Control of breaking strong versusweak bonds of BaFCH3by femtosecond IR VIS laser pulses: theory and experiment

Intense 80 GW cm−2 ultrashort 100 fs infrared IR laser pulses may be employed for excitation of a high frequency 3500 cm−1 local mode vibration in a molecule. Subsequently, an intense 16–256 GW cm−2, ultrashort visible VIS laser pulse yields electronic excitation with near adiabatic transfer of the vibrational energy, which has been accumulated by the IR pulse. The net result of these sequential IR VIS laser pulses may be the breaking of a strong molecular bond close to the pre-excited one. In contrast, exclusive excitation by just a visible laser pulse breaks a competing weak bond. The effects of IR VIS laser pulse control may be considered as an extension of vibrationally mediated chemistry, from ns pulses or continuous wave cw excitations to sub-ps laser pulses, and from direct vibrational pre-excitation of the bond to be broken to a neighboring bond, thus exploiting intramolecular vibrational redistribution IVR from the pre-excited local mode to the bond to be broken in the electronic excited state. The mechanism is demonstrated by quantum simulations for the model system BaFCH3, where BaF-, FC- and CH3play the roles of the weak and strong bonds to be broken, and the vibrationally pre-excited CH3stretch. The theoretical predictions are confirmed experimentally. Various extensions of the control by IR VIS laser pulses include the control of the branching ratio of weak versusstrong bond breaking, as well as isotopomer selectivity depending on the vibrational pre-excitations.