Separation of vibrational and rotational coherences with polarized femtosecond time-resolved four-wave mixing spectroscopy

One of the degrees of freedom available to femtosecond time-resolved four-wave mixing spectroscopy is the geometry with which the polarization of the three independent lasers employed in this technique are oriented relative to one another. In the work presented here, different polarization geometries were used to demonstrate the selectivity that can be achieved with respect to the rotational and vibrational dynamics observed in a femtosecond time-resolved four-wave mixing transient. Two of the polarization geometries presented here allow for the simultaneous observation of rotational and vibrational coherences and furthermore illustrate the mechanism with which the rotational dynamics enter into the signal. A third 'magic angle' polarization geometry allows for the complete elimination of the rotational coherences from the signal, which makes full characterization of the vibrational dynamics in the four-wave mixing signal possible. This is shown for the dynamics in both an electronically excited and the electronic ground state of gaseous iodine, which serves as a model system. The results are discussed in a wave-packet picture which applies under the present experimental conditions where any damping mechanism can safely be neglected. Copyright (C) 2000 John Wiley & Sons, Ltd.