiments on
the undoped cuprates. The recombination mechanism consists of two essential
ingredients: the formation of a metastable s-type bound holon-doublon pair,
i.e. the Mott exciton, and the decay of such an excitonic state via the
multimagnon emission. In spite of the large gap that requires many bosons to be
emitted, latter process is fast due to large exchange scale and strong
charge-spin coupling in planar systems. As the starting microscopic model we
consider the single-band Hubbard model, and then more realistic three-band
model for cuprates, both leading to the same minimal one. The decay rate of the
exciton is evaluated numerically via the Fermi golden rule, having consistency
also with the direct time-evolution calculation. The decay rate reveals
exponential dependence on the ratio of the Mott-Hubbard gap and the exchange
coupling - the result qualitatively reproduced also within a toy exciton-boson
model.
