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Quantum Electronics

Effect of amplified spontaneous emission on selectivity of laser photoionisation of the177Lu radioisotope

Aleksei B D'yachkov, Aleksei A. Gorkunov, Anton V. Labozin, Sergei M. Mironov, Vladislav Ya. Panchenko, Valerii A Firsov, Gleb O. Tsvetkov

Published Jun 6, 2016• 12 citations
Publisher

Abstract

A significant deselecting effect of amplified spontaneous emission has been observed in the experiments on selective laser photoionisation of the radioisotope according to the scheme autoionisation state . The effect is conditioned by involvement of non-target isotopes from the lower metastable level into the ionisation process. Spectral filtering of spontaneous emission has allowed us to significantly increase the selectivity of the photoionisation process of the radioisotope and to attain a selectivity value of when using saturating light intensities.

DOI
10.1070/qel16021

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A significant deselecting effect of amplified spontaneous emission has been observed in the experiments on selective laser photoionisation of the radioisotope according to the scheme autoionisation state . The effect is conditioned by involvement of non-target isotopes from the lower metastable level into the ionisation process. Spectral filtering of spontaneous emission has allowed us to significantly increase the selectivity of the photoionisation process of the radioisotope and to attain a selectivity value of when using saturating light intensities.
A theory is developed of multistage isotopically selective ionisation of ytterbium ions. This theory is based on the rate equations for the level populations and on the transfer equation for the laser line intensities. An analysis of the time characteristics of multistage photoionisation is used to find the optimal laser radiation intensities for different ionisation stages. A multilevel model of multistage photoionisation developed for a natural mixture of isotopes is used to identify the ranges of wavelengths where the photoionisation selectivity is the highest. A mechanism of degradation of a short but powerful laser pulse is considered and the length of an efficiently excited medium is found.
Atoms in an autoionisation state can be excited to higher states in the continuum as well as performing spontaneous transitions to bound states by laser-induced autoionisation. Both effects are investigated by means of the reaction matrix. Results obtained by Armstrong et al (1978) using the same method are extended. The shape of the photoelectron spectrum is different from that of the photon spectrum emitted from the autoionisation state but is similar to that of the photon spectrum emitted from the same bound state which is excited to the ionisation states in the former case. It is predicted that the shape of the fluorescence yield with the pump laser detuning is Lorentzian. The multiphoton process contributes to the photoelectron spectrum in a complicated way.
Strong laser-field-induced autoionisation in the presence of both photoionising and radiative decay of the autoionising state (AS) is investigated, focusing on the laser intensity dependence of the photoemission and photoelectron spectra. In contrast to previous predictions, power broadening and increasing reduction of the doublet peak heights with field strength are found in the photoemission spectrum. Similar effects leading to considerable suppression and even complete disappearance of the lowest-order peaks in the photoelectron spectrum, together with peak switching, are also demonstrated, which are closely related to above-threshold ionisation. In addition, it is suggested that the total number of energetic photoelectrons may serve as an alternative to measuring the atomic parameters of the AS. All these effects are attributed to the presence of the strong 'probe': laser-induced decay of the AS.
State- and isotope-selective perpendicular excitation of a thermal energy, mixed metastable Ne(2p53s 3P2, 3P0) atom beam with a polarised 640.2 nm laser beam yields a well defined sample of 20Ne(2p53p 3D3) atoms, which are ionised by UV photons (wavelength range 331-412 nm). Core-switching processes are found to be negligible: autoionisation features in the region between the two Ne+(2P3/2,1/2) fine-structure thresholds and photoelectrons corresponding to Ne+(2P1/2) ions could not be detected. The application of these results for the production of state-selected, polarised Ne+(2p5 2P3/2) ions is briefly discussed.
To extend the capabilities of the existing spectroscopic methods of investigation and diagnostics of gaseous media, we consider a macroscopic effect of an increase in the rate of spontaneous emission, which results in the transition of a considerable part of excited molecules to lower-lying optically accessible states with light pulse emission along the direction of propagation of the exciting laser radiation. As an application of the effect of amplified spontaneous emission, we propose the population with its help of optically inaccessible electronic states of atoms and molecules.
Mathematical simulation methods are used to optimise of three-stage isotopically selective photoionisation of atoms of the ytterbium-168 isotope. The influence of nonlinear deformation of laser beams is taken into account. It is shown that exact tuning of the laser radiation frequencies to the frequencies of the lines of radiative transitions in the ytterbium-168 atom results in absorption of the laser radiation by atoms of other isotopes and in self-focusing of the laser beams. Introduction of a two-photon detuning reduces considerably the influence of the absorption and self-focusing. A determination is reported of the two-photon detuning, and of the dimensions and the spatial distribution of the power in the laser beam required to ensure that the absorption and nonlinear self-focusing of laser beams do not reduce significantly the ionisation efficiency.