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Physical Review A

New Measurements of Excitation Transfer in Low-Density Helium Gas

Daniel Krause, Edward A. Soltysik

Published Jun 1, 1972• 2 citations
Publisher

Abstract

We have measured the effect of the transfer of excitation on the population of the $3^{3}D$ 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 $3^{3}D$ 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 $n^{1}P$ state to the corresponding $n^{3}F$ state times the branching ratio ${B}_{\mathrm{n}}$ for radiative cascade to the $3^{3}D$ state, and obtain $\ensuremath{\Sigma}{B}_{n}{\ensuremath{\sigma}}_{c}(n)=2.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ ${\mathrm{cm}}^{2}$. This measurement enables us to relate the excitation transferred to the $3^{3}D$ state ${Q}_{T} (3^{3}D)$ to the excitation cross section of the $4^{1}P$ 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 $N$.

DOI
10.1103/physreva.5.2488

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Physical Review A•May 1, 1982•13 citations
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Precise observations have been made on light emission from low triplet levels of helium gas traversed by a monoenergetic electron beam, with varying helium pressure and electron energy. In the past it has been customary to assign this triplet excitation to direct transfer from 1P^{1}P1P states in flagrant violation of the Wigner rule and seemingly excessive cross sections for the transfer have been inferred.A new process of excitation transfer is proposed which minimizes the conflict with the Wigner rule by reducing the sizes of the cross sections required to values close to the gas-kinetic cross section. It is hypothesized that many 1P^{1}P1P states, including those with large quantum number nnn, transfer excitation energy to neighboring triplet states having closely corresponding principal quantum numbers. The triplet states thus formed in turn populate low-level triplet states by radiative transitions. It is found that states lying between n=4n=4n=4 and n=15n=15n=15 would play the dominant role in the transfer process.Satisfying qualitative explanations of several additional excitation-transfer-process phenomena are derived from this new multiple-state-transfer process.
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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.
The cross section for the transfer of excitation from the 23P02^{3}P_{0}23P0​ level of helium to the 23P12^{3}P_{1}23P1​ and 23P22^{3}P_{2}23P2​ levels as a result of collisions with the ground-state helium atom (11S01^{1}S_{0}11S0​) has been measured with a conventional resonance-fluorescence technique. A He4{\mathrm{He}}^{4}He4 source excited the $2^{3}S_{1}\ensuremath{-}2^{3}P_{0}$ transition in a He3{\mathrm{He}}^{3}He3 absorption cell. The scattered light was examined with a high-resolution grating spectrometer and the ratio of the $2^{3}P_{0}\ensuremath{\rightarrow}2^{3}S_{1}$ to the $2^{3}P_{1,2}\ensuremath{\rightarrow}2^{3}S_{1}$ intensities was measured as a function of cell pressure from 0.05 to 10 Torr. The cross section for collision-induced transitions out of the 23P02^{3}P_{0}23P0​ level is found to be (68\ifmmode\pm\else\textpm\fi{}3) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}16}$ cm2{\mathrm{cm}}^{2}cm2 at room temperature.
Cross sections for $7^{2}D_{\frac{3}{2}}\ensuremath{\leftrightarrow}7^{2}D_{\frac{5}{2}}$ transfer in Rb, induced in collisions with He, Ne, Ar, and with ground-state Rb atoms, have been determined using methods of atomic fluorescence. Rb vapor, pure or mixed with a noble gas, was irradiated in a glass fluorescence cell with pulses of 660-nm radiation from a N2{\mathrm{N}}_{2}N2​-laser-pumped dye laser, populating one of the 2D^{2}D2D states by two-photon absorption. The resulting fluorescence included a direct component emitted in the decay of the optically excited state and a sensitized component arising from the collisionally populated state. Relative intensities of the components yielded the cross sections for 72D7^{2}D72D mixing: $Q(^{2}D_{\frac{3}{2}}\ensuremath{\rightarrow}^{2}D_{\frac{5}{2}})=8.8,6.5,10.4, \mathrm{and} 30$; $Q(^{2}D_{\frac{3}{2}}\ensuremath{\leftarrow}^{2}D_{\frac{5}{2}})=5.8,4.0,6.9, \mathrm{and} 18$, in units of ${10}^{\ensuremath{-}14}$ cm2{\mathrm{cm}}^{2}cm2 for He, Ne, Ar, and Rb, respectively. Cross sections for the effective quenching of the 2D^{2}D2D states were also determined.