  Photoassociation of ultracold atoms induced by chirped picosecond pulses is
analyzed in a non-perturbative treatment by following the wavepackets dynamics
on the ground and excited surfaces. The initial state is described by a
Boltzmann distribution of continuum scattering states. The chosen example is
photoassociation of cesium atoms at temperature T=54 $\mu K$ from the $a^3
\Sigma_u^+(6s,6s)$ continuum to bound levels in the external well of the
$0_g^-(6s+6p_{3/2})$ potential. We study how the modification of the pulse
characteristics (carrier frequency, duration, linear chirp rate and intensity)
can enhance the number of photoassociated molecules and suggest ways of
optimizing the production of stable molecules.
