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Physica Scripta

Two-step photoionization of barium atoms through excited P 1 1 , 3 levels in optical dense approximation

Н. И. Косарев

Published Dec 19, 2022• 0 citations
Publisher

Abstract

Abstract The kinetics of two-step photoionization of barium atoms through excited P 1 1 , 3 levels was studied by numerical simulation method in the optically dense approximation. The first laser at wavelength λ 1 = 553.5 nm ( λ 1 = 791 nm) pumps transition S 0 1 ↔ P 1 1 ( S 0 1 ↔ P 1 3 ). The second laser with λ 2 = 266 nm ( λ 2 = 310 nm) ionizes atoms from the level P 1 1 ( P 1 3 ). The third probe laser with wavelength λ i = 493.41 nm causes resonant fluorescence of barium ions. It is shown that the metastable D 2 3 from the D 1 , 2 , 3 3 triplet in barium is strongly populated through P 1 <

DOI
10.1088/1402-4896/acaa0f

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AIP conference proceedings•Jan 1, 1995•1 citation
Abstract The kinetics of two-step photoionization of barium atoms through excited P 1 1 , 3 levels was studied by numerical simulation method in the optically dense approximation. The first laser at wavelength λ 1 = 553.5 nm ( λ 1 = 791 nm) pumps transition S 0 1 ↔ P 1 1 ( S 0 1 ↔ P 1 3 ). The second laser with λ 2 = 266 nm ( λ 2 = 310 nm) ionizes atoms from the level P 1 1 ( P 1 3 ). The third probe laser with wavelength λ i = 493.41 nm causes resonant fluorescence of barium ions. It is shown that the metastable D 2 3 from the D 1 , 2 , 3 3 triplet in barium is strongly populated through P 1 <
The kinetics of ionization and recombination of an ultracold barium plasma created in a two-step process, taking into account the transfer of resonant radiation in 3D cylindrical geometry, is studied by numerical simulation. At the first step, a pump laser excites the upper level of the resonant transition 6s2 S10↔6s6p P11 (λ1=553.5 nm). At the second step, the laser with quantum energy exceeding the ionization potential from the level 6s6p P11 (λ2=417.79 nm) ionizes the atoms. A scheme is proposed for increasing the efficiency of electron yield: at the second ionization step, the laser radiation with frequency corresponding to the continuum from the metastable D32 is used. The electron temperature from the initial value 0.1 K during the action of the pump and ionizing lasers increases by more than 200 times due to superelastic processes. As a result, the time of three-body recombination of plasma increases significantly. The results of numerical simulation indirectly confirm the fact of Killian et al. [Phys. Rev. Lett. 83(23), 4776 (1999)] that the deceleration of recombination of ultracold xenon plasma can be explained by the heating of electrons in superelastic quenching collisions.
A model for numerical simulation of the solar pumping of a simple atomic system is constructed and compared with the exact analytical solution. The process is then extended to cover the more complex 5-level 5-transition barium ion term scheme and the 61-level 86-transition barium neutral term scheme. An advantage of the step-wise simulation is that in addition to yielding the equilibrium relative level populations and transition intensities, it also permits the dynamic grow-in to equilibrium to be studied. The neutral barium system has also been studied with the inclusion of photoionization from each of several metastable levels.
Multiphoton ionization (MPI) of barium atom using an excimer-pumped pulsed dye laser has been studied in the wavelength range 570-608 nm at a laser intensity approximately 10.0 GW cm -2 . Resonances in MPI were recorded by monitoring either the ions produced in the multiphoton process using a thermionic detector or fluorescence from the ions formed in their various excited states of Ba + . The observation of photoion and fluorescence spectra were made on Ba vapour in the heat-pipe oven in the moderate-density regime ( approximately 10 13 atoms/cm 3 ) so that the collisional effects are negligible. Fine-structure resolution of the final ionic state (6p 2 P 1/2,3/2 ), produced after four-photon autoionizing decay was obtained. The decay of the Ba atom after four-photon absorption preferentially populates the 6p 2 P 3/2 state relative to the 6p 2 P 1/2 state.
Dense barium vapour (10 19 -10 21 m -3 ) produced in a heatpipe oven is irradiated by a laser light pulse (4.6*10 8 W m -2 , 300 ns, 30 pm) tuned to the resonant transition at lambda =553.55 nm. The transient population densities of various Ba I and Ba II states are measured and time resolved by means of the hook method. Simultaneously, the IR forward scattering (1.5 mu m; 1.13 mu m; 2.55 mu m) in the barium vapour is measured. When the laser pulse is tuned out of resonance the ion density decreases resonantly whereas the population densities of the low-lying Ba I states do not. This resonant ionisation behaviour is interpreted by means of radiative processes.
The line contour of the autoionizing resonance of 6P7P configuration in barium atoms was experimentally studied in the range of energies. 52,100 cm−1 with the help of a two‐step ionization of via intermediate states 6s6p (1P1), 6s8p (1P1). Using various combinations of polarization of the first‐ and second‐step laser, both linear and circular dichroism were meausred in the process of ionization from an excited state. Theoretical expressions governing linear and circular dichroism are derived. Using these expression, the values of two parameters used in the theory (the ratios of dipole matrix elements from the excited state, squared) were obtained from the experimental data.