Pressure effects in the multiphoton ionization of atoms and molecules

Thirty years ago R.H. Dicke wrote ''In the usual treatment of spontaneous radiation by a gas, the radiation process is calculated as though the separate molecules radiate independently of each other. This simplified picture overlooks the fact that all the molecules are interacting with a common radiation field and hence cannot be treated as independent. The model is wrong in principle and many of the results obtained from it are incorrect''. Many subsequent results of magnetic resonance spectroscopy, and more recently of laser interactions have shown that Dicke stated a fundamental truth and that a collective treatment of molecules in a radiation field is often necessary. Notable examples in nonlinear optics include third harmonic generation and coherent antistokes Raman spectroscopy. We would like to present here another experimental effect which we feel is best explained by treating non-interacting molecules as a collective system. This effect is the quenching of the AC Stark shift upon an increase in the partial pressure of the molecule interacting with the radiation field. 15 references, 5 figures