Fluorescence-detected wave packet interferometry. II. Role of rotations and determination of the susceptibility

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Abstract
The recently developed technique of time‐resolved spectroscopy with phase‐locked optical pulse pairs is further explored with additional experimental data and more detailed comparison to theory. This spectroscopic method is sensitive to the overall phase evolution of an optically prepared nuclear wave packet. The phase locking scheme, demonstrated for the B←X transition of gas phase molecular iodine, is extended through the use of in‐quadrature locked pulses and by examination of the dispersed fluorescence signal. The excited state population following the interaction with both pulses is detected as the resultant two‐field‐dependent fluorescence emission from the B state. The observed signals have periodically recurring features that result from rovibrational wave packet dynamics of the molecule on the excited state electronic potential energy curve. Quantum interference effects cause the magnitude and sign of the periodic features to be strongly modulated. The two‐pulse phase‐locked interferograms are interpreted with first order time‐dependent perturbation theory. Excellent agreement is found between the experimental interferograms and those calculated from literature values of the parameters governing the electronic, vibrational and rotational structure of I2. A relationship between the phase‐locked interferograms and the time‐dependent linear susceptibility is obtained. The in‐phase and in‐quadrature phase‐locked interferograms together provide a complete record of the optical free induction decay. Thus by combining the in‐phase and in‐quadrature data, we obtain the contributions to both the absorptive and dispersive linear susceptibilities arising from transitions within the pulse spectrum.