Enhancing the non-adiabatic transfer with heavier isotopomers: Effect of mass in the ultrafast electron-nuclear dynamics in LiH induced by few-cycle IR pulses

Exciting molecules using few-cycle intense optical pulses leads to non-equilibrium electron density and induces ultrafast nuclear response on the femtosecond timescale. By simulating the coupled electron-nuclear dynamics of the LiH molecule and isotopomers, we show that non adiabatic coupling is favored in heavier isotopomers of LiH.
When a coherent superposition of coupled electronic states is pumped by the pulse, the rate of transfer is modulated by the overlap of the nuclear wave packets and the momentum acquired on the electronic states involved in the non-adiabatic dynamics. Since the non-adiabatic coupling strength is inversely proportional to the reduced mass one would expect that the increase in mass decreases the importance of non-adiabatic transfer. However, our quantum dynamical simulations show the overlap between the nuclear wave packets of heavier isotopomers is larger and that they acquire a larger momentum on the repulsive region of the potential energy curves. This leads to a significant transient bistate isotope effect and an increase in non-adiabatic transfer for the heavier isotopomers.