The effect of resonant two-photon ionisation processes on the AC Stark-split spontaneous emission spectrum is determined using master equation methods.
For pt.I see ibid., vol.17, p.4135 (1984). Using a previously described new method to evaluate light-induced level shifts, it is experimentally demonstrated that the two possible intermediate 6D 2 states in four-photon resonant five-photon ionisation of mercury are mixed at high laser intensities. Due to this mixing, the AC Stark shifts of these levels are no longer quadratic in field amplitude. This is in good agreement with a simple theoretical model. This makes a direct determination of the oscillator strengths for two mercury lines possible.