Dynamics of Polar Polarizable Rotors acted upon by Unipolar Electromagnetic Pulses: From the Sudden to the Adiabatic Regime

Populations of rotational states of a quantum pendulum initially in its ground state, emparted by ultrashort excitations for different values of the two kick strength parameters.

Marjan Mirahmadi, Burkhard Schmidt, Mallikarjun Karra, and Bretislav Friedrich

We study, analytically as well as numerically, the dynamics that arises from the interaction of a polar polarizable rigid rotor with single unipolar electromagnetic pulses of varying length, Δτ, with respect to the rotational period of the rotor, τr. In the sudden, non-adiabatic limit, Δτ < τr, we derive analytic expressions for the rotor's wavefunctions, kinetic energies, and field-free evolution of orientation and alignment. We verify the analytic results by solving the corresponding time-dependent Schrödinger equation numerically and extend the temporal range of the interactions considered all the way to the adiabatic limit, Δτ > τr, where general analytic solutions beyond the field-free case are no longer available. The effects of the orienting and aligning interactions as well as of their combination on the post-pulse populations of the rotational states are visualized as functions of the orienting and aligning kick-strengths in terms of populations quilts. Quantum carpets that encapsulate the evolution of the rotational wavepackets provide the space-time portraits of the resulting dynamics. The population quilts and quantum carpets reveal that purely orienting, purely aligning, or even-break combined interactions each exhibit a sui generis dynamics. In the intermediate temporal regime, we find that the wavepackets as functions of the orienting and aligning kick-strengths show resonances that correspond to diminished kinetic energies at particular values of the pulse duration.

J. Chem. Phys. 149 (17), 174109 (2018)