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Journal of Physics B Atomic and Molecular Physics

Integrity of dressed states in multiphoton ionisation

Piers Coleman, P. L. Knight

Published Jul 14, 1981• 26 citations
Publisher

Abstract

The dressed eigenstates of an atom interacting with a near-resonant radiation field are used to study the dynamics and final-state photoelectron spectra of resonant two-photon ionisation for arbitrary initial conditions and detunings. The authors find that under some circumstances (and particularly for intense fields) the dressed states preserve their integrity whilst decaying as distinguishable states, leading to AC Stark-split photoelectron energy spectra. In general, however, there exists an off-diagonal coupling between dressed states via the continuum so that each dressed state cannot decay independently of the existence of the other. This can modify lowest order expectations, particularly for weak fields and can entirely alter final-state spectra.

DOI
10.1088/0022-3700/14/13/012

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Journal of Physics B Atomic and Molecular Physics•Dec 28, 1983•2 citations
The dressed eigenstates of an atom interacting with a near-resonant radiation field are used to study the dynamics and final-state photoelectron spectra of resonant two-photon ionisation for arbitrary initial conditions and detunings. The authors find that under some circumstances (and particularly for intense fields) the dressed states preserve their integrity whilst decaying as distinguishable states, leading to AC Stark-split photoelectron energy spectra. In general, however, there exists an off-diagonal coupling between dressed states via the continuum so that each dressed state cannot decay independently of the existence of the other. This can modify lowest order expectations, particularly for weak fields and can entirely alter final-state spectra.
The effects of saturation of a bound-bound atomic resonance on bound-continuum decays in investigated using the dressed state concept. Two special cases are discussed: the modification of spontaneous radiative decay by coupling to a third bound atomic state, and the changes in single-photon photoionisation when the initial discrete state is coupled by an external field to another bound state. For strong external fields Rabi oscillations and dynamic Stark splittings profoundly alter the 'decay'; but for weak fields the alterations at line centre are no less dramatic.
Perturbation theory of multiphoton ionisation due to a classical electromagnetic field is modified to allow for intermediate resonances with bound states. Complex energies, generally associated with resonances, do not enter into this formalism. For a monochromatic field of frequency u, a constant ionisation rate can then be defined unambiguously and only such continuum states are excited which correspond to the absorption of energy in integral multiples of hu. As an application, differential and total cross sections for the two-photon ionisation of hydrogen, for frequencies below the n = 3 resonance region, are obtained in closed form. Existing data for generalised cross sections, calculated numerically using the complex coordinate method, are in good agreement with the present results. Finally, the mean fractional ionisation resulting from a pulse of finite duration is estimated on the basis of the associated power spectrum. For short pulses, the time dependence of ionisation exhibits a departure from that expected of a time-independent rate.
Complex dressed states and photoelectron spectra are considered for a model system containing two discrete states that are coupled by separate lasers to a continuum that supports a resonance such as an autoionizing state. Spontaneous radiative decay from the structured continuum has been included and its effects are examined. It is shown that population can be trapped within the model atom even when radiative decay is included provided (1) the two Fano q values which characterize the discrete-state--continuum interactions are equal, and (2) the two-photon resonance condition is satisfied.
Non-perturbative completely analytic calculations of the spin polarisation of photoelectrons produced in resonant two-photon ionisation of alkali atoms with exciting and ionising light beams are presented. A well justified alkali atom model composed of four discrete states and the continuum is applied. Contrary to simpler models it enabled one to obtain results valid when the effective resonance Rabi frequency exceeds the frequency separation between the two components forming a given fine-structure energy doublet at resonance. It is shown that in this limit the photoelectron polarisation depends critically on whether a light or heavy alkali atom is ionised. In the former case the photoelectron polarisation is proved to be a copy of the electronic polarisation of the atom, whereas in the case of heavier alkalis it departs appreciably from that of the ionised atom due entirely to spin-orbit perturbation of radial wavefunctions of P discrete states. It is also shown how the author's present results go over into those previously obtained for weaker exciting fields.