Selective bond breakage within the HOD molecule using optimized femtosecond ultraviolet laser pulses

With the HOD molecule initially in its vibrational ground state, we theoretically analyze the laser-induced control of the $\mathrm{O}\mathrm{D}∕\mathrm{O}\mathrm{H}$ branching ratio $\mathrm{D}+\mathrm{O}\mathrm{H}\leftarrow \mathrm{H}\mathrm{O}\mathrm{D}\to \mathrm{H}+\mathrm{O}\mathrm{D}$ in the first absorption band. In the weak-field limit, any form of UV-pulse shaping control leads to a branching ratio larger than $\sim 2$. We obtain in the strong-field limit (peak intensities $\sim 10\mathrm{TW}∕{\mathrm{cm}}^2$) a branching ratio significantly less than 2. The optimized pulses operate by a pump-dump-pump mechanism, where the dumping to the electronic ground state creates nonstationary vibrational states in HOD.

Figure 1

(Color online) The ground and first excited state potential energy surfaces for water. The two potentials asymptotically approach the same value, but we have here separated the surfaces for graphical purposes. The Franck-Condon wave packet is shown right after an instantaneous excitation.

Figure 2

The upper panel shows the optimized pulse (cycle-averaged peak intensity $18\mathrm{TW}∕{\mathrm{cm}}^2$). The lower panel shows the Husimi transform of the pulse.

Figure 3

The upper panel shows the population in the ground and excited state for the pulse in Fig. 2. The lower panel shows the time-resolved “branching ratio,” see text. The result corresponding to excitation by an infinitely short $\delta$ pulse is also shown.

Figure 4

(Color online) Plots of the vibrational probability density in the electronic ground state during the action of the pulse in Fig. 2.