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Physical Review A

Dynamic properties of neutral excitations produced in electron-bombarded superfluid helium. I. The He(2S3) and He2(aΣ3) atomic and molecular metastable states

John W. Keto, F. J. Soley, M. Stockton, W. A. Fitzsimmons

Published Sep 1, 1974• 74 citations
Publisher

Abstract

We have studied the dynamic properties of the $\mathrm{He}(2^{3}S)$ and the ${\mathrm{He}}_{2}(a^{3}\ensuremath{\Sigma}_{u}^{+})$ atomic and molecular metastable states of helium that are produced in electron-bombarded superfluid helium. The steady-state concentration of ${\mathrm{He}}_{2}(a^{3}\ensuremath{\Sigma}_{u}^{+})$ molecules is shown to be limited by collisions between pairs of these molecules. Measurements of the rate coefficient for this bimolecular reaction as a function of temperature and pressure show that the reaction is diffusion limited and that the diffusion coefficient for the metastable molecules in the superfluid is determined by ${\mathrm{He}}_{2}(a^{3}\ensuremath{\Sigma}_{u}^{+})$-roton collisions. The bilinear destruction of metastable molecules has been monitored for times as long as 0.1 sec after the initial population of the state, and this establishes a lower bound of 0.1 sec on the radiative lifetime of the $(v=0) {\mathrm{He}}_{2}(a^{3}\ensuremath{\Sigma}_{u}^{+})$ state in superfluid helium. The lifetime and decay mode of the $\mathrm{He}(2^{3}S)$ atoms are distinctly different from that of the corresponding ${\mathrm{He}}_{2}(a^{3}\ensuremath{\Sigma}_{u}^{+})$ molecule. In particular, the characteristic lifetime of the $\mathrm{He}(2^{3}S)$ state in superfluid helium is essentially independent of the temperature between 2.08 and 1.4 \ifmmode^\circ\else\textdegree\fi{}K and is measured to be about 15 \ensuremath{\mu}sec. Our experimental observations indicate that the destruction of metastable atoms results in the population of high vibrational levels of the $a^{3}\ensuremath{\Sigma}_{u}^{+}$ molecular state. However, it is shown that the production of $(v=0) {\mathrm{He}}_{2}(a^{3}\ensuremath{\Sigma}_{u}^{+})$ molecules responds immediately to the electron beam (response time 3 \ensuremath{\mu}sec) and thus the formation of this state in the liquid does not depend upon the destruction of metastable atoms. Despite the fact that the lifetimes and decay modes of the metastable $\mathrm{He}(2^{3}S)$ atoms and $(v=0) {\mathrm{He}}_{2}(a^{3}\ensuremath{\Sigma}_{u}^{+})$ molecules are not related, the production rates for these states are nearly equal and correspond to about 450 and 500 per incident 160-keV electron, respectively.

DOI
10.1103/physreva.10.872

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Beams of metastable helium produced by electron bombardment generally contain atoms in both the 21S and 23S states. It is important in many experiments to distinguish between these states, or at least to know the population ratio. A beam of 23S and ground state helium atoms has been produced by electron bombardment and subsequent resonant quenching of 21S atoms with 21S−21P 2 μ radiation. This radiation mixes the 21S and 21P states, the latter decaying primarily to the 11S state by emission of 584 Å radiation. The quenching has been studied as a function of beam atom velocity. The results indicate that beams of metastable helium containing less than 1% 21S atoms can be produced.

