  We investigate collisional loss in an ultracold mixture of $^{40}$K$^{87}$Rb
molecules and $^{87}$Rb atoms, where chemical reactions between the two species
are energetically forbidden. Through direct detection of the KRb$_{2}^{*}$
intermediate complexes formed from atom-molecule collisions, we show that a
$1064$ nm laser source used for optical trapping of the sample can efficiently
deplete the complex population via photo-excitation, an effect which can
explain the universal two-body loss observed in the mixture. By monitoring the
time-evolution of the KRb$_{2}^{*}$ population after a sudden reduction in the
$1064$ nm laser intensity, we measure the lifetime of the complex ($0.39(6)$
ms), as well as the photo-excitation rate for $1064$ nm light ($0.50(3)$
$\mu$s$^{-1}($kW/cm$^{2})^{-1}$). The observed lifetime is ${\sim}10^{5}$ times
longer than recent estimates based on the Rice-Ramsperger-Kassel-Marcus
statistical theory, which calls for new insight to explain such a dramatic
discrepancy.
