Real-time observation of ultrafast molecular rotation in weakly bound dimers

Abstract

The fragmentation dynamics of dicationic dimers of acetylene molecules initiated upon strong-field laser ionization is studied. Time-resolved pump-probe experiments with femtosecond laser pulses, accompanied by ab initio dynamical calculations, allow us to evaluate the detailed behavior of molecular ions during the dissociation process. The dynamical properties of the intermediate C_{2}H_{2}^{+}⋯C_{2}H_{2}^{+} state created by the pump pulse are probed by a second pulse which causes further ionization. The time-dependent yield of a coincident C_{2}H_{2}^{+} + C_{2}H_{2}^{2+} ion pair exhibits an oscillation feature with a periodicity of 240 ± 30 fs. Our studies demonstrate that this is caused by an ultrafast rotation of C_{2}H_{2}^{+} cations driven by intermolecular Coulomb force together with the orientation-dependent ionization rate. We suggest that the present observation of ultrafast molecular rotation can be a general phenomenon occurring in a wide variety of systems.