Adiabatic theory of multiphoton ionisation. II. Resonant phenomena

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Abstract
For pt.I, see ibid., vol.20, no.11, p.2397 (1987). The adiabatic approach to the problem of multiphoton ionisation is extended to the case when the resonant phenomena are important. Mathematically this implies inclusion of new types of singularities (the poles) in the three-term recurrence relations defining the system quasi-energy states. The regimes of strong and weak coupling of the resonating states are discussed. It is shown that in a strong external field the quasi-energy state can be stabilised. The resonance with the weakly bound state (its role can also be played by the Rydberg series of levels) can lead to the substantial modification of the ejected electron energy spectrum. It decreases more slowly with energy and all peaks exhibit a similar dependence on the non-stationary field strength. These features are in qualitative agreement with recent experimental observations on the above-threshold ionisation.