Phase-dependent field-free molecular alignment and orientation

Abstract

We investigated the temporal behavior of alignment and orientation of LiH following a femtosecond laser pulse excitation comprising two fields at center frequencies $\ensuremath{\omega}$ and $2\ensuremath{\omega}$ (e.g., $E(t)=E[cos(\ensuremath{\omega}t)+cos(2\ensuremath{\omega}t+\ensuremath{\Phi})]$) shifted by a phase $\ensuremath{\Phi}$. The effects of repopulations and rephasing of rotational states on the resulting alignment and orientation were evaluated. The population distribution of rotational states is only changed during the exciting pulse. Afterwards the established rotational state distribution is maintained in the absence of collisions. The phases of rotational states play the most crucial role in determining the time evolution of molecular alignment and orientation. Equal alignment and rotational populations are obtained when the phases are chosen $\ensuremath{\Phi}=0$ and $\ensuremath{\Phi}=\ensuremath{\pi}$. However, orientation is different due to the fact that in the case $\ensuremath{\Phi}=\ensuremath{\pi}$ the mutual phases of even rotation states are not changed but the phases of odd rotational states are shifted by $\ensuremath{\pi}$, comparing with that of $\ensuremath{\Phi}=0$. The effect of temperature on molecular orientation was also addressed. It was shown that an efficient field-free molecular orientation can be observed even at room temperature.