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Journal of Physics B Atomic and Molecular Physics

Measurement of cross sections for collisions of the second kind in helium

Joseph S. Bakos, János Szigeti

Published Nov 1, 1968• 14 citations
Publisher

Abstract

We have measured the cross sections for collisions of the second kind between helium atoms in the n 1 P ( n = 3,4) state and in the normal state. As a consequence of the collision the atoms are transferred from the n 1 P to the n 1 D, n 3 D or n 3 F state. The n 1 P states were excited selectively by optical pumping. The collision cross sections for the transitions 3 1 P -> 3 1 D, 3 1 P -> 3 3 D, 4 1 P -> 4 1 D, 4 1 P -> 4 3 D and 4 1 P -> 4 3 F were measured as 3.2 × 10 -16 , 2.2 × 10 -19 , 2.8 × 10 -16 , 2.2 × 10 -16 and 9.5 × 10 -17 cm 2 , respectively. Thus they are smaller by several orders of magnitude than those reported previously.

DOI
10.1088/0022-3700/1/6/315

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Optical Pumping of Neon P23 Metastable Atoms

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The $^{3}P_{2}$ metastable atoms of neon have been optically oriented. This represents the first case of the optical pumping of non-$S$ state atoms. The optical-pumping process is described, and depolarization cross sections for collisions with ground-state neon and ground-state helium atoms measured. The cross section for collisional depolarization with helium is 4.30 \ifmmode\pm\else\textpm\fi{} 0.22 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}17}$ ${\mathrm{cm}}^{2}$, and with neon is 166 \ifmmode\pm\else\textpm\fi{} 8 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}17}$ ${\mathrm{cm}}^{2}$ at 300\ifmmode^\circ\else\textdegree\fi{}K. The metastable $^{3}P_{2}$ atoms of Ar and Xe have also been optically pumped. As the polarizability of the noble-gas buffer increases, the collisional depolarization cross section approaches the diffusion cross section.

