Controlling ground-state rotational dynamics of molecules by shaped femtosecond laser pulses

We report controlled excitation of ground-state rotational wave packet by pulse-shaping technique. The experiment is conducted in nitrogen (N₂) at room temperature and atmospheric pressure. A femtosecond laser pulse produces rotational coherences in the vibronic ground state of N₂ through an impulsive Raman process. The laser pulse is tailored using a spatial light modulator producing spectral phase modulation. Periodic phase steps are applied in order to control the excitation of specific rotational Raman transitions. The outcome is the modification of the relative excitation between odd and even rotational states which allows the control of the symmetry and rephasing period of the wave packet.