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Shaped ultrafast laser pulses were used to study and control the ionizationdynamics of electronically excited pyrazine in a pump and probe experiment. Forpump pulses created without feedback from the product signal, the ion growthcurve (the parent ion signal as a function of pump/probe delay) was describedquantitatively by the classical rate equations for internal conversion of the$S_2$ and $S_1$ states. Very different, non-classical behavior was observedwhen a genetic algorithm (GA) was used to minimize the ion signal at somepre-determined target time, T. Two qualitatively different control mechanismswere identified for early (T$<1.5$ ps) and late (T$>1.5$ ps) target times. Inthe former case, the ion signal was largely suppressed for $t<T$, while for $t\gg T$ the ion signal produced by the GA-optimized pulse and a transformlimited (TL) pulse coalesced. In contrast, for $T>1.5$ ps the ion growth curvefollowed the classical rate equations for $t<T$, while for $t \gg T$ thequantum yield for the GA-optimized pulse was much smaller than for a TL pulse.We interpret the first type of behavior as an indication that the wave packetproduced by the pump laser is localized in a region of the $S_2$ potentialenergy surface where the vertical ionization energy exceeds the probe photonenergy, whereas the second type of behavior may be described by a reducedabsorption cross section for $S_0 \rightarrow S_2$ followed by incoherent decayof the excited molecules.

Coherent Phase Control of Internal Conversion in Pyrazine