Metastable Hydrogen Research Articles

A Lamb-shift polarized ion source for the TUNL tandem accelerator

Construction details and operating conditions are given for the Lamb-shift polarized ion source installed at TUNL. A spin filter is used to polarize the metastable hydrogen or deuterium beam.

Small-angle scattering of metastable hydrogen formed by collisions of neutral hydrogen atoms with gas targets

An experimental investigation has been made of the scattering of metastable hydrogen formed in collisions of neutral hydrogen with helium; projectile energies were in the range 1 to 20 keV and scattering angles investigated range from 0.2\ifmmode^\circ\else\textdegree\fi{} to 1.5\ifmmode^\circ\else\textdegree\fi{}. In addition angular distributions of scattering H${}^{0}$ and ${\mathrm{H}}^{+}$ were measured in an extended angular range of 0.2\ifmmode^\circ\else\textdegree\fi{} - 5.0\ifmmode^\circ\else\textdegree\fi{}. Total cross sections for $\mathrm{H}(2s)$ formation in helium and argon targets have also been measured. The total cross sections for $\mathrm{H}(2s)$ formation in helium are in good agreement with coupled-state theoretical predictions at energies above 4.0 keV; the measured differential cross sections show the same angular dependence as theory at 10-keV impact energy. Also, at low energies, a low-angle peak is found in the angular distribution of $\mathrm{H}(2s)$; this is in accordance with theoretical predictions.

Destruction of fast metastable hydrogen atoms passing through atomic hydrogen

The first Born approximation is used to calculate cross sections for excitation or de-excitation to discrete states from fast metastable hydrogen atoms passing through atomic helium. Full account is taken of double transitions and the calculations are performed using length and velocity formulation values of the Born helium matrix elements. Close coupling values in the impact parameter theory for the specific transitions 2s to nl(nl=1s, 2p, 3s, 3p) in hydrogen with the target helium atom unexcited indicate that coupling to excited states and to the hydrogen continuum is not important for these particular destruction processes for impact energies greater than 10 keV. Addition of the present Born approximation results to those of Bell and Kingston (1971) for the electron loss contribution provide total destruction cross sections which are a factor of about 2.5 lower than the experimental data (5-30 keV) of Gilbody et al (1971). For electron loss, the recent data of Hughes and Choe suggests that the first Born approximation may be satisfactory for impact energies greater than 100 keV.

Formation of metastable hydrogen atoms by charge exchange of protons in cesium vapor

Formation of an intense beam of hydrogen atoms in the metastable $2s$ state is not only of basic interest, but is also useful for polarized-ion sources of the Lamb-shift type. The nearly resonant reaction ${\mathrm{H}}^{+}+\mathrm{Cs}\ensuremath{\rightarrow}\mathrm{H}(2s)+{\mathrm{Cs}}^{2}$ is often used for this purpose. The fraction $f$ of metastable atoms relative to the number of neutral atoms in all $n=2$ states formed in the collision of a proton in a thin Cs-vapor target is reported here for the energy range 0.4-3.0 keV. An apparent maximum of $f=0.43\ifmmode\pm\else\textpm\fi{}0.03$ at 0.5 keV is found. For energies above 0.75 keV, $f=0.25\ifmmode\pm\else\textpm\fi{}0.01$, which would be expected if the $n=2$ states of H are statistically populated. Previously reported values for ${\ensuremath{\sigma}}_{+m}$, the cross section for electron pickup in the metastable $2s$ state of hydrogen for ${\mathrm{H}}^{+}$ + Cs collisions, differ by two orders of magnitude for a given energy. It can be assumed that essentially all electron pickup is into the $n=2$ states at low energy. Thus measured values of $f$ can be multiplied by reported values of ${\ensuremath{\sigma}}_{+0}$, the cross section for electron pickup into any neutral state of the H atom, to find values for ${\ensuremath{\sigma}}_{+m}$. As this method is independent of calibration or normalization of the photon measurements, these values for ${\ensuremath{\sigma}}_{+m}$ are more reliable than those previously reported. Our results for ${\ensuremath{\sigma}}_{+m}$ are between 4.3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}15}$ and 1.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{2}$ in the energy range 0.5-3.0 keV.

Destruction of metastable hydrogen in collisions with helium

The cross section for collisional destruction of metastable hydrogen by helium has been measured for velocities between 4 x lo6 and 5 x lo8 cm s -l . Cross sections at 43 individual velocities have been obtained by an experimental method which combines time-of-flight techniques with a new kind of gas target pressure modulation. Results are compared with the existing rather elaborate theoretical work on the problem as well as with the limited experimental information.

