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Journal of Physics B Atomic and Molecular Physics

Two-photon ionisation of caesium

Constantine E. Theodosiou, L Armstrong

Published Feb 14, 1979• 14 citations
Publisher

Abstract

The authors study the time dependence of the two-photon ionisation rate for atomic caesium calculated using a sudden approximation. For pulse lengths similar to the ones used in the measurements by Granneman and Van de Wiel (1975) second-order perturbation theory gives results different from those obtained with the authors' model. Finally they utilise molecular data to resolve the discrepancy still outstanding between theory and experiment.

DOI
10.1088/0022-3700/12/3/004

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Journal of Physics B Atomic and Molecular Physics•Jul 11, 1975•43 citations
The authors study the time dependence of the two-photon ionisation rate for atomic caesium calculated using a sudden approximation. For pulse lengths similar to the ones used in the measurements by Granneman and Van de Wiel (1975) second-order perturbation theory gives results different from those obtained with the authors' model. Finally they utilise molecular data to resolve the discrepancy still outstanding between theory and experiment.
For pt.I see ibid., vol. 17, p.4643 (1984). The authors apply the theoretical results obtained in a previous paper in order to study the two-photon resonant ionisation of atomic caesium in the neighbourhood of the fine-structure doublet 7P 1/2 -7P 3/2 when the incident light is right-hand circularly polarised. This formalism allows one to consider these two resonant states on an equal footing and it includes the effects of the infinite summations involved in higher-order perturbation theory. The numerical results show that the two-photon resonant ionisation of caesium takes place through an anticrossing of levels, at least in the intensity range studied. The temporal behaviour of the probability exhibits two distinct temporal regimes: linear for long interaction times and cubic for the shorter ones. The intermediate regime is characterised by quantum beats. At high intensity values, it is found that the induced light shift of the resonances is time dependent. It is also shown that the contribution of the higher-order effects may explain the slight departures between the theoretical and experimental results in the off-resonant two-photon ionisation of atomic caesium.
By using a perturbative treatment based on the central field approximation, the authors have calculated multiphoton ionisation probabilities in several cases of experimental interest (two-, three- and four-photon ionisation of the caesium and potassium ground states at the doubled Nd glass laser frequency and one-, two- and three-photon ionisation of some s, p, d and caesium Rydberg states). Some of these probabilities have been calculated beyond the minimum non-vanishing order. They have also performed some calculations for hydrogen. The comparison of their results with previous ones obtained by analytical methods provides a good test of the numerical techniques involved in the calculations.
Orer a small range of light intensity the three-photon ionization rate of caesium atoms by ruby laser light decreases when the incident photon flux is increased. This result may be quantitatively explained by an induced energy shift of the 913 levels of the caesium atom proportional to the square of the light intensity. Thus perturbations of bound electron states by an intense light beam can be of importance in determining the multiphoton ionization rate. In this paper the functional dependence of the ionization rate on the photon flux is calculated for caesium, using a perturbation theory somewhat different from that usually employed.
Measurements are reported of two-photon ionization probabilities in atomic Cs at the nine wavelengths of the Ar ion laser (5145-4545 AA), the first step of the two-photon process occurring to intermediate levels in the vicinity of the 7 2 P 1/2,3/2 levels. Additional ionization due to molecular Cs was monitored by ion time-of-flight analysis and found to be negligible when choosing proper oven temperatures. An absolute scale for the two-photon rates is provided by normalization on a one-photon UV cross section. Comparison of our data with calculations based on quantum defect Coulomb wavefunctions shows discrepancies of up to four orders of magnitude.