This paper presents model numerical simulations of photoassociation and ionization of cold $\mathrm{Rb}+\mathrm{Cs}$ atoms steered by broadband optical pulses operating within the weak-field limit over a picosecond time scale. The primary focus of the work is on generation of RbCs molecules in bound levels of the $\left(1\right)0^{+}$ $\left(X{}^1\Sigma ^{+}\right)$ electronic ground state through a sequence of pump-dump transitions between $\left(1\right)0^{+}$ and $\left(4\right)0^{+}$ states driven by a field centered at $811\mathrm{nm}$ with different phase modulations. It is found that chirped fields generate substantially bound oscillator levels of the $X{}^1\Sigma ^{+}$ state by avoiding promotion of amplitude to vibrational levels in the upper state whose Franck-Condon factors for stimulated emission to levels in the $X{}^1\Sigma ^{+}$ state are detrimentally low, but which otherwise come into play when the driving field is transform limited. Optimal generation of molecules in the electronic ground state irrespective of vibrational level selectivity is calculated to occur when the temporal phase of the driving field is modulated by the classical energy difference for promotion of $\left(4\right)0^{+}\leftarrow \left(1\right)0^{+}$ photon absorption. Conversely, generation of deeply bound vibrational eigenlevels of the $X{}^1\Sigma ^{+}$ state is optimally promoted by a field whose temporal phase enhances $\left(4\right)0^{+}\to \left(1\right)0^{+}$ stimulated emission. Driving fields phase modulated by the classical difference potential or linearly down-chirped enhance molecule formation vis-à-vis the thermal collision; fields whose temporal phase is modulated according to the shape of the $\left(1\right)0^{+}$ potential energy curve or which are linearly up-chirped suppress molecule formation relative to the thermal probability, but generate deeply bound $X{}^1\Sigma ^{+}$ vibrational levels. Application of a bichromatic field comprising temporally overlapped components centered 811 and $622\mathrm{nm}$ also improves the probability of generating deeply bound vibrational levels, but at the expense of increased ionization. Model calculations of transition probabilities between neutral and ionic states of the collision lead to the conclusions that: transitions to individual vibrational levels of the $X{}^1\Sigma ^{+}$ state occur with a higher probability than resonant three-photon ionization prompted by a single-color field at $811\mathrm{nm}$; and application of bichromatic light at 811 and $622\mathrm{nm}$ preferentially ionizes the incipient molecule via a two-color two-photon absorption, the likelihood of which masks the occupation of deeply bound $X{}^1\Sigma ^{+}$ oscillator levels.

• Received 4 July 2008

DOI:https://doi.org/10.1103/PhysRevA.78.053404