Lifetime measurement of hyperfine-structure levels of metastable hydrogen in static electric fields

A time-of-flight technique has been used to select metastable hydrogen atoms with uniform velocity and the quenching of polarized and unpolarized metastable hydrogen in static electric fields has been measured.

Electron-loss cross sections of lithium atoms in gases

Electron-loss cross sections sigma 01 for ground-state lithium atoms traversing hydrogen, helium, nitrogen, neon, argon, krypton, and xenon within the range of impact energies from 20 to 500 keV have been measured by beam attenuation in a transverse electrostatic field. These experimental cross sections for loss of the loosely bound valence electron in neutral lithium are compared with the theoretical estimates, based upon the semi-empirical classical impulse approximation of Bates and Walker (1966). Further, the present cross sections for lithium atoms are compared with published experimental electron-loss cross sections for metastable hydrogen and helium atoms. On the basis of the theory mentioned above, similarities between these cross sections are expected. The overall agreement between the cross sections presented and the theoretical estimates, as well as the experimental electron-loss sections for metastable hydrogen and helium atoms is reasonable. A more detailed comparison is, however, made difficult by the fact that results from different laboratories in some cases show poor agreement.

Destruction of fast metastable hydrogen atoms passing through atomic helium

The first Born approximation is used to calculate cross sections for excitation or de-excitation to discrete states from fast metastable hydrogen atoms passing through atomic helium. Full account is taken of double transitions and the calculations are performed using length and velocity formulation values of the Born helium matrix elements. Close coupling values in the impact parameter theory for the specific transitions 2s to nl(nl=1s, 2p, 3s, 3p) in hydrogen with the target helium atom unexcited indicate that coupling to excited states and to the hydrogen continuum is not important for these particular destruction processes for impact energies greater than 10 keV. Addition of the present Born approximation results to those of Bell and Kingston (1971) for the electron loss contribution provide total destruction cross sections which are a factor of about 2.5 lower than the experimental data (5-30 keV) of Gilbody et al (1971). For electron loss, the recent data of Hughes and Choe suggests that the first Born approximation may be satisfactory for impact energies greater than 100 keV.

Polarization of Lyman-alpha radiation from the quenching of metastable hydrogen atoms in an external electric field

Abstract Polarization of Lyman-alpha radiation resulting from the quenching of metastable hydrogen atoms in the 2s state has been measured as a function of electric field strength. Results are in agreement with an adiabatic theory.

Collisional Quenching of Metastable Hydrogen Atoms by Rare Gases

Collisional Quenching of Metastable Hydrogen Atoms by Rare Gases

Collisional Quenching of Metastable Hydrogen

Measurements of the collisional quenching of the $2S$ state of atomic hydrogen by helium, neon, argon, and krypton have been made. The energy range was in the thermal region ($v\ensuremath{\sim}{10}^{6}$ cm/sec). The cross sections were determined by measuring the attenuation of a beam of $\mathrm{H}(2S)$ atoms in passing through targets of the various gases. Time-of-flight techniques were used to measure the velocity dependences of the cross sections. For an incident $\mathrm{H}(2S)$ velocity $\ensuremath{\approx}1\ifmmode\times\else\texttimes\fi{}{10}^{6}$ cm/sec, the measured cross sections are $\ensuremath{\sigma}(\mathrm{helium})=81\ifmmode\pm\else\textpm\fi{}12$ ${\mathrm{\AA{}}}^{2}$/atom, $\ensuremath{\sigma}(\mathrm{neon})=80\ifmmode\pm\else\textpm\fi{}12$ ${\mathrm{\AA{}}}^{2}$/atom, $\ensuremath{\sigma}(\mathrm{argon})=91\ifmmode\pm\else\textpm\fi{}14$ ${\mathrm{\AA{}}}^{2}$/atom, and $\ensuremath{\sigma}(\mathrm{krypton})=108\ifmmode\pm\else\textpm\fi{}16$ ${\mathrm{\AA{}}}^{2}$/atom. The results for helium are 10-15% higher than theoretical results obtained by Slocomb, Miller, and Schaefer and 20-25% higher than a calculation by Byron and Gersten using a pseudopotential method.

