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Journal of Physics B Atomic and Molecular Physics

Multiphoton absorptions above the ionisation threshold

Yves Gontier, Michel Poirier, M. Trahin

Published Apr 14, 1980• 63 citations
Publisher

Abstract

It is shown that the absorption of photons by electrons above the ionisation threshold can take place through one- and two-step processes. These competing mechanisms show quite different features. Diagnostic methods based on the intensity and pressure dependences and on the magnitude and the angular distribution of the probabilities are proposed.

DOI
10.1088/0022-3700/13/7/013

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Near-threshold behaviour of multiphoton ionisation probabilities

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Journal of Physics B Atomic Molecular and Optical Physics•Oct 14, 1989•12 citations
It is shown that the absorption of photons by electrons above the ionisation threshold can take place through one- and two-step processes. These competing mechanisms show quite different features. Diagnostic methods based on the intensity and pressure dependences and on the magnitude and the angular distribution of the probabilities are proposed.
Previous results for the shift of the ionisation potential of an atom in a laser field are used to investigate the kinematics of multiphoton ionisation in either a laser beam or a laser pulse. Previous results for a laser beam show essentially no shifts of the 'above-threshold ionisation' peaks in the electron energy distribution and only small widths. Results for a laser pulse show the possibility of large shifts and widths growing as the pulse becomes shorter. Electrons which absorb a larger number of photons are predicted to show less shifted and narrower peaks.
In a recent experiment involving multiphoton ionisation of Xe it was found that the photoelectron spectrum contained peaks which corresponded to absorption of more than the minimum number of photons required ionise. The effective orders of nonlinearity, furthermore, showed a striking uniformity. The authors investigated these effects with a model approximation consisting of a single bound state and m continua. Simple analytic expressions are obtained for the quantity measured in the experiment and the limit m to infinity is obtained. The results show good qualitative agreement with experiment.
The general expression for the amplitude of the probability of ionisation by a multiphoton absorption process is derived. Its non-relativistic limit is taken and the bipolar approximation is used for calculating the ionisation cross-section of hydrogen-like atoms. This latter involves the summation over intermediate virtual states by means of: a) a recursion relationship concerning angular functions, b) a particular technique which when applied to radial functions makes it possible to solve a system of inhomogeneous first-order differential equations. (authors) [French] On etablit l'expression generale de l'amplitude de probabilite d'ionisation par un processus d'absorption multiphotonique. On en prend la limite non-relativiste et l'on utilise l'approximation dipolaire avant de calculer la section efficace d'ionisation d'atomes hydrogenoides. Cette derniere fait intervenir des sommations sur des etats virtuels intermediaires effectuees a l'aide: a) d'une relation de recurrence qui concerne les fonctions angulaires, b) d'une technique particuliere qui, appliquee aux fonctions radiales, conduit a resoudre un systeme d'equations differentielles inhomogenes du premier ordre. (auteur)
The near-threshold behaviour of four-photon ionisation is studied in an energy region where the third laser photon crosses the Rydberg threshold. Particular emphasis is given to the role played by the Stark shift (raising of the ionisation potential) in a Gaussian laser pulse. As a function of the laser frequency the ionisation probability shows (i) a region where individual Rydberg resonances are well resolved, followed by (ii) a smoothly decreasing ionisation probability in the bound-state region, and (iii) an almost energy-independent above-threshold ionisation signal.