Coherent Control of Molecular Ion Production in Cold Rb Vapor*

When Rb vapor is exposed to pulses of light from an ultra-fast laser having a central wavelength of about 800~nm, the result is a very large number of atomic Rb ions. This is because, within the bandwidth of the laser pulse, resonant three-photon ionization takes place along the ladder Rb(5s) $\rightarrow$ Rb (5p) $\rightarrow$ Rb(5d) $\rightarrow$~$\epsilon l$. The first transition is at 780~nm, the second is at 776~nm, and the third is anything shorter than 1252~nm. Virtually no molecular ions are formed in the interaction of the optical pulse with the Rb vapor. In a series of experiments we show that this natural trend can be reversed, with greatly reduced Rb$^+$ production and greatly increased Rb$_2^+$ production. Partly this is accomplished by introducing a weak, quasi-cw diode laser, nearly resonant with the Rb(5p) $\rightarrow$ Rb(4d) transition. However, an important component in switching from Rb$^+$ to Rb$_2^+$ production is shaping the ultra-fast pulse in the frequency and phase domains, putting ``notches" in the beam at crucial wavelengths and adjusting the ``chirp" of the pulse. For example, removing wavelengths near the D1 and Rb (5p) $\rightarrow$ Rb(5d) transitions reduced Rb$^+$ production by nearly two orders of magnitude. A detailed discussion of these and related results will be given.