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-w ave mixing spectroscopy is the geometry with which the polarization of the three independent lasers employed in this technique are oriented relative t o one another. In the work presented here, different polarization geometrie s were used to demonstrate the selectivity that can be achieved with respec t to the rotational and vibrational dynamics observed in a femtosecond time -resolved four-wave mixing transient. Two of the polarization geometries pr esented here allow for the simultaneous observation of rotational and vibra tional coherences and furthermore illustrate the mechanism with which the r otational dynamics enter into the signal. A third 'magic angle' polarizatio n geometry allows for the complete elimination of the rotational coherences from the signal, which makes full characterization of the vibrational dyna mics in the four-wave mixing signal possible. This is shown for the dynamic s in both an electronically excited and the electronic ground state of gase ous 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 Jo hn Wiley & Sons, Ltd.