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Laser Physics

Isotope-selective laser excitation and field ionization of thenF5/2Rydberg states in a thallium beam

Petr A. Bokhan, Nikolai V. Fateev, V A Kim, Dm. E. Zakrevsky

Published Apr 16, 2013• 1 citation
Publisher

Abstract

The effective isotope-selective excitation and field ionization of the 16F5/2 thallium Rydberg states have been studied experimentally. The 16F5/2 states were excited by the two-step scheme 6P1/2→6D3/2→nF5/2 in a dense collimated atomic beam by means of monochromatic radiation from two frequency tunable lasers (λ1 = 276.9 nm, λ2 = 773–808 nm) with a mean power density of more than 1 W.The largest selectivity of excitation in this scheme takes place in the first step 6P1/2→6D3/2. The experiments showed that the main reason for the limiting selectivity excitation of the Rydberg states is the field broadening of the absorption resonances at the first and second steps of the excitation scheme. The excitation efficiency of this scheme is considerably governed by the parameters of the laser radiation at the second step and an electric field pulse.

DOI
10.1088/1054-660x/23/5/055702

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The effective isotope-selective excitation and field ionization of the 16F5/2 thallium Rydberg states have been studied experimentally. The 16F5/2 states were excited by the two-step scheme 6P1/2→6D3/2→nF5/2 in a dense collimated atomic beam by means of monochromatic radiation from two frequency tunable lasers (λ1 = 276.9 nm, λ2 = 773–808 nm) with a mean power density of more than 1 W.The largest selectivity of excitation in this scheme takes place in the first step 6P1/2→6D3/2. The experiments showed that the main reason for the limiting selectivity excitation of the Rydberg states is the field broadening of the absorption resonances at the first and second steps of the excitation scheme. The excitation efficiency of this scheme is considerably governed by the parameters of the laser radiation at the second step and an electric field pulse.
Numerical computations of ionization efficiency and isotope selectivity of 176 L u have been carried out for the following three-step ladder type photoionization scheme, 5 d 6 s 2 2 D 3 / 2 ( 0.0 c m − 1 ) − − − ⟶ 540.4068 n m ⁡ 5 d 6 s 6 p 4 F 5 / 2 o ( 18504.58 c m − 1 ) − − − ⟶ 535.0626 n m 5 d 6 s 7 s 4 D 3 / 2 ( 37193.98 c m − 1 ) − − − ⟶ 618.0061 n m 53375 c m − 1 A u t o i o n i z a t i o n S t a t e → L u + , by invoking the density matrix formalism for the laser–atom interactions. Equations of motion have been derived for the odd isotopes by inclusion of hyperfine levels. The line shapes and their effect on the ionization efficiency and isotope selectivity have been discussed in detail. The effects of power of excitation and ionization lasers, spectral bandwidths, pulse delays, Doppler broadening of the atomic ensemble on the ionization efficiency, and degree of enrichment have been presented. Optimum conditions for the power of excitation, detunings, and other system parameters for the selective yet efficient photoionization of 176 L u have been identified.
The selectivity of three-step pulse photo-ionization of atoms versus the detuning delta of exciting radiation frequencies from intermediate resonance is investigated in powerful laser fields where the power broadening of the resonant transitions is at least of the order of isotope shifts. The Doppler broadening of the resonant transitions is taken into account and it is assumed that the sum frequency of both exciting laser radiations is tuned to the line centre of the two-photon transition of a selected isotope. It is shown that in this situation the photo-ionization selectivity increases so greatly with the absolute detuning mod delta mod that even in such strong fields the process of photo-ionization can be highly selective. The optimization of selective photo-ionization of atoms is discussed in connection with the decrease in the selective isotope yield with increase of the absolute detuning mod delta mod . Analytical consideration of selective three-step pulse photo-ionization in powerful laser fields, based on the density matrix formalism, is presented together with the results of numerical simulation of the 40 Ca- 48 Ca laser isotope separation process as an example.
Selective photoionization of ytterbium isotope is studied numerically based on a three-step photoionization scheme, 4f146s2 1S0 (0 cm−1) → 4f146s6p 3P1 (17992.008 cm−1) → (4f136s26p) (7/2, 3/2)2 (35196.98 cm−1) → auto-ionization state (52353 cm−1) → Yb+, by the density matrix theory with the consideration of atomic hyperfine structures and magnetic sublevels. To examine the physical model, the numerical isotopic abundance of ytterbium is compared with that from mass spectroscopy experiment, showing that they are in good agreement with each other. The excitation process and ionization process of ytterbium, especially for odd isotopes, are discussed and analyzed in detail on this basis. The effects of frequency detuning, power densities, spectral bandwidths, polarization of two excitation lasers, and atomic Doppler broadening on the total ionization yield and isotopic abundance are investigated numerically and the optimal excitation conditions for 176Yb enrichment are identified semi-quantitatively.
We have excited helium atoms from their metastable 2 3S{}^{3}\phantom{\rule{-0.16em}{0ex}}S3S state to Rydberg states in the range 13<n<5013<n<5013<n<50 in a two-step process via the 3 3P{}^{3}\phantom{\rule{-0.16em}{0ex}}P3P state using light at $\ensuremath{\lambda}=389$ nm and 785--815 nm. Atoms in a thermal beam (100 K) cross partially overlapping laser beams of the appropriate frequencies in the counterintuitive order to exploit the high efficiency of stimulated rapid adiabatic passage. The interaction region is between two plates that can be used for Stark tuning in a few V/cm field or for field ionization. At fields much too low for field ionization, we observe signals attributed to ionization by blackbody radiation. Multiple tests confirm this attribution as the cause of ionization. For example, by heating the plates we observe the expected signal increases. Our experiments reinforce previous work where the interaction between Rydberg atoms and room temperature blackbody radiation is important for experiments.
Isotopically selective resonance ionization of mercury atoms has been accomplished by three-step excitation via the 6 3P1 and 8 1S0 excited states. Three collinear beams, respectively 254, 286, and 532 nm, from two dye lasers and a pulsed neodymium:yttrium aluminum garnet (ND:YAG) pump laser, were passed through a closed mercury vapor cell containing electrodes to which the ions were drawn by an electric field. The ion current and the 254-nm fluorescence radiation were measured as a function of laser wavelengths. Hyperfine structures for the 254- and 286-nm transitions were observed.
Rydberg positronium (Ps) atoms have been prepared in selected Stark states via two-step (1s→2p→nd/ns) optical excitation. Two methods have been used to achieve Stark-state selection: a field ionization filter that transmits the outermost states with positive Stark shifts, and state-selected photoexcitation in a strong electric field. The former is demonstrated for n=17 and 18 while the latter is performed for n=11 in a homogeneous electric field of 1.9 kV/cm. The observed spectral intensities and their dependence on the polarization of the laser radiation are in agreement with calculations that include the perturbations of the intermediate n=2 manifold. Our results pave the way for the generation of Rydberg Ps atoms with large electric dipole moments that are required for the realization of schemes to control their motion using inhomogeneous electric fields, an essential feature of some proposed Ps free-fall measurements requiring focused beams of long-lived atoms.