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Journal of Applied Physics

Slow metastable atomic hydrogen beam by optical pumping

Kenneth C. Harvey

Published May 1, 1982• 17 citations
Publisher

Abstract

A beam source of atomic hydrogen is described which produces metastable atoms in the 2S1/2 state by optical pumping. A beam flux of 1016 atoms/s is generated in the ground state. The atoms in the beam pass in front of a lamp producing Lyman-β (1026 Å) radiation, where some of them are excited to the 3P level and cascade with a branching ratio of 12% to the 2S1/2 state. The number of metastable atoms produced is measured by quenching them with an electric field and detecting the emitted Lyman-α (1216 Å) radiation. Beams of 106 metastable atoms/s were obtained. Using the Bethe-Lamb theory for the quenching process, a metastable beam effective temperature of 100 K was measured.

DOI
10.1063/1.331180

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arXiv:physics/0612046•Review of Scientific Instruments•Feb 1, 2007•25 citations
A beam source of atomic hydrogen is described which produces metastable atoms in the 2S1/2 state by optical pumping. A beam flux of 1016 atoms/s is generated in the ground state. The atoms in the beam pass in front of a lamp producing Lyman-β (1026 Å) radiation, where some of them are excited to the 3P level and cascade with a branching ratio of 12% to the 2S1/2 state. The number of metastable atoms produced is measured by quenching them with an electric field and detecting the emitted Lyman-α (1216 Å) radiation. Beams of 106 metastable atoms/s were obtained. Using the Bethe-Lamb theory for the quenching process, a metastable beam effective temperature of 100 K was measured.
The interaction between a metastable H(2s) atomic hydrogen beam and an external electric field leads to the emission of the Lyman-α line. It originates in the Stark mixing of the near-degenerate 2s1/2 and 2p1/2 levels separated by the Lamb shift. The quenched radiation proportional to the square of the electric field amplitude is recovered in vacuum by using such an atomic probe beam. We observe the strong enhancement of the signal when the field is oscillating at the Lamb shift frequency. This technique is applied in a plasma, offering an alternative way to measure weak electric fields by direct and non-intrusive means.
The interaction between a metastable H(2s) atomic hydrogen beam and an external electric field leads to the emission of the Lyman-α line. It originates in the Stark mixing of the near-degenerate 2s1/2 and 2p1/2 levels separated by the Lamb shift. The quenched radiation proportional to the square of the electric field amplitude is recovered in vacuum by using such an atomic probe beam. For larger electric field, saturation is observed and related to the beam finite transit time. We also observe the strong enhancement of the signal when the field is oscillating at the Lamb shift frequency. This technique is applied in a plasma, offering an alternative way to measure weak electric fields by direct and non-intrusive means.
The polarizations of Lyman-$\ensuremath{\alpha}$ radiation produced (i) by electron impact on atomic and molecular hydrogen and (ii) by electric field quenching of metastable 2S2S2S hydrogen atoms have been measured. The polarization analyzer was a LiF crystal set at the Brewster angle, with reflected radiation being detected by an iodine-vapor photon counter. Generally good agreement between the measured polarization fractions and recent theoretical predictions was found. The radiation produced by quenching metastable atoms in an electric field was found to be polarized with a polarization fraction of -0.30\ifmmode\pm\else\textpm\fi{}0.02, which compares with the theoretical value -0.323.
We have observed second-harmonic generation in atomic hydrogen in the Lyman-α region, with and without an applied dc electric field, Ea. Experiments with a continuous atomic beam were carried out by using a pulsed dye laser tuned to 243 nm for excitation of the n = 2-1 transition. The laser beam was focused 0.4 mm below the H-source nozzle, where the H-atom density was estimated to be 5 × 1013 cm-3. Measurements of second-harmonic generation (SHG) at ~122 nm were made at various electric fields up to ~27 kV/cm. It was shown that the SH signal produced at E a = 0 occurs because of a charge-separation field of ~800 V/cm that is caused by three-photon ionization. Measurements of the SHG and ion current were found to vary as I2 L and I3 L , respectively. At high fields (E a > 10 kV/cm) the SH signal at the center of the Stark-split components decreased somewhat from the peak value, but the ion current (corresponding to photon absorption) approached zero. This observation is a clear demonstration of a similar effect, predicted by Harris et al.1 of efficient nonlinear conversion with induced transparency, and it suggests the possibility of obtaining intense Lyman-a radiation.
A room-temperature beam of krypton atoms in the metastable 5s[3/2]2 level is demonstrated via an optical excitation method. A Kr-discharge lamp is used to produce vacuum ultraviolet photons at 124 nm for the first-step excitation from the ground level 4p6 1S0 to the 5s[3/2]1 level. An 819 nm Ti:sapphire laser is used for the second-step excitation from 5s[3/2]1 to 5s[3/2]2 followed by a spontaneous decay to the 5s[3/2]2 metastable level. A metastable atomic beam with an angular flux density of 3 x 10(14) s(-1) sr(-1) is achieved at the total gas flow rate of 0.01 cm3/s at STP (or 3 x 10(17) at./s). The dependences of the flux on the gas flow rate, laser power, and lamp parameters are investigated.