Field-free permanent alignment of ionized nitrogen molecules

Abstract

Ionization occurs when strong femtosecond laser pulses propagate in nitrogen gas. The ionized nitrogen molecules are in ground electronic state as well as excited electronic states. In this paper, we study the polarization distribution of the fluorescence emission correspoding to the transition of N2 + (B 2 Σu + → X 2 Σg + ). It is found that the fluorescence polarization exihibits an anisotropic angular distribution relative to the pump laser polarization, which can be attributed to the permanent alignment caused by angulardependent ionization of neutral nitrogen molecules in strong femtosecond laser fields. The permanent alignment charactireizd by reaches 3/7 in our experiment where θ is the angle between the molecular axis and the pump laser polarization.