Formation of HeH+ from Low-Energy Collisions of Metastable Helium and Molecular Hydrogen

A merging-beams technique has been used to study HeH+ formation resulting from the interaction of a composite beam of He(21S) and He (23S) with a beam of H2. The experiments were conducted over a range of interaction energy W from 0.05 to 10 eV. It was learned that the dominant process for producing HeH+ is the rearrangement ionization reaction He*+H2→HeH++H+e, where He* represents the metastable helium. The experiment consisted of measuring the lab-energy distributions of HeH+. Relative and absolute cross sections were obtained from these distributions. The distributions indicate that, in the center-of-mass system, most of the HeH+ is scattered in the direction of the incident He*. The energies associated with the HeH+ at the peaks of these distributions are very close to those expected for spectator stripping. Conservation of energy for the reaction can only be achieved by assuming that the electron carries away a substantial portion of the available energy. It is shown that the two-step model He*+H2→He+H2+†+e→HeH++H+e (where † indicates ground-state or vibrationally excited H2+) satisfactorily explains the measured lab-energy distributions near W=0.05 eV but does not agree completely with the results for W≥ 2 eV.

Angular Distribution of Metastable Hydrogen Formed by Dissociation ofH2+

A study has been made of ${\mathrm{H}}_{2}^{+}$ dissociation by impact on targets of He, Ar, ${\mathrm{H}}_{2}$, and ${\mathrm{N}}_{2}$. Cross sections were measured for the angular distributions of metastable hydrogen, protons, and neutral hydrogen atoms resulting from dissociation. Projectile energies are in the range 4-12 keV; the angular range of the measurements extended from 0.5\ifmmode^\circ\else\textdegree\fi{} to 5.0\ifmmode^\circ\else\textdegree\fi{}. The angular distributions of metastables are consistent with their formation through excitation of the ${\mathrm{H}}_{2}^{+}$ projectile into the $2s{\ensuremath{\sigma}}_{g}$ and $3p{\ensuremath{\sigma}}_{u}$ repulsive states of ${\mathrm{H}}_{2}^{+}$. In all cases the dissociation cross section appears to be strongly peaked towards small angles between the internuclear axis of the molecule and the direction of the ${\mathrm{H}}_{2}^{+}$ projectile; in the case of $\mathrm{H}(2s)$ formed by dissociation on He, the cross section appears to vary as approximately the square of the cosine of that angle. Angular distributions vary quite markedly from one target to another.

Angular Distribution of Metastable Hydrogen Formed by Electron Capture in a Helium Target

A study has been made of the angular distribution of scattered particles induced by proton impact on a target of helium. Cross sections are presented for scattering of protons, of neutral hydrogen atoms, and of metastable hydrogen atoms. Impact energies range from 4 to 20 keV; scattering angles are from 0\ifmmode^\circ\else\textdegree\fi{} to 2.0\ifmmode^\circ\else\textdegree\fi{}. Measurements of the fractional metastable content of the scattered neutral flux rise from 1% or less at 0\ifmmode^\circ\else\textdegree\fi{} to as much as 8% at angles of 1\ifmmode^\circ\else\textdegree\fi{}. Theoretical predictions of the cross sections for scattering of all particles, neutrals and protons, show reasonable agreement with experiments. Theoretical predictions, by Colegrave and Stevens, of the probability for forming a neutral atom disagree with the experimental values by a constant phase factor. Measurements of the probability for forming a metastable atom do not agree with available theoretical predictions.

Absolute Detection and Collisional Destruction of 2.5-keV Metastable Hydrogen Atoms Produced by a Charge-Exchange Process in Cesium Vapor

