Dynamic Stark Control of Torsional Motion by a Pair of Laser Pulses

The torsional motion of a molecule composed of two substituted benzene rings, linked by a single bond, is coherently controlled by a pair of strong ($3\times 10^{13}\mathrm{W}{\mathrm{cm}}^{-2}$), nonresonant (800 nm) 200-fs-long laser pulses—both linearly polarized perpendicular to the single-bond axis. If the second pulse is sent at the time when the two benzene rings rotate toward (away from) each other the amplitude of the torsion is strongly enhanced (reduced). The torsional motion persists for more than 150 ps corresponding to approximately 120 torsional oscillations. Our calculations show that the key to control is the strong transient modification of the natural torsional potential by the laser-induced dynamic Stark effect.

Figure 1

(a) Schematic of the experiment showing the FBCBP molecule (atoms labeled by color), the dihedral angle ${\varphi }_d$, the detector with a raw ${\mathrm{F}}^{+}$ ion image, the two kick laser pulses (vertical polarization) that induce torsional motion and the probe laser pulse (horizontal polarization) that Coulomb explodes the molecules such that the instantaneous ${\varphi }_d$ value can be determined from the ${\mathrm{F}}^{+}$ and ${\mathrm{Br}}^{+}$ ion images. A third laser pulse (not shown) keeps the molecule aligned as illustrated. Details are given in the text. (b),(c) Covariance images showing the emission direction of ${\mathrm{F}}^{+}$ ions with respect to the ${\mathrm{Br}}^{+}$ ion recoil (vertical). (b) is recorded at $t=4.00\mathrm{ps}$ and is obtained from the ${\mathrm{F}}^{+}$ image on the detector in (a). (c) is obtained at $t=2.17\mathrm{ps}$.

Figure 2

Calculated time-dependent torsional potential (red line), $V$, and norm square of the wave packet (filled blue area) subject to the dynamic Stark shift from two, 200-fs-long kick laser pulses centered at time 0.00 ps and 1.22 ps. (a) Prior to the first kick pulse. (b) During the first kick pulse. (c) Maximal displacement after the first kick pulse. (d) Shortly before the second kick pulse. (e) During the second kick pulse. (f) Maximal displacement after the second kick pulse. Dotted black lines show the center of the initial wave function.

Figure 3

The average dihedral angle as a function of time. (a) A single kick pulse. (b) Two kick pulses separated by 1.22 ps. (c) Two kick pulses separated by 0.54 ps. Black dots: experimental results. Red lines: calculated results. Blue shapes: intensity profiles of the kick pulses.

Figure 4

The measured (black dots) and simulated (red lines) average dihedral angle as a function of time for two kick pulses separated by 1.22 ps. The inset in (c) shows the Fourier transform, $\mathcal{F}$, (arbitrary units) of $⟨{\varphi }_d⟩$ for the calculated results (red) in the interval 0–2500 ps and the experimental results (black) in the interval 0–20 ps.