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Physical Review A

Velocity-dependent neutralization cross sections of Ba+ ground and metastable states by Na

Ludo Vermeeren, Peter Lievens, Roger E. Silverans

Published Oct 1, 1990• 2 citations
Publisher

Abstract

A combined optical-pumping--laser-induced-fluorescence method was developed to determine the fraction of metastable-state ions in a ${\mathrm{Ba}}^{+}$ beam. This allowed us to measure the ratio of the charge-exchange cross sections for collisions of ${\mathrm{Ba}}^{+}$(6s) and ${\mathrm{Ba}}^{+}$(5d) ions with a sodium vapor for impact energies ranging from 3 to 29 keV. For a collision energy of 5 keV, the ratio of the cross sections for ${\mathrm{Ba}}^{+}$(5d $^{2}$${\mathit{D}}_{3/2}$) and ${\mathrm{Ba}}^{+}$(5d $^{2}$${\mathit{D}}_{5/2}$) was measured. All results are compared to theoretical near-resonance predictions.

DOI
10.1103/physreva.42.3901

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Cross sections for the excitation energy transfer induced by collisions with He, Ar and Na atoms

Vlasta Horvatić, Mladen Movre, Cedomil Vadla

The cross sections for the excitation energy transfer induced by collisions with He, Ar and Na ground-state atoms have been measured. The sodium atoms in the 4D state have been produced by two-photon excitation using cw laser radiation at 578.89 nm. Within a model which takes into account collisional population and depopulation of the 4F level as well as radiative relaxation (direct or cascade) of the 3D, 4P, 4D and 4F states ending in the 3P level, the rate for the collisional transfer turns out to be related to the ratio of the steady-state populations created in 3D and 4D states. They have been determined by measuring the fluorescence intensities of the and transitions. The measurements yield the following cross section values for those processes investigated: , and , at .

Journal of Physics B Atomic Molecular and Optical Physics•Nov 14, 1997•1 citation
A combined optical-pumping--laser-induced-fluorescence method was developed to determine the fraction of metastable-state ions in a Ba+{\mathrm{Ba}}^{+}Ba+ beam. This allowed us to measure the ratio of the charge-exchange cross sections for collisions of Ba+{\mathrm{Ba}}^{+}Ba+(6s) and Ba+{\mathrm{Ba}}^{+}Ba+(5d) ions with a sodium vapor for impact energies ranging from 3 to 29 keV. For a collision energy of 5 keV, the ratio of the cross sections for Ba+{\mathrm{Ba}}^{+}Ba+(5d ^{2}$${\mathit{D}}_{3/2}) and Ba+{\mathrm{Ba}}^{+}Ba+(5d ^{2}$${\mathit{D}}_{5/2}) was measured. All results are compared to theoretical near-resonance predictions.
The ratio of the charge exchange cross sections for Ba + (5d) and Ba + (6s) on Na, Rb and Cs, for Sr + (4d) and Sr + (5s) on Na and Rb, and for Ca + (3d) and Ca - (4s) on Na have been measured for ion impact energies between 0.5 keV and 30 keV. For the collisions with Na, absolute charge exchange cross section values are derived using data for the resonant process Na+Na + as a reference. The results are compared to theoretical near-resonance predictions. A method is proposed to study the polarization dependence of the neutralization cross sections for reactions involving ionic D states.
Optical pumping of the ground states of sodium can radically alter the shape of the laser-induced fluorescence excitation spectrum, complicating measurements of temperature, pressure, etc., which are based on these spectra. Modeling of the fluorescence using rate equations for the eight hyperfine states of the sodium D manifolds can be used to quantify the contribution to the ground state pumping of transitions among the hyperfine excited states induced by collisions with buffer gas atoms. This model is used here to determine, from the shape of experimental spectra, cross sections for $\ensuremath{\Delta}F$ transitions of the P3/2{P}_{3/2}P3/2​ state induced by collisions with helium and argon atoms, for a range of values assumed for the ${P}_{1/2}, \ensuremath{\Delta}F$ cross sections. The hyperfine cross sections measured using this method, which to our knowledge is novel, are compared with cross sections for transitions involving polarized magnetic substates mF{m}_{F}mF​ measured previously using polarization sensitive absorption. Also, fine-structure transition $(\ensuremath{\Delta}J)$ cross sections were measured in the pumped vapor, giving agreement with previous measurements made in the absence of pumping.
We report our measurements of the cross sections ${\ensuremath{\sigma}}_{5S}$ and ${\ensuremath{\sigma}}_{4D}$ for the electronic-energy-transfer process $\mathrm{Na}(3P)+\mathrm{Na}(3P)\ensuremath{\rightarrow}\mathrm{Na}(5S,4D)+\mathrm{Na}(3S)$. To obtain these cross sections we have measured the fluorescence intensity for the transitions $5S\ensuremath{\rightarrow}3P$, $4D\ensuremath{\rightarrow}3P$, and $3P\ensuremath{\rightarrow}3S$ in sodium vapor excited with a cw dye laser. The study of this electronic energy transfer is complicated and there is a large disagreement (\ensuremath{\gtrsim}104{10}^{4}104) among reported measurements. We believe we have avoided most of the difficulties of previous experiments. Our results are ${\ensuremath{\sigma}}_{5S}=(2.0\ifmmode\pm\else\textpm\fi{}0.7)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ cm2{\mathrm{cm}}^{2}cm2 and ${\ensuremath{\sigma}}_{4D}=(3.2\ifmmode\pm\else\textpm\fi{}1.1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{\ensuremath{-}2}$, and they are compared with the results of other experiments.
The key features of the cross sections for electron-impact excitation out of the ground levels of the rare-gas atoms are summarized and application of these cross section data is shown. This is followed by discussions of recent experiments on determination of electron-impact cross sections for excitation out of the metastable levels of He and Ar. The metastiable atom targets are produced by two different methods, i.e., hollow-cathode discharges and charge-exchange collisions of rare-gas ions with Cs atoms. The former method is simple but is limited to low electron energies because of the very small metastable concentration, whereas the latter method yields a fast target beam with predominantly metastable atoms allowing measurements up to the keV range. The measured cross sections show features very different from those of excitation out of the ground levels. Another new development in electron-atom collision is the use of atom traps for measuring ionization cross sections. The experiment is conducted by passing an electron beam pulse through magneto-optically trapped Rb atoms while the trap is momentarily turned off. Turning the trap back on immediately after the electron beam pulse recaptures the unionized atoms allowing only the ions to escape. The ionization cross section is determined bymore » measuring the electron beam current density at the trapped atom region and the fractional loss of trapped atoms due to the electron beam. This method avoids measurements of the absolute number of the target atoms and the overlap between the atomic and electron beams and therefore eliminates the major sources of uncertainty associated with the crossed-beam method for measuring ionization cross section.« less
The cross sections for the excitation energy transfer induced by collisions with He, Ar and Na ground-state atoms have been measured. The sodium atoms in the 4D state have been produced by two-photon excitation using cw laser radiation at 578.89 nm. Within a model which takes into account collisional population and depopulation of the 4F level as well as radiative relaxation (direct or cascade) of the 3D, 4P, 4D and 4F states ending in the 3P level, the rate for the collisional transfer turns out to be related to the ratio of the steady-state populations created in 3D and 4D states. They have been determined by measuring the fluorescence intensities of the and transitions. The measurements yield the following cross section values for those processes investigated: , and , at .