Multiphoton transitions in a colored vacuum: coupling of the ac Stark shift with spontaneous decay and the Lamb shift

In free space, where the vacuum mode density varies relatively slowly in the spectral region of an atomic transition, an atom's real-field and vacuum interactions are effectively decoupled. Consequently, spontaneous decay and the Lamb shift are, for all practical purposes, independent of any real-field atomic perturbation. However, in a colored vacuum (i.e., a low-Q cavity) the vacuum mode density can change significantly in the vicinity of an atomic transition, so that a Stark shift will alter an atom's vacuum environment and thereby couple real-field and vacuum effects. Since the ac Stark shift is an inherent aspect of multiphoton processes, this coupling is unavoidable for highly nonlinear field-atom interactions that occur in cavities. Here we consider this effect for 3 + 2 resonance-enhanced multiphoton ionization of xenon. (C) 2002 Optical Society of America.