Coherent Control of Internal Conversion in Strong-Field Molecular Ionization

We demonstrate coherent control over internal conversion during strong-field molecular ionization with shaped, few-cycle laser pulses. The control is driven by interference in different neutral states, which are coupled via non-Born-Oppenheimer terms in the molecular Hamiltonian. Our measurements highlight the preservation of electronic coherence in nonadiabatic transitions between electronic states.

Figure 1

Top panel: Schematic diagram illustrating the experiment. Two short pulses intersect a molecular beam in a VMI spectrometer. The electrons produced by the laser-molecule interaction are velocity map imaged to the detector and VMI images are recorded for varying phases between pulses. Two Abel-inverted images are shown for the different phases between pulses on the left (blue and red color coding). Bottom panel: Illustration of interfering pathways which lead to phase-dependent ionization yields.

Figure 2

Main panel: ${\mathrm{CH}}_2\mathrm{IBr}$ photoelectron yield as a function of photoelectron energy and phase at a 15 fs delay for pump and probe pulses with fixed energies (i.e., no intensity compensation). Lower panel: Lineouts for the two states of interest $D_1$ and $D_3$ as a function of phase, in purple and orange, respectively. Right panel: Lineouts as a function of energy normalized to the total number of detected electrons, i.e., photoelectron spectra for two different phases.

Figure 3

Potential energy curves and electronic configurations for relevant neutral and ionic states of ${\mathrm{CH}}_2\mathrm{IBr}$. Left panel: potential energy as a function of the ${\mathrm{CH}}_2$ wagging coordinate for neutral Rydberg states, $R_0$ (green), $R_1$ (purple), and $R_3$ (orange), and ionic states, $D_0$ (green), $D_1$ (purple), and $D_3$ (orange). Cartoon wave packets for the pump (black) and probe (white) are shown for a 15 fs pump-probe delay. Right panel: Electronic configurations for these same states. HOMO corresponds to highest occupied molecular orbital and LUMO corresponds to lowest unoccupied molecular orbital.

Figure 4

Depth of modulation ($M_D$) vs pump probe delay ($\tau$). The top panel shows results for ${\mathrm{CH}}_2\mathrm{IBr}$ and the bottom panel for ${\mathrm{CH}}_2\mathrm{BrCl}$. The data points of the $M_D$ were fit to an exponential decay $e^{-\gamma \tau }$. The coherence time is displayed in the legend.