Quantum ignition of intramolecular rotation by means of IR + UV laser pulses

Quantum ignition of intramolecular rotation may be achieved as follows: first, a few-cycle infrared (IR) laser pulse excites the torsional vibration in an oriented molecule. Subsequently, a well-timed ultrashort ultraviolet (UV) laser pulse induces a Franck–Condon type transition from the electronic ground to the excited state with approximate conservation of the intramolecular angular momentum. As a consequence, the torsional motion is converted into a unidirectional intramolecular rotation, with high angular momentum (≈100ℏ). The mechanism is demonstrated by means of representative laser driven wave packets which are propagated on ab initio potential energy curves of the model system (4-methyl-cyclohexylidene)fluoromethane.