Physical Review•Apr 5, 1969•26 citations
We have measured the cross sections for collisions of the second kind between helium atoms in the n 1 P ( n = 3,4) state and in the normal state. As a consequence of the collision the atoms are transferred from the n 1 P to the n 1 D, n 3 D or n 3 F state. The n 1 P states were excited selectively by optical pumping. The collision cross sections for the transitions 3 1 P -> 3 1 D, 3 1 P -> 3 3 D, 4 1 P -> 4 1 D, 4 1 P -> 4 3 D and 4 1 P -> 4 3 F were measured as 3.2 × 10 -16 , 2.2 × 10 -19 , 2.8 × 10 -16 , 2.2 × 10 -16 and 9.5 × 10 -17 cm 2 , respectively. Thus they are smaller by several orders of magnitude than those reported previously.
Collisions of the second kind between caesium and caesium, caesium and helium, and caesium and xenon atoms, leading to P p the transfer of excitation between the 6 6 P^yg states in caesium, have been investigated by studying the sensitized fluorescence in caesium vapor at pressures in the range 2x10*^ • 7x10*5 mm Hg and in vapor-inert gas mixtures.These low caesium densities, at which trapping of resonance radiation is virtually absent, were previously Inaccesible to such measurements.The use of a high intensity light source and an improved fluorescence cell in conjunction with a very sensitive detection system, permitted accurate measurements of extremely low fluorescent intensities.The total collision cross sections for the processes ô^P^yg -> and 6 %]^yg<r-G^P^yg equal 0.64x10*^^ cm® and 3.1x10*15 ^^2 for caesium-caesium collisions, 0.41x10*®® cm® and 2.7x10*®® cm® for caesium-helium collisions, and 7 .6x10 *®! cm^ and 2.9x10*1^ cm® for caesium-xenon collisions respectively.The ratios of these cross sections agree well with the values predicted by the principle of detailed balancing except for the caesiumxenon collisions, where a shielding effect has been postulated to account for the discrepancy.ill
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.
Absolute velocity-resolved total integral cross sections have been measured for collisions of helium singlet (2 1 S 0 ) and triplet (2 3 S 1 2) metastable (He*) atoms with ground-state helium, neon, and argon atoms in the thermal velocity range of 1.0-3.5*10 5 cm s -1 . Additional measurements on the He*-Ne system with a large input aperture at the detector to suppress the sensitivity of the apparatus to elastic scattering failed to show the previously predicted (Arathoon) sharply rising velocity structure in the inelastic excitation transfer cross sections.
An experiment is described which measures the collision cross section for transfer of excitation in rubidium by helium. Rubidium atoms are pumped to either the 2P12^{2}P_{\frac{1}{2}}2P21​​ or 2P32^{2}P_{\frac{3}{2}}2P23​​ state by irradiation with one of the resonance lines. Some excited rubidium atoms are transferred to the other PPP state by collisions with helium atoms. The rubidium atoms then radiate both resonance lines. Measurements of the intensities of both resonance lines as a function of helium pressure are used to calculate the collision cross section. The cross section for 2P12^{2}P_{\frac{1}{2}}2P21​​ to 2P32^{2}P_{\frac{3}{2}}2P23​​ is 1.0\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}17}$ cm2{\mathrm{cm}}^{2}cm2\ifmmode\pm\else\textpm\fi{}10% and for 2P32^{2}P_{\frac{3}{2}}2P23​​ to 2P12^{2}P_{\frac{1}{2}}2P21​​ is 1.2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}17}$ cm2{\mathrm{cm}}^{2}cm2\ifmmode\pm\else\textpm\fi{}10%.
We have measured the effect of the transfer of excitation on the population of the 33D3^{3}D33D state of helium. Through careful diagnosis of a proton beam passing through a long target chamber containing helium gas, we have been able to separate the various contributions to the light intensity from the 33D3^{3}D33D state. Our measurements were performed at target-gas pressures roughly two orders of magnitude lower than those of previously reported measurements. We measure the summation of the excitation-transfer cross sections ${\ensuremath{\sigma}}_{c}(n)$ for the transfer from an n1Pn^{1}Pn1P state to the corresponding n3Fn^{3}Fn3F state times the branching ratio Bn{B}_{\mathrm{n}}Bn​ for radiative cascade to the 33D3^{3}D33D state, and obtain $\ensuremath{\Sigma}{B}_{n}{\ensuremath{\sigma}}_{c}(n)=2.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ cm2{\mathrm{cm}}^{2}cm2. This measurement enables us to relate the excitation transferred to the 33D3^{3}D33D state QT(33D){Q}_{T} (3^{3}D)QT​(33D) to the excitation cross section of the 41P4^{1}P41P state $\ensuremath{\sigma}(4^{1}P)$. We obtain ${Q}_{T} (3^{3}D)=(1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15} {\mathrm{cm}}^{3})N\ensuremath{\sigma}(4^{1}P)$, an expression, which for the low pressures used in this experiment, is linear in the number density NNN.
The 3P2^{3}P_{2}3P2​ metastable atoms of neon have been optically oriented. This represents the first case of the optical pumping of non-SSS state atoms. The optical-pumping process is described, and depolarization cross sections for collisions with ground-state neon and ground-state helium atoms measured. The cross section for collisional depolarization with helium is 4.30 \ifmmode\pm\else\textpm\fi{} 0.22 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}17}$ cm2{\mathrm{cm}}^{2}cm2, and with neon is 166 \ifmmode\pm\else\textpm\fi{} 8 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}17}$ cm2{\mathrm{cm}}^{2}cm2 at 300\ifmmode^\circ\else\textdegree\fi{}K. The metastable 3P2^{3}P_{2}3P2​ atoms of Ar and Xe have also been optically pumped. As the polarizability of the noble-gas buffer increases, the collisional depolarization cross section approaches the diffusion cross section.