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A combined experimental and theoretical study on realizing and using laser controlled torsion of molecules
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Image of FIG. 1.
FIG. 1.

To illustrate the principle of laser-induced torsion and time-resolved deracemization we show the calculated kick pulse induced time-dependent torsional potential as a function of time measured with respect to the center of the pulse and dihedral angle between the Br- and F-phenyl planes. The asymmetry in the potential is obtained by orienting the molecules (here with the Br-phenyl plane out of the paper), 3D aligning them, and by polarizing the kick pulse at an angle of 13° with respect to the SMPA (see Fig. 2). The red dotted curve at large times illustrates the laser-free time-independent torsional potential. For the enantiomer, starting out with , the time varying potential induces an oscillatory motion (gray curve) corresponding to torsion confined within the initial well. By contrast, due to the induced asymmetry between the two wells, the initial enantiomer is traversing the central torsional barrier, and ends up as an enantiomer undergoing internal rotation (black curve). The kick pulse has an intensity of and a duration of 1.0 ps (FWHM). The torsional motion may be monitored by fs time-resolved Coulomb explosion imaging. The inset shows a model of the 3,5DFDBrBPh molecule with the sterogenic axis marked by red (gray).

Image of FIG. 2.
FIG. 2.

Model of 3,5DFDBrBPh along with the MF and the LF coordinate axes. The axis points into the plane of the paper. The dotted line indicates the second most polarizable axis (SMPA), which is located 11° from the Br-phenyl ring and 28° from the F-phenyl ring.

Image of FIG. 3.
FIG. 3.

The field-free torsional states in units of . The (scaled) torsional potential is indicated with a dashed line. The potential barriers are 76 and 87 meV. (a) The four first almost degenerate energy eigenstates lie 1.71 meV above the minimum of the torsional potential. From linear combinations of these we obtain the corresponding localized states shown in (b). The calculated energy differences are of the order , and hence the lifetimes of the localized states are tens of milliseconds.

Image of FIG. 4.
FIG. 4.

(a) Synthesis of 3,5DFDBrBPh by Pd-catalyzed cross-coupling. [(b)–(d)] NMR-spectra of synthetic 3,5DFDBrBPh. Integrated signals are indicated by green (gray) curves, and the corresponding numbers are situated directly below the respective curves.

Image of FIG. 5.
FIG. 5.

Schematic representation of the experimental setup, showing the vacuum system and the three laser beam paths. The Nd:YAG (alignment) pulses and the probe pulses are polarized vertically, i.e., perpendicular to the detector plane, whereas the fs kick pulse is polarized in the plane of the detector.

Image of FIG. 6.
FIG. 6.

(a) Ion images of and fragments at probe times . The ns pulse is polarized perpendicularly to the image (detector) plane and the , 0.7 ps (FWHM) kick pulse is polarized horizontally. (b) Angular distribution of the ions, at , obtained by radially integrating the corresponding ion image. The splitting of the pairwise peaks is twice the average angle, , between the ion recoil (and thus the F-phenyl plane) and the kick pulse polarization. (c) as a function of , for times where a clear four-peak structure is visible in the angular distributions. The curve is a fit of the sum of a linear and a harmonic function to the experimental points (squares).

Image of FIG. 7.
FIG. 7.

Comparison of the classical model for rotation at various intensities with the experiment of Ref. 33. The theory captures the laser-induced rotation within the first 12 ps, but generally overestimates the degree of angular confinement.

Image of FIG. 8.
FIG. 8.

Angular distributions of (a) F-phenyl and (b) Br-phenyl rings at , 1.47, and 2.47 ps. (c) Expectation value of the dihedral angle for a molecule starting out with the SMPA (see Fig. 2) aligned along the kick pulse polarization. The kick pulse is as in Fig. 6.

Image of FIG. 9.
FIG. 9.

Time evolution of the dihedral angle for a molecule starting out as (a) an or (b) an enantiomer. Initially, the molecule is 3D oriented with the Br-phenyl end pointing out of the paper and the SMPA aligned at an angle of 13° with respect to the kick pulse polarization. The kick pulse triggering the torsional motion has a peak intensity of and duration (FWHM) of 1.0 ps. The torsional potential is scaled down by 1/4 rather than by increasing the kick strength. In practice reduction of the torsional barrier is possible, for instance, by modifying the aromatic rings or by using halogen substituted biphenylacetylene (see also Fig. 1).


Generic image for table
Table I.

The table lists the relevant polarizability components, , of 3,5DFDBrBPh in the MF frame as a function of the dihedral angle, . The components are -periodic and fulfill , and . Also, .

Generic image for table
Table II.

Vibrational frequencies and relative Raman cross sections of the eight lowest lying normal modes of 3,5DFDBrBPh, computed at B3LYP/TZVPP level.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: A combined experimental and theoretical study on realizing and using laser controlled torsion of molecules