Optimal orientation of an asymmetric top molecule with terahertz pulses

Terahertz pulses effects are investigated in an asymmetric top C_2v molecule using numerical simulations. The average value of the direction cosine Φ_Zx is computed solving the time dependent Schrödinger equation for several types of pulses. The H₂S molecule taken as a test case is first subject to two short terahertz pulses with a duration smaller than 5 ps, an identical maximum value of the electric field of 2 MV/cm, but a different shape. The thermal average ⟨⟨Φ_Zx⟩⟩ is calculated for several temperatures, and non-periodic time variations are found even for the lowest temperature. For a given temperature, the maximum orientation achieved is shown to be dependent on the overlap between the absorption spectrum of the molecule and the Fourier transform of the pulse. The maximum orientation is also shown to be closely related to the molecular energy increase. In a second step, the optimal control theory is used to build a 14 ps long few-cycle pulse with the same maximum value of the electric field allowing us to reach a large maximum value of ⟨Φ_Zx⟩ equal to 0.93. A fairly good understanding of the wavefunction describing the molecule after the pulse was achieved.