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Journal of Physics B Atomic and Molecular Physics

Measurement of the photoionization cross section from the laser-populated3D metastable levels in barium

John L. Carlsten, Thomas J. Mcilrath, W. H. Parkinson

Published Jun 1, 1974• 16 citations
Publisher

Abstract

The authors have measured the absolute photoionization cross section from the 6s5d 3 D metastable level of barium. The 3 D levels were selectively populated with a high-power, tuneable dye laser. The cross section obtained at 303 nm was (18+or-4)*10 -18 cm 2 . The number density was determined by observing the resulting depopulation of the ground state when pumping occurred.

DOI
10.1088/0022-3700/7/8/003

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Measurement of photoionization cross section in atomic uranium using simultaneous observation of laser-induced photoionization and fluorescence signals

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Laser-induced photoionization and fluorescence signals were simultaneously observed in atomic uranium using a single Nd:YAG-pumped dye laser. These signals were recorded in two specific cases. In the first case, the dye laser was resonant to the first-step transition (0–16900.38 cm−1). In the second case, the laser was near-resonant to the first-step transition with a slight detuning (0.15 cm−1) so that it became two-photon resonant at 33801.06 cm−1. The uranium atoms in the ground state were ionized by a single-color, three-photon photoionization technique resulting in the photoionization signal, and the fluorescence signal was simultaneously obtained from the first excited state involved in the photoionization process. The photoionization and the fluorescence signals in the above-mentioned cases were also estimated theoretically for several values of the photoionization cross section for the transition between the second excited state at 33801.06 cm−1 and the autoionization state at 50701.59 cm−1 using density matrix formalism. From the comparison of theoretically calculated ratios of fluorescence signals in the two specific cases with the experimentally obtained values, the photoionization cross section for the 33801.06–50701.59 cm−1 transition has been obtained, which is found to be (5±1)×10−16 cm2.

