Field-free alignment in repetitively kicked nitrogen gas

Abstract

We demonstrate a high level of laser-induced transient alignment in room temperature and density ${\text{N}}_{2}$ with a technique that avoids laser field ionization. Our measured alignment shows an improvement over previous one-pulse or two-pulse experimental alignment results and approaches the theoretical maximum value. We employ eight equally spaced ultrafast laser pulses with a separation that takes advantage of the periodic revivals for the ensemble of quantum rotors. Each successive pulse increases the transient alignment $[⟨{\text{cos}}^{2}\text{ }\ensuremath{\theta}⟩(t)]$ and also moves the rotational population away from thermal equilibrium. These measurements are combined with simulations to determine the value of $⟨{\text{cos}}^{2}\text{ }\ensuremath{\theta}⟩$, the $J$-state distributions, and the functional dependencies of the alignment features.