Rotation-Induced Breakdown of Torsional Quantum Control

Control of the torsional angles of nonrigid molecules is key for the development of emerging areas like molecular electronics and nanotechnology. Based on a rigorous calculation of the rotation-torsion-Stark energy levels of nonrigid biphenyl-like molecules, we show that, unlike previously believed, instantaneous rotation-torsion-Stark eigenstates of such molecules, interacting with a strong laser field, present a large degree of delocalization in the torsional coordinate even for the lowest energy states. This is due to a strong coupling between overall rotation and torsion leading to a breakdown of the torsional alignment. Thus, adiabatic control of changes on the planarity of this kind of molecule is essentially impossible unless the temperature is on the order of a few Kelvin.

Figure 1

In Case I, the average value of the square of the torsional function, as defined in Eq. (3), for the $M=0$, $A_{1g}^{+}$, $n=1$, and $n=3$ rotation-torsion-Stark levels and for two nonzero intensities of the laser field. For $I=2.2\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$, the same plot should be used for both levels.

Figure 2

Thermal average $⟨⟨\cos 4\gamma ⟩⟩$ as a function of the intensity of the laser field for $T=0.1\mathrm{K}$ (solid line) and $T=5\mathrm{K}$ (dotted line). The curves corresponding to Cases I, II, and III are plotted with circles, triangles, and squares, respectively. In the last case, only the portion of the curve corresponding to positive values is plotted.