High Degrees of Impulsive Alignment in Repetitively Excited $\mbox{N}_2$ at STP

\newcommand{\trace}[1]{\mbox{tr}\left( #1 \right)} We demonstrate a high degree of both transient and time-independent alignment in Nitrogen at STP resulting from multiple impulsive Raman excitations with linearly polarized light. The alignment is optimized by exploiting the structure of the density matrix, $\rho(J,m_J)$. Our experiment demonstrates a time-independent population alignment, defined as the time average of $\langle\cos^2\theta\rangle$, that exceeds the single pulse transient coherent alignment. We compare our experimental results to a quantum calculation, which suggests that transient alignment following multiple excitations can exceed $\langle\cos^2\theta\rangle\sim0.6$. Under impulsive excitation the entropy and quantum purity remain constant, but both the energy of the ensemble and the $J$-state distribution move markedly away from a thermal distribution. Transient alignment is related to rotational coherence $C_2 = \left(1 - \frac{ \trace{\mbox{diag}(\rho)^2}} { \trace{\rho^2} } \right)^{1/2}$. We show that this $C_2$ coherence grows monotonically with our train of eight impulses.