COHERENT ION DIP SPECTROSCOPY OF THE GROUND-STATE BENZENE-AR COMPLEX - VIBRATION-ROTATION LEVELS UP TO 130 CM(-1) INTERMOLECULAR ENERGY

Coherent ion dip spectroscopy (CIS) provides high sensitivity and high resolution; for the investigation of vibrational overtones in molecul ar electronic ground states. For a special time sequence of two cohere nt narrow-band Fourier transform limited nanosecond UV light pulses, w ith a modest delay of the pump pulse of 6.4 ns, a complete blocking of the population transfer to the upper state is achieved in the lambda- type three-level system ion dip experiment. This leads to ion dips wit h a depth as large as 95% and each dip represents an individual rovibr onic transition. In this work, CIS is applied for the first time to a weakly bound van der Waals complex, benzene-Ar. We are able to observe six new van der Waals vibrational states up to an excess energy of 13 0 cm(-1). The assignments are made by comparison with recent S-1 excit ed state data of benzene-Ar and p-difluorobenzene-Ar and by analysis o f the positions and intensities of the observed individual rotational lines. The frequency positions of the intermolecular vibrational state s display a regular pattern up to 130 cm(-1). (C) 1995 American Instit ute of Physics.