We have studied a near-resonant charge-exchange process between incident protons and a cesium vapor target. The cross section ${\ensuremath{\sigma}}_{+0}$ for the production of all the neutral states of hydrogen decreases slightly with increasing energy. It varies from (10\ifmmode\pm\else\textpm\fi{}3) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{2}$ at 0.5 keV to (6.4\ifmmode\pm\else\textpm\fi{}1.6) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{2}$ at 2.5 keV. The cross section ${\ensuremath{\sigma}}_{+m}$ for the production of the $2{S}_{\frac{1}{2}}$ metastable state of hydrogen is (1.7\ifmmode\pm\else\textpm\fi{}0.6) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{2}$ at 2.4 keV. The percentage of metastable atoms in the outgoing neutral beam is found to be 0.27\ifmmode\pm\else\textpm\fi{}0.08 for cesium target thickness less than ${10}^{13}$ atoms/${\mathrm{cm}}^{2}$. The outgoing fraction of $\mathrm{H}(2{S}_{\frac{1}{2}})$ atoms reaches a maximum equal to 0.13 for a cesium thickness of 1.2\ifmmode\times\else\texttimes\fi{}${10}^{14}$ atoms/${\mathrm{cm}}^{2}$. The collisional quenching processes studied are the electron loss of $\mathrm{H}(2{S}_{\frac{1}{2}})$ on the noble gases ${\mathrm{H}}_{2}$, ${\mathrm{N}}_{2}$, and HI at 2.5 keV and the electron attachment of $\mathrm{H}(2{S}_{\frac{1}{2}})$ on ${\mathrm{N}}_{2}$ at the same energy. The electron-loss cross sections ${\ensuremath{\sigma}}_{m+}$, in units of ${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$, and known with a 35% uncertainty, are 4.1 for He, 2.7 for Ne, 2.9 for Ar, 2.7 for Kr, 6 for Xe, 3.4 for ${\mathrm{H}}_{2}$, and 5 for ${\mathrm{N}}_{2}$. The cross section ${\ensuremath{\sigma}}_{m+}$ is always greater than ${\ensuremath{\sigma}}_{g+}$, the electron-loss cross section for the ground state of hydrogen. The maximum ratio 9.7 of these two cross sections is obtained with the halogen compound target HI, which is an interesting gas to selectively ionize $\mathrm{H}(2{S}_{\frac{1}{2}})$ in a beam containing the two species $\mathrm{H}(2{S}_{\frac{1}{2}})$ and $\mathrm{H}(1{S}_{\frac{1}{2}})$. A comparison of the data is made with theoretical predictions using an impulse approximation. Attachment cross sections of $\mathrm{H}(2{S}_{\frac{1}{2}})$ and $\mathrm{H}(1{S}_{\frac{1}{2}})$ have been measured on ${\mathrm{N}}_{2}$ at 2.5 keV. We obtain ${\ensuremath{\sigma}}_{m\ensuremath{-}}=(1.2\ifmmode\pm\else\textpm\fi{}0.4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ and ${\ensuremath{\sigma}}_{g\ensuremath{-}}=(9.7\ifmmode\pm\else\textpm\fi{}2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}18}$ ${\mathrm{cm}}^{2}$.

Electron capture by metastable hydrogen

Cross sections for electron capture by H(2s) in collisions with argon, nitrogen and oxygen have been measured in the energy range 1.4-8 keV. At 1.4 keV the capture cross section is as large as 2*10-15 cm2 for argon and roughly an order of magnitude smaller for oxygen and nitrogen. In all three cases the cross section for capture by metastables decreases rapidly as a function of energy and approaches the cross section for electron capture by ground state hydrogen at the high energy end of the range. Using earlier measurements on electron loss from metastable hydrogen a lower bound to the total destruction cross section is established.

Collisional Quenching of Metastable Hydrogen Atoms

We have measured the cross section for the quenching (i.e., depopulation) of the $2s$ (metastable) state of atomic hydrogen by collisions with atoms of helium and argon. The energy range covered was 0.25-30.0 keV. The experimental technique was to measure the attenuation of a beam passing through the various gases, the detector being a Lyman-$\ensuremath{\alpha}$ counter viewing a section of the beam where a dc electric field mixed $2s$ and $2p$ states, which essentially converted metastable atoms into Lyman-$\ensuremath{\alpha}$ photons. For helium as target gas, the cross section was also calculated using a pseudopotential and the eikonal approximation. For helium, the measured cross section falls rapidly as the energy increases from 0.25 to 2.5 keV and remains roughly constant at $7.5\ensuremath{\pi}{a}_{0}^{2}$ from 2.5 to 30 keV. The theoretical curve is about a factor of 4 below the measured value. Implications of this discrepancy are discussed in the text. For argon, the measured cross section increases from $20\ensuremath{\pi}{a}_{0}^{2}$ to $27\ensuremath{\pi}{a}_{0}^{2}$ as the energy increases from 1 to 30 keV.

Electron loss from metastable hydrogen

Cross sections for electron loss from metastable hydrogen in collisions with argon, molecular hydrogen, nitrogen, and oxygen have been measured in the energy range 2-60 keV. The results are compared with recent experimental work of Gilbody et al. and with theoretical predictions based on the semiempirical classical impulse approximation of Bates and Walker and, in the case of hydrogen, with born approximation calculations of Bell and Kingston. While a satisfactory agreement between theory and the present experiment is found, measured cross sections are significantly larger that those of Gilbody et al. (see abstr. A23337 of 1971).

Quenching of Metastable Hydrogen by Helium

Quenching of Metastable Hydrogen by Helium

Collisional Quenching of Metastable Hydrogen Atoms

Quenching of metastable hydrogen atoms by low energy collisions with atoms and molecules is considered, the actual process being H(2s)+X→H(2p)+X. The Born approximation, with long-range multipole–multipole interactions, is used to describe collisions of H(2s) with molecules, and simple formulas for the cross section result. Collisions with spherically symmetric species (i.e., rare gas atoms) are treated in the adiabatic approximation, and the process is seen to be formally identical to symmetric charge transfer. Numerical results for collisional quenching by helium, based on accurately computed potential curves, are presented.