Control of breaking strong versus weak bonds of BaFCH3 by femtosecond IR + VIS laser pulses: theory and experiment

Abstract

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 CH₃ play the roles of the weak and strong bonds to be broken, and the vibrationally pre-excited CH₃ stretch. 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 versus strong bond breaking, as well as isotopomer selectivity depending on the vibrational pre-excitations.