Journal of the Optical Society of America B•Mar 16, 2012•4 citations
The authors have measured the absolute photoionization cross section from the 6s5d 3 D metastable level of barium. The 3 D levels were selectively populated with a high-power, tuneable dye laser. The cross section obtained at 303 nm was (18+or-4)*10 -18 cm 2 . The number density was determined by observing the resulting depopulation of the ground state when pumping occurred.
A simple theory describing selective excitation of a three-level atom with a tuneable laser is developed. Effects of number density, line widths and laser parameters on the final populations of the levels with laser excitation are discussed. An experiment is described in which a tuneable dye laser is used to pump large numbers of barium atoms into the 6s5d 3 D metastable term from the (bs) 2 1 S 0 ground state via the 6s6p 3 P term. The theory for the interaction of the laser with this 'three-level system' predicts that approximately 80% of the barium atoms should be excited into the 3 D term. Experimentally the largest amount of excitation to the 3 D term observed was (78+or-5)%.
We demonstrate an experimental technique that selects the nuclear spin I=0 isotope 138Ba^{138}\mathrm{Ba}138Ba for spectroscopic measurements from a beam of natural barium containing seven stable isotopes. A cw dye laser is used to optically pump the I=3/2 isotopes 135Ba^{135}\mathrm{Ba}135Ba and 137Ba^{137}\mathrm{Ba}137Ba into the metastable 6s5d 1^{1}1D2{\mathit{D}}_{2}D2​ and 3^{3}3D1,2{\mathit{D}}_{1,2}D1,2​ levels via the 6s6p 1^{1}1P1{\mathit{P}}_{1}P1​ excited state, leaving nearly pure 138Ba^{138}\mathrm{Ba}138Ba in the ground state. Photoionization spectra of the excited 5d6p 3^{3}3D1{\mathit{D}}_{1}D1​ even isotope state recorded with optical pumping of 135Ba^{135}\mathrm{Ba}135Ba and 137Ba^{137}\mathrm{Ba}137Ba to the metastable levels are essentially devoid of forbidden features associated with the hyperfine interaction in the odd isotopes.
We describe a set of experiments that investigate diffusion of barium 6s5d3{\mathrm{}}^{3}3DJ{\mathit{D}}_{\mathit{J}}DJ​ metastable level and 6s2{\mathit{s}}^{2}s21{\mathrm{}}^{1}1S0{\mathit{S}}_{0}S0​ ground-state atoms through the noble gases: He, Ne, Ar, Kr, and Xe. The barium metastable levels were populated through optical pumping of the 6s2{\mathit{s}}^{2}s21{\mathrm{}}^{1}1S0{\mathit{S}}_{0}S0​\ensuremath{\rightarrow}6s6p3{\mathrm{}}^{3}3P1o{\mathit{P}}_{1}^{\mathit{o}}P1o​ intercombination transition with a pulsed laser, followed by spontaneous or stimulated emission into the 6s5d3{\mathrm{}}^{3}3D1,2{\mathit{D}}_{1,2}D1,2​ levels. Collisional mixing then distributed population throughout the 6s5d3{\mathrm{}}^{3}3DJ{\mathit{D}}_{\mathit{J}}DJ​ levels. A weak narrow-band cw laser was tuned to appropriate transitions to probe the spatial distribution of metastable level atoms, as well as the hole created in the ground-state spatial distribution, by the pulsed laser. Time-dependent absorption coefficients were obtained by measuring the transmission of the probe laser at several different pump-probe spatial separations. From these absorption data, spatial number density profiles were mapped out for various times following the firing of the pump laser. These density profiles were used to determine the diffusion coefficient for each state at a particular buffer-gas pressure and temperature. From these pressure- and temperature-dependent diffusion coefficients D, we determined the thermally averaged diffusion cross sections ${\mathrm{\ensuremath{\sigma}}}_{\mathit{D}}$(6s5d3{\mathrm{}}^{3}3DJ{\mathit{D}}_{\mathit{J}}DJ​) and ${\mathrm{\ensuremath{\sigma}}}_{\mathit{D}}$(6s2{\mathit{s}}^{2}s21{\mathrm{}}^{1}1S0{\mathit{S}}_{0}S0​) for the metastable and ground-state atoms diffusing through the noble gases, respectively. We also report values for the scaled standard diffusion coefficients D0{\mathit{D}}_{0}D0​. Our experimental values of the thermally averaged diffusion cross sections ${\mathrm{\ensuremath{\sigma}}}_{\mathit{D}}$(6s5d3{\mathrm{}}^{3}3DJ{\mathit{D}}_{\mathit{J}}DJ​) and ${\mathrm{\ensuremath{\sigma}}}_{\mathit{D}}$(6s2{\mathit{s}}^{2}s21{\mathrm{}}^{1}1S0{\mathit{S}}_{0}S0​) for the barium-helium system are compared with values calculated from published, theoretical barium-helium potentials and the agreement is excellent. \textcopyright{} 1996 The American Physical Society.
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 <
We describe a new technique for accurate measurement of absolute photoionization cross sections. By measuring the loss rateof atoms from a laser trap in the presence of ionizing light, we directly measure the photoionization rate. The only quantities requiring absolute calibration are the ionizing laser intensity and the fractional population in the relevant state. Our technique is capable of detecting extremely small ionization rates, which means that low-power cw sources can be used. We have applied this method to photoionization from the 5P(3/2) state of rubidium at wavelengths of 413 and 407 nm.The cross sections are 1.36(12) x 10(-17) and 1.25(11) x 10(-17) cm(2) , respectively.
Laser-induced photoionization and fluorescence signals were simultaneously observed in atomic uranium using a single Nd:YAG-pumped dye laser. These signals were recorded in two specific cases. In the first case, the dye laser was resonant to the first-step transition (0–16900.38 cm−1). In the second case, the laser was near-resonant to the first-step transition with a slight detuning (0.15 cm−1) so that it became two-photon resonant at 33801.06 cm−1. The uranium atoms in the ground state were ionized by a single-color, three-photon photoionization technique resulting in the photoionization signal, and the fluorescence signal was simultaneously obtained from the first excited state involved in the photoionization process. The photoionization and the fluorescence signals in the above-mentioned cases were also estimated theoretically for several values of the photoionization cross section for the transition between the second excited state at 33801.06 cm−1 and the autoionization state at 50701.59 cm−1 using density matrix formalism. From the comparison of theoretically calculated ratios of fluorescence signals in the two specific cases with the experimentally obtained values, the photoionization cross section for the 33801.06–50701.59 cm−1 transition has been obtained, which is found to be (5±1)×10−16 cm2.