  We investigate the possibility of forming deeply bound ultracold RbCs
molecules by a two-color photoassociation experiment. We compare the results
with those for Rb_2 in order to understand the characteristic differences
between heteronuclear and homonuclear molecules. The major differences arise
from the different long-range potential for excited states. Ultracold 85Rb and
133Cs atoms colliding on the X^1Sigma+ potential curve are initially
photoassociated to form excited RbCs molecules in the region below the Rb(5S) +
Cs(6P_1/2) asymptote. We explore the nature of the Omega=0^+ levels in this
region, which have mixed A^1Sigma^+ and b^3Pi character. We then study the
quantum dynamics of RbCs by a time-dependent wavepacket (TDWP) approach. A
wavepacket is formed by exciting a few vibronic levels and is allowed to
propagate on the coupled electronic potential energy curves. For a detuning of
7.5 cm-1, the wavepacket for RbCs reaches the short-range region in about 13
ps, which is significantly faster than for the homonuclear Rb_2 system; this is
mostly because of the absence of an R^-3 long-range tail in the excited-state
potential curves for heteronuclear systems. We give a simple semiclassical
formula that relates the time taken to the long-range potential parameters. For
RbCs, in contrast to Rb_2, the excited-state wavepacket shows a substantial
peak in singlet density near the inner turning point, and this produces a
significant probability of deexcitation to form ground-state molecules bound by
up to 1500 cm-1. Our analysis of the role of spin-orbit coupling concerns the
character of the mixed states in general and is important for both
photoassociation and stimulated Raman deexcitation.
