Nuclear spin blockade of laser ignition of intramolecular rotation in the model boron rotor B 13 + 11 .

The boron rotor B 13 + 11 consists of a tri-atomic inner "wheel" that may rotate in its pseudo-rotating ten-atomic outer "bearing"-this concerted motion is called "contorsion." B 13 + 11 in its ground state has zero contorsional angular momentum. Starting from this initial state, it is a challenge to ignite contorsion by a laser pulse. We discover, however, that this is impossible, i.e., one cannot design any laser pulse that induces a transition from the ground to excited states with non-zero contorsional angular momentum. The reason is that the ground state is characterized by a specific combination of irreducible representations (IRREPs) of its contorsional and nuclear spin wavefunctions. Laser pulses conserve these IRREPs because hypothetical changes of the IRREPs would require nuclear spin flips that cannot be realized during the interaction with the laser pulse. We show that all excited target states of B 13 + 11 with non-zero contorsional angular momentum have different IRREPs that are inaccessible by laser pulses. Conservation of nuclear spins thus prohibits laser-induced transitions from the non-rotating ground to rotating target states. We discover various additional constraints imposed by conservation of nuclear spins, e.g., laser pulses can change clockwise to counter-clockwise contorsions or vice versa, but they cannot stop them. The results are derived in the frame of a simple model.

Laser-Controlled Torsions: Four-Dimensional Theory and the Validity of Reduced Dimensionality Models.

A multitude of possible applications along with unique coherence, chirality, and symmetry properties makes the control of molecular torsion with moderately strong, nonresonant laser pulses a fascinating subject. A description of combined rotation and torsion requires at least four angular degrees of freedom, which is challenging for the majority of systems. Lower-dimensional models have been proposed but also questioned. Here, we develop a four-dimensional model for the coupled rotational-torsional motions of molecules consisting of two identical moieties. By comparing four-dimensional calculations with a two-dimensional model, we define conditions under which the lower-dimensional model is valid. In particular, we point to the crucial role of coordinate dependence of the polarizability tensor. Our results do not agree with those of previous four-dimensional calculations but support the conclusions of recent experiments.

Nuclear spin selective alignment of ethylene and analogues.

We investigate the alignment of ethylene and of some of its analogues via short, non-resonant laser pulses and show that it depends crucially on the nuclear spin of the molecules. We calculate the time-dependent alignment factors of the four nuclear spin isomers of ethylene and analyze them by comparison with the symmetric top molecule allene. Moreover, we explore how the nuclear spin selective alignment depends on the asymmetry of the molecules and on the intensity of the laser pulse. As an application, we discuss how nuclear spin selective alignment could be applied in order to separate different isotopomers of ethylene.

Quantum-dynamical consequences of the permutation symmetry of methyl groups.

We identify the nuclear spin isomers of nitromethane (CH(3)NO(2)) and discuss how symmetry arguments may be used to determine the spectrum and eigenfunctions of the spatial and spin-dependent Hamiltonians in an elegant way. Furthermore, we explore the effect of nuclear spin on the dynamics of the methyl group induced by a time-dependent magnetic field. We demonstrate that dipolar interactions between the protons can give rise to rotation of the methyl group and show within a one-dimensional model and first order time-dependent perturbation theory that the induced motion is unidirectional and nuclear-spin selective.