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We have studied the dynamic properties of the He(23S)\mathrm{He}(2^{3}S)He(23S) and the He2(a3\ensuremathΣu+){\mathrm{He}}_{2}(a^{3}\ensuremath{\Sigma}_{u}^{+})He2​(a3\ensuremathΣu+​) atomic and molecular metastable states of helium that are produced in electron-bombarded superfluid helium. The steady-state concentration of He2(a3\ensuremathΣu+){\mathrm{He}}_{2}(a^{3}\ensuremath{\Sigma}_{u}^{+})He2​(a3\ensuremathΣu+​) molecules is shown to be limited by collisions between pairs of these molecules. Measurements of the rate coefficient for this bimolecular reaction as a function of temperature and pressure show that the reaction is diffusion limited and that the diffusion coefficient for the metastable molecules in the superfluid is determined by He2(a3\ensuremathΣu+){\mathrm{He}}_{2}(a^{3}\ensuremath{\Sigma}_{u}^{+})He2​(a3\ensuremathΣu+​)-roton collisions. The bilinear destruction of metastable molecules has been monitored for times as long as 0.1 sec after the initial population of the state, and this establishes a lower bound of 0.1 sec on the radiative lifetime of the (v=0)He2(a3\ensuremathΣu+)(v=0) {\mathrm{He}}_{2}(a^{3}\ensuremath{\Sigma}_{u}^{+})(v=0)He2​(a3\ensuremathΣu+​) state in superfluid helium. The lifetime and decay mode of the He(23S)\mathrm{He}(2^{3}S)He(23S) atoms are distinctly different from that of the corresponding He2(a3\ensuremathΣu+){\mathrm{He}}_{2}(a^{3}\ensuremath{\Sigma}_{u}^{+})He2​(a3\ensuremathΣu+​) molecule. In particular, the characteristic lifetime of the He(23S)\mathrm{He}(2^{3}S)He(23S) state in superfluid helium is essentially independent of the temperature between 2.08 and 1.4 \ifmmode^\circ\else\textdegree\fi{}K and is measured to be about 15 \ensuremath{\mu}sec. Our experimental observations indicate that the destruction of metastable atoms results in the population of high vibrational levels of the a3\ensuremathΣu+a^{3}\ensuremath{\Sigma}_{u}^{+}a3\ensuremathΣu+​ molecular state. However, it is shown that the production of (v=0)He2(a3\ensuremathΣu+)(v=0) {\mathrm{He}}_{2}(a^{3}\ensuremath{\Sigma}_{u}^{+})(v=0)He2​(a3\ensuremathΣu+​) molecules responds immediately to the electron beam (response time 3 \ensuremath{\mu}sec) and thus the formation of this state in the liquid does not depend upon the destruction of metastable atoms. Despite the fact that the lifetimes and decay modes of the metastable He(23S)\mathrm{He}(2^{3}S)He(23S) atoms and (v=0)He2(a3\ensuremathΣu+)(v=0) {\mathrm{He}}_{2}(a^{3}\ensuremath{\Sigma}_{u}^{+})(v=0)He2​(a3\ensuremathΣu+​) molecules are not related, the production rates for these states are nearly equal and correspond to about 450 and 500 per incident 160-keV electron, respectively.
We have studied the production, destruction, and lifetimes of the ${\mathrm{He}}_{2}(a{^{3}\ensuremath{\Sigma}_{\mathrm{u}}}^{+})$ and He(23S)\mathrm{He}(2^{3}S)He(23S) metastable states of helium that are produced in electron-bombarded superfluid helium. The characteristic lifetime of a metastable molecule in the superfluid is shown to exceed 0.1 sec, while the lifetime of a metastable atom is only about 15 \ensuremath{\mu}sec. The production rates for the ${\mathrm{He}}_{2}(a{^{3}\ensuremath{\Sigma}_{\mathrm{u}}}^{+})$ and He(23S)\mathrm{He}(2^{3}S)He(23S) metastable states are about 500 and 450 per incident 160-keV electron, respectively.
The temperature dependence of conversion of $\rm He(2\sp3S\sb1)$ metastable atoms to $\rm He\sb2(a\sp3\Sigma\sbsp{u}{+})$ metastable molecules in the three-body reaction $\rm He(2\sp3S\sb1)+ 2He(1\sp1S\sb0)\to He\sb2(a\sp3\Sigma\sbsp{u}{+}) + He(1\sp1S\sb0)$ has been investigated over the temperature range 65K-700K. This reaction is thermally activated as a consequence of a long range repulsive barrier in the $\rm He(2\sp3S\sb1)-He(1\sp1S\sb0)$ interaction potential. The data reveal that there are two reaction channels with distinctly different activation energies. The temperature dependence of the measured rate coefficient k$\sb{\rm s}($T) is accurately described by $\rm k\sb{s}(T) = \lbrack 87.4\ T\ exp({-750}/T) + 4.1 T\ exp({-200}/T)\rbrack \times 10\sp{-37}\ cm\sp{-6}sec\sp{-1}$. The first activation energy, 750 ±\pm± 70K (63 ±\pm± 6meV), is equal to the known $\rm He(2\sp3S\sb1)$-$\rm He(1\sp1S\sb0)$ repulsive barrier height. The second activation energy is 17 ±\pm± 2 meV. The temperature dependences of the rate constants for collision-induced mixing among $\rm He(2\sp3P\sb{J,m\sb{J}})$ levels, and for conversion of He(2$\sp3$P) atoms to $\rm He\sb2(b\sp3\Pi\sb{g})$ molecules in the three body reaction $\rm He(2\sp3P) + 2He(1\sp1S)\to He\sb2(b\sp3\Pi\sb{g}) + He(1\sp1S)$ have been investigated over the range 1.4$\sim$300K. The