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Journal of Physics B Atomic and Molecular Physics

Three-photon ionisation of potassium atoms near the 42P1/2,3/2resonances

Zdzisław Stryła, R. Parzyński

Published Mar 14, 1986• 4 citations
Publisher

Abstract

One-photon resonant three-photon ionisation of atomic potassium via the 4P 1/2,3/2 doublet is studied in experiments using one and two light beams. Dispersion curves of the ionisation efficiency and the ratio of the total-cross sections for ionisation using circularly and linearly polarised light are determined. The character of the dependences measured is found to be influenced strongly by the field broadening of the 4P 1/2,3/2 states due to the optical Autler-Townes effect.

DOI
10.1088/0022-3700/19/5/014

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One-photon resonant three-photon ionisation of atomic potassium via the 4P 1/2,3/2 doublet is studied in experiments using one and two light beams. Dispersion curves of the ionisation efficiency and the ratio of the total-cross sections for ionisation using circularly and linearly polarised light are determined. The character of the dependences measured is found to be influenced strongly by the field broadening of the 4P 1/2,3/2 states due to the optical Autler-Townes effect.
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.
Two-photon ionisation of atomic hydrogen in the ground state with linearly and circularly polarised light is considered. In the region above the one-photon ionisation threshold expressions for the two-photon ionisation transition matrix elements are obtained using the non-relativistic dipole approximation. The two-photon ionisation rate is calculated above and below the one-photon ionisation threshold. Above threshold the total cross section for two-photon ionisation has no resonances and diminishes more rapidly than the photoionisation cross section as the wavelength decreases.
We study the multiphoton ionization of potassium atoms in 800 nm and 400 nm femtosecond laser fields. In the 800 nm laser field, the potassium atom absorbs three photons and emits one electron via one photon resonance with the 4p intermediate state with the help of the ac-Stark shift. The resonance feature is clearly shown as an Autler–Townes (AT) splitting and is mapped out in the electron kinetic energy spectrum. In a 400 nm laser field, although one photon resonance is possible with the 5p state, no splitting is observed. The different transition amplitudes between 4s–4p and 4s–5p explain the observed results. Due to the AT effect, an unexpected peak in the photoelectron energy spectrum that violates the dipole transition rule is observed. A preliminary explanation involving the spin-orbit interaction in the p state is given to account for this component. The observed AT-splitting in the electron kinetic energy distribution can be used as an effective method to calibrate the intensity of a laser field.
In the two-photon ionisation of Cs atoms resonances may be expected which correspond to a quadrupole 6s-nd transition associated with the first photon. At the 6D 5/2 and 6D 3/2 resonances the ratio of photoionisation cross sections for circularly polarised and for linearly polarised light (ion asymmetry) and the spin polarisation of the photoelectrons ejected by circularly polarised light have been measured. The ratios of the radial transition matrix elements from the 6D to the epsilon P and epsilon F continuum states were derived from the experimental results.