Observation of rotational revivals for iodine molecules in helium droplets using a near-adiabatic laser pulse

A160-ps near-Gaussian, linearly polarized laser pulse is used to align iodine (I-2) molecules embedded in helium nanodroplets. The rise time of the laser pulse is sufficiently long and smooth that the alignment, characterized by < cos(2) theta(2D)>, behaves adiabatically during the pulse turnon. However, after the laser pulse has turned off < cos(2) theta(2D)> stays above 0.50 and a recurrence structure occurs similar to 650 ps later. Measurements on isolated (I-2) molecules with identical laser pulses are used to identify, through analysis of the observed half-and full-rotational revivals, that the nonadiabatic postpulse alignment dynamics results from a mild truncation of the trailing edge of the laser pulse. This truncation establishes a well-defined starting time for coherent rotation, which leads to the revival structures observed both for isolated molecules and molecules in He droplets. In the latter case the time-dependent < cos(2) theta(2D)> trace recorded here is compared to that obtained previously for a 450-fs alignment pulse. It is found that the observed revivals are very similar.