  We demonstrate that judicious shaping of a nanosecond-time-scale frequency
chirp can dramatically enhance the formation rate of ultracold $^{87}$Rb$_{2}$
molecules. Starting with ultracold $^{87}$Rb atoms, we apply pulses of
frequency-chirped light to first photoassociate the atoms into excited
molecules and then, later in the chirp, de-excite these molecules into a high
vibrational level of the lowest triplet state, $a \, ^{3}\Sigma_{u}^{+}$. The
enhancing chirp shape passes through the absorption and stimulated emission
transitions relatively slowly, thus increasing their adiabaticity, but jumps
quickly between them to minimize the effects of spontaneous emission.
Comparisons with quantum simulations for various chirp shapes support this
enhancement mechanism.
