Rotational coherence phenomena

The development of seeded, supersonic molecular-beam techniques [1] has made it possible to study dynamical phenomena that would be very difficult to study otherwise. The time-domain manifestations of the free rotational motion of large molecular species subsequent to pulsed excitation comprise one such phenomenon [2, 3]. Both the vibrational and rotational cooling that pertain to a seeded-beam sample render the observation of rotational dynamics in the time domain much easier in these samples than in higher temperature gases. The vibrational cooling allows for the spectral isolation of the rotational bands associated with different vibronic (or vibrational) resonances. These bands can be individually excited by ultrafast laser pulses, with the ensuing dynamics due only to rotation. The rotational cooling significantly limits the range of rotational states that can contribute to the dynamics (though many states may still contribute for large species). The effect of this is primarily to slow down the rotational dynamics such that they can be readily observed with picosecond resolution.