Line shapes in coherent ion dip spectroscopy of polyatomic molecules

Using two narrow-band Fourier-transform limited, tunable ns laser pulses the observation of coherent adiabatic Raman processes becomes feasible for polyatomic molecules. Recent experiments reveal the possibility of an effective suppression of ionization in an ion dip experiment using a lambdatype resonant coherent optical-optical UV-double-resonance process and suggest its application in molecular spectroscopy. The experiments, including the observed ionization enhancement for off-resonance conditions, are quantitatively described using a few-level density matrix approach.