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Journal of Physics B Atomic and Molecular Physics

Two-photon ionization measurements on atomic caesium by means of an argon ion laser

Ernst H.A. Granneman, Marnix J. van der Wiel

Published Jul 11, 1975• 43 citations
Publisher

Abstract

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.

DOI
10.1088/0022-3700/8/10/014

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Journal of Physics B Atomic and Molecular Physics•Nov 11, 1976•20 citations
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.
Measurements are reported of the ratio of the ion count rates obtained in two-photon ionization of caesium atoms for linearly and circularly polarized light at the nine wavelengths of the argon-ion laser (514.5-454.5 nm). Comparison of this data with the recent theoretical values of Teague et al. (1976) and Flank et al. (1976) shows discrepancies at all photon energies except near the 7P resonance. In addition, upper limits are presented for the absolute two-photon rates in the rhodamine-6G wavelength region (563-635 nm).
The relative cross sections of the production of Cs + and Cs 2 + ions through the two-photon ionization of Cs 2 were measured in the 5300-6200 Å wavelength range. A linear polarized dye laser was operated in the CW mode as a light source. The results showed that (1) the cross sections for Cs + ions have a broad peak at around 5570 Å and the threshold at around 6010 Å, and (2) those for Cs 2 + ions have the largest peak at around 5940 Å and many structures in the wavelength range longer than it. Through consideration of the above results, we suggested that Cs + ions are produced not only by two-photon ionization of Cs 2 , but also by a participation of a third photon. The threshold for Cs + ions at around 6010 Å is concerned with the intermediate excited-state Cs 2 (C 1 ∏ u + (ν ′ )), which can be excited from the ground state X 1 Σ g + (ν ′′ =0).
An experiment is reported on two-step photoionization of Cs via the 6P 3/2 intermediate state using circularly polarized radiation from a GaAs laser for the first step (8521 AA) and from an Ar-ion laser for the second step. Values for the ratio of ion production rates for right- and left-handed circular polarization in the second step and for the spin polarization of the photoelectrons are presented. The measurements are interpreted on the basis of a calculation which takes into account the hyperfine splitting of the levels involved in the first transition. A description in terms of fine-structure splitting of the 6P state is shown to be totally inadequate, even when allowing for possibly large spin-orbit effects in the continuum. The process is evaluated as a possible source of a spin-polarized electron beam.
Very highly excited p states of Cs with principal quantum numbers 30<or=n<or=85 were produced by one-step photoexcitation with frequency-doubled light from a flash-lamp-pumped dye laser. Field ionization was employed for selective detection. Ionization occurs at field strengths E>or=E min with a sharp threshold. The relation E min (n*) varies as (n*) -4 was verified over a wide range. Two-photon excitation to a very high state via a virtual intermediate state leads to immediate ionization of most of the excited atoms; in this case the ion signal as a function of the light intensity I is not proportional to I k , k>or=2, whereas a direct two-photon transition into the continuum yields proportionality with I 2 .
This paper will review briefly a recent experiment which measured the photodetachment probability for I-ions due to simultaneous absorption of two ruby quanta. We have calculated the two-quantum photoionization rate for atomic cesium as a function of wavelength. The "transition" from the initial state to the bound intermediate state is expressed in terms of oscillator strengths. The second "transition," from these excited valence states into the coulomb continuum is expressed as a cross section. We have observed what appear to be genuine two-photon ionization signals in three cases: 1) near resonance in the10P_{3/2}state using the second-harmonic ofQ-switched ruby, 2) away from resonances, using the 5300 Å second-harmonic ofQ-switched neodymium laser, and 3) away from resonances using the first anti-Stokes line at 5750 Å from ruby-pumped D2gas. Present experiments aim at scanning the interesting range near resonance, e.g., by thermally tuning the ruby.
The authors have investigated the two-step process 6 2 S 1/2 to 6 2 P 3/2 to (epsilon 2 S 1/2 , epsilon 2 D 3/2 , epsilon 2 D 5/2 ) in Cs for circular polarization of the radiation in both steps. Theoretical expressions are presented for the photoelectron spin polarization P z and the ion count-rate 'asymmetry' A i (ratio of ion rates for right or left circular polarization in the second step), based on the hyperfine-coupling scheme for the first step. These expressions contain ratios of the three radial matrix elements involved in the ionizing step. The experimental determination of P z and A i , for different groups of hyperfine transitions, permits an evaluation of these ratios. The experiment was performed on a Cs atomic beam using a pulsed GaAs laser at 8521 AA for the first step and a pulsed Ar-ion laser at 5017-4545 AA for the second step. Effects of saturation in the first transition are studied and optimal conditions for use of the process in a spin-polarized electron source are discussed.