Coherent control of the ultrafast dissociative ionization dynamics of bromochloroalkanes

Abstract

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.