We present a quantum-mechanical study of the dynamics of collisions of
cold atoms in a (magneto-) optical trap valid in the linear intensity
regime. We find further evidence for a discrepancy with results of th
e semiclassical optical-Bloch-equation method at lower collision energ
ies and detunings, referred to as ''quantum suppression'' in previous
work. We find a strong dependence on the detuning: for the lowest ener
gies considered the suppression factor ranges from more than three ord
ers of magnitude for DELTA = -GAMMA(at) to a factor 2 at DELTA = -10GA
MMA(at). The fraction of atoms surviving at small interatomic distance
s for a single l value can in good approximation be described by a sim
ple formula in the form of a product of a Landau-Zener excitation func
tion and a WKB survival factor. Summed over partial waves this formula
allows a relatively easy calculation of loss rates in a two-channel l
inear-intensity description. The role of such an approach in a future
more complete treatment including hyperfine channels is pointed out.