Strong-field phase-dependent molecular dissociation

We show how wave functions with the same energy and probability distribution but different momenta have very different dissociation probabilities when two molecular potentials are strongly coupled by an intense laser field. Our measurements of dynamics in a molecular family, in conjunction with molecular structure and wave-packet calculations, highlight the role of the wave-packet momentum, or spatially varying phase, in the dynamics leading to dissociation. Phase-dependent dynamics are central to coherent control of laser driven chemistry.

Figure 1

(Color online) A cartoon of three important potential-energy surfaces in the ${\text{CH}}_{2}X\text{I}$ family of molecules: the ground neutral state $V_0^{\text{neu}}$, the ground ionic state $V_0^{\text{ion}}$ (reached via strong-field ionization—note the broken vertical axis), and an excited ionic state $V_3^{\text{ion}}$. A wave packet launched high on the latter surface can dissociate along the C-I bond.

Figure 2

(Color online) Pump-probe signals for several molecules. (a) ${\text{CH}}_2{\text{BrI}}^{+}$ and ${\text{CH}}_2{\text{Br}}^{+}$ from ${\text{CH}}_2\text{BrI}$. Note that the oscillations in the parent and daughter ions are perfectly $\pi$ out of phase. (b) Two parent ions for comparison: ${\text{CH}}_2{\text{BrI}}^{+}$ and ${\text{CH}}_2{\text{ClI}}^{+}$. Inset: magnitude of the Fourier transforms for the data shown in panel (b).

Figure 3

Calculated potential-energy surfaces for ${\text{CH}}_2\text{BrI}$ as a function of the bending normal coordinate $u_B$. The shading of the curves is roughly proportional to the strength of the transition dipole moment from $V_0^{\text{ion}}$. The dotted line indicates ${\tilde{V}}_0^{\text{ion}}$, the $V_0^{\text{ion}}$ potential modified to emulate the effects of spin-orbit coupling.

Figure 4

(Color online) (a) The calculated population transferred from ${\tilde{V}}_0^{\text{ion}}$ to $V_3^{\text{ion}}$ for left- (at 80 fs) and right-going (at 250 fs) wave packets as a function of peak field strength. Also shown are the calculated ratios of population transferred for left- and right-going wave packets as a function of field strength. The ratios are shown with (green dotted) and without (blue solid) volume and orientation averaging. (b) The experimental population transfer, measured as a depletion from the parent (depth of modulation in the oscillations). The thick red solid and dashed lines represent population transferred for two different left-going wave packets. The thin black lines represent population transferred for two different right-going wave packets. (c) Adiabatic dressed states calculated from ${\tilde{V}}_0^{\text{ion}}$ and $V_3^{\text{ion}}$ in 2 GV/m (purple dashed lines) and 5 GV/m (green dotted lines) probe fields. Schematic wave packets are shown. (d) The calculated probability density for wave packets in ${\tilde{V}}_0^{\text{ion}}$ at 80 fs (left moving) and 250 fs (right moving).