Spinning a pseudorotating molecular top by means of a circularly polarized infrared laser pulse: Quantum simulations for 114CdH2

A circularly polarized infrared (IR) laser pulse which propagates along the axis of a pre-aligned linear polyatomic molecule may excite unidirectional pseudorotations corresponding to excitation of the molecular bend and rotation of the bent molecule around that axis. This preparation and spinning a molecular top is, in part, analogous to the generation of unidirectional electron circulation in molecules by means of circularly polarized ultraviolet (UV) laser pulses [I. Barth, J. Manz, Angew. Chem. 118 (2006) 3028; Angew. Chem., Int. Ed. 45 (2006) 2962], allowing to apply similar methods for the laser driven wavepacket dynamics. It is demonstrated here by means of quantum simulations of the laser driven wavefunction Ψ(t) of the model system 114CdH2(X1Σg+), where Ψ(t) is expanded in terms of vibrational eigenfunctions Ψvl which are labeled by quantum numbers v,l=(vs=0,vbl,va=0) for the vibrations (v) including the symmetric stretch (s), bend (b), antisymmetric stretch (a), and for the pseudorotation (l). The corresponding vibrational and pseudorotational eigenenergies and matrix elements for the model 114CdH2 are adapted as approximations from accurate values which have been determined previously, for simulations of high resolution IR spectra. These dipole matrix elements and related selection or propensity rules imply dominant ladder climbing vbl=00→11→22→⋯ or 00→1-1→2-2→⋯, i.e., sequential populations of the states with increasing quantum numbers for the bends and pseudorotations vbl, where l=+vb or l=-vb depending on the right (+) or left (-) circular polarization of the IR laser pulse, respectively.