measured thermally-averaged cross section for $\rm He(2\sp3P\sb{J,m\sb{J}})$ mixing in collisions with ground state helium atoms are described by the function $\rm\sigma\sb{pm}(T) = (4.4 + 20.6/T\sp{1/3})\times 10\sp{-15}cm\sp2,$ and can be understood in terms of Langevin theory. The measured rate coefficients for the three body reaction exhibit a strong inverse temperature dependence, $\rm k\sb{p}(T) = (0.04 + 2.18/T)\times 10\sp{-30}\ cm\sp6{\cdot}s\sp{-1},$ which suggests that, unlike conversion of $\rm He(2\sp3S\sb1)$ to $\rm He\sb2(a\sp3\Sigma\sbsp{u}{+}),$ there is no activation energy required for this reaction. A magneto-optical trap for helium 2$\sp3$S metastable atoms has been designed and constructed, utilizing superconducting magnet gradient coils and a Ti:Sapphire ring laser for pumping the helium 2$\sp3$S-2$\sp3$P transition. He(2$\sp3$S) atoms are produced by a weak discharge in helium gas at temperature 1.3K. The discharge products flow through an orifice into the trap cell, where the He(2$\sp3$S) atoms are trapped and ground state helium atoms are rapidly cryopumped by zeolite pellets that cover most of the cell bottom. Preliminary experimental results suggest that $\sim101010\sp6$ atoms are trapped, with a trap lifetime of about 0.2 sec limited by He(2$\sp3$S) - He(2$\sp3$P) Penning reactions. Ultimately, it is estimated that a substantial number of atoms can be trapped and cooled for much longer times in a near-perfect vacuum. Measurements of decay times of the trapped atoms should yield rates for $\rm \sp{4,3}He(2\sp3S)$ - $\sp{4,3}$He(2$\sp3$S) and resonantly-enhanced He(2$\sp3$S) - He(2$\sp3$P) Penning reactions in the ultra-cold quantum regime, and perhaps the He(2$\sp3$S) natural lifetime.
Long-lived phosphorescence has been observed emanating from superfluid helium following excitation by ionizing radiation. This extreme ultraviolet light is predominantly from the radiative decay of metastable ${\mathrm{He}}_{2}(a{}^{3}{\ensuremath{\Sigma}}_{u}^{+})$ molecules and is detected through frequency down-conversion and photon counting techniques. This allows measurement of the phosphorescence lifetime in a low-molecular-density regime that is not dominated by two-body reactions. A phosphorescence lifetime of $13\ifmmode\pm\else\textpm\fi{}2 \mathrm{s}$ is observed.
Long-lived metastable ${\mathrm{He}}_{2}^{\ensuremath{-}}$ negative ions have been observed for the first time. They were produced from He2+{\mathrm{He}}_{2}^{+}He2+​ by two-step electron capture in a Cs vapor target. The apparent autodetachment decay rates have been measured at several delay times after formation, and indicate the presence of more than one state, with the lifetime of the major component exceeding 100 \ensuremath{\mu}s. The most likely configurations are ${^{4}\ensuremath{\Sigma}_{g}}^{+}$ and/or $^{4}\ensuremath{\Pi}_{g}$ states bound below ${\mathrm{He}}_{2}({^{3}\ensuremath{\Sigma}_{g}}^{+})$.
Calculations of low-energy collisions of antiprotons p‾\overline{p}p​ with helium atoms in the ground (11S)(1\phantom{\rule{4pt}{0ex}}^{1}\mathit{S})(11S) and metastable (23S(2\phantom{\rule{4pt}{0ex}}^{3}\mathit{S}(23S and 21S)2\phantom{\rule{4pt}{0ex}}^{1}\mathit{S})21S) states are carried out by using a semiclassical method and by further assuming that one of the two electrons is passive. Antiproton capture to form antiprotonic helium $(\ensuremath{\rightarrow}\overline{p}{\mathrm{He}}^{+}+e)$ and breakup ionization $(\ensuremath{\rightarrow}\overline{p}+{\mathrm{He}}^{+}+e)$ are investigated. The energy dependence of the capture and ionization cross sections and the distributions of the product states are presented and compared with previous theoretical calculations. For the metastable helium targets, most of the p‾He+\overline{p}{\mathrm{He}}^{+}p​He+ atoms produced in the capture have long lifetimes against subsequent Auger decays and can be observed experimentally. For the ground-state targets, however, the formation of such long-lived states is rather a rare event at low collision energies.
Beams of metastable helium produced by electron bombardment generally contain atoms in both the 21S and 23S states. It is important in many experiments to distinguish between these states, or at least to know the population ratio. A beam of 23S and ground state helium atoms has been produced by electron bombardment and subsequent resonant quenching of 21S atoms with 21S−21P 2 μ radiation. This radiation mixes the 21S and 21P states, the latter decaying primarily to the 11S state by emission of 584 Å radiation. The quenching has been studied as a function of beam atom velocity. The results indicate that beams of metastable helium containing less than 1% 21S atoms can be produced.