LARGE COLLECTIVE LAMB SHIFTS IN HIGH-PRESSURE NOBLE-GASES

When three-photon atomic excitation is produced by two laser beams of angular frequencies omega-L1 and omega-L2, which are crossed at an angle theta, large co-operative pressure dependent shifts can be produced in the peak position of the excitation lineshape. By studying an experimental situation in xenon the theoretical predictions are shown to be in precise agreement with experiment for several angles and a wide range of pressures. The possibility of observing related shifts in nonlinear processes (such as backward stimulated hyper-Raman generation) is discussed.