Selective Bond Breaking of HOD by Nonresonant Infrared Pulse Trains and a Delayed Ultraviolet Pulse

Tailored ultrafast laser pulses provide a powerful means of selectively breaking bonds in polyatomic molecules. However, traditional laser control strategies often assume that reactions begin from pure eigenstates or thermal ensembles. Here, we present a theoretical framework that combines an ultrashort infrared (IR) pulse train with a time-delayed ultraviolet (UV) pulse to achieve selective photodissociation in HOD─a prototypical system for mode-selective chemistry. Our approach utilizes nonresonant infrared (NIR) pulse trains to drive impulsive stimulated Raman scattering (ISRS), thereby generating coherent superpositions of the O-H and O-D stretching vibrational modes in the ground electronic state. These vibrational coherences enable UV-induced quantum interference: a time-delayed UV pulse projects the prepared vibrational states onto a repulsive electronic state, resulting in bond-selective photodissociation. By tuning the pulse delay and adjusting molecular orientation, we can steer the reaction pathway and, under optimal conditions, reduce the H/D product ratio to 0.3. Our findings underscore the critical roles of vibrational coherence and pulse shaping in controlling chemical reactivity and offer an analytically transparent perspective that connects vibrational-state preparation to dissociation branching ratios.