Quantum Simulations for Isotope Effects of IR + UV Laser Pulses on Symmetry and Selective Hydrogen Bond Breaking

Intense few-cycle femtosecond (fs) infrared (IR) laser pulses yield dynamical symmetry breaking of oriented strong symmetric hydrogen bonds A···H···A due to nearly coherent vibrational excitation. Consequently, the system evolves with alternate stretches or compressions of the competing bonds A···H and H···A. For a specific stretch of, say, the H···A bond, an ultrashort ultraviolet (UV) laser pulse may induce a nearly instantaneous electronic excitation and/or photo-detachment by means of a Franck–Condon(FC)-type transition to a dissociative potential energy surface of the excited state. The stretched bond, H····A, will then dissociate selectively, yielding preferably the products AH+A. A minor fraction of the other products A+HA may also be formed due to competing wavepacket dispersion. The corresponding isotope effects are investigated by means of the representative laser driven molecular wavepackets which are propagated on ab initio potential energy surfaces and with ab initio dipole coupling for the model systems, FHF− versus FDF−. The resulting quantum dynamics for both systems are nearly equivalent if the driving IR laser fields are scaled with decreasing carrier frequency and amplitudes and increasing durations for the corresponding increasing masses of the isotopomers.