Rotational decoherence in a dense gas of multiply kicked N$_2$

\newcommand{\expect}[1]{\left\langle #1 \right\rangle} We use a recently developed multiple-kick scheme to study relaxation and dephasing in rotational wavepackets. When properly timed, multiple pulses can produce substantial coherent alignment as well as significant Raman redistribution. Transient peaks in $\expect{\cos^2\theta}$ reflect coherent alignment while a time-average of $\expect{\cos^2\theta} > 1/3$ reflects Raman redistribution. We observe both features in nitrogen gas at 300K and 1 ATM following impulsive Raman excitation by a train of up to eight, 50 fs, 800 nm laser pulses. Using a quantum calculation to help disentangle population relaxation from phase decoherence [1], we experimentally investigate rotational decoherence in the context of $J$-changing and $M$-changing collisions as a function of both rotational energy and gas density. [1] S. Ramakrishna and T. Seideman, Phys.~Rev.~Lett.~{\bf 95}, 113001 (2005).