We report on the coherent control of the ultrafast ionization and fragmentation dynamics of the bromochloroalkanes C2H4BrCl and C3H6BrCl using shaped femtosecond laser pulses. In closed-loop control experiments on bromochloropropane (C3H6BrCl) the fragment ion yields of CH2Cl+, CH2Br+, and C3H3+ are optimized with respect to that of the parent cation C3H6BrCl+. The fragment ion yields are recorded in additional experiments in order to reveal the energetics of cation fragmentation, where laser-produced plasma radiation is used as a tunable pulsed nanosecond vacuum ultraviolet radiation source along with photoionization mass spectrometry. The time structure of the optimized femtosecond laser pulses leads to a depletion of the parent ion and an enhancement of the fragment ions, where a characteristic sequence of pulses is required. Specifically, an intense pump pulse is followed by a less intense probe pulse where the delay is 0.5 ps. Similarly optimized pulse shapes are obtained from closed-loop control experiments on bromochloroethane (C2H4BrCl), where the fragment ion yield of CH2Br+ is optimized with respect to that of C2H4BrCl+ as well as the fragment ion ratios C2H2+/CH2Br+ and C2H3+/C2H4Cl+. The assignment of the underlying control mechanism is derived from one-color 804 nm pump–probe experiments, where the yields of the parent cation and several fragments show broad dynamic resonances with a maximum at Δt = 0.5 ps. The experimental findings are rationalized in terms of dynamic ionic resonances leading to an enhanced dissociation of the parent cation and some primary fragment ions.
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