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AIP conference proceedings

Two‐photon excitation and ionization of H‐atoms with tunable VUV at Lyman‐α

H. Rottke, H. Zacharias, K. H. Welge

Published Jan 1, 1982• 0 citations
Publisher

Abstract

Two kinds of experiments with tunable VUV laser light have been carried out with H atoms: (1) High resolution resonant two‐photon ionization, H(1 2S1/2)+hνα→H(2 2P1/2,3/2)+ hνuv→H++e with tunable Lyman‐α light and (2) resonant tow photon excitation of high Rydberg states. H(1 2S1/2)+hνα→H(2 2P)+hνuv →H(n 2S, n 2D)→H++e followed by ionization through collisions. In both cases an atomic beam configuration has been employed and H+ ions were detected with a mass filter. The smallest bandwidth of tunable Lyman‐α radiation achieved was ∼1.0 Ghz, enough for partial resolution of the hyperfine structure doublet of the 1 2S1/2 ground state. The application to produce polarized protons by high resolution two photon ionization is discussed. Rydberg states have been excited up to n∼48 and detected by collisional ionization under the given experimental conditons.

DOI
10.1063/1.33738

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Two kinds of experiments with tunable VUV laser light have been carried out with H atoms: (1) High resolution resonant two‐photon ionization, H(1 2S1/2)+hνα→H(2 2P1/2,3/2)+ hνuv→H++e with tunable Lyman‐α light and (2) resonant tow photon excitation of high Rydberg states. H(1 2S1/2)+hνα→H(2 2P)+hνuv →H(n 2S, n 2D)→H++e followed by ionization through collisions. In both cases an atomic beam configuration has been employed and H+ ions were detected with a mass filter. The smallest bandwidth of tunable Lyman‐α radiation achieved was ∼1.0 Ghz, enough for partial resolution of the hyperfine structure doublet of the 1 2S1/2 ground state. The application to produce polarized protons by high resolution two photon ionization is discussed. Rydberg states have been excited up to n∼48 and detected by collisional ionization under the given experimental conditons.
We report investigations with tunable VUV radiation, employing VUV generation in regions down to the cut-off wavelength of lithium fluorid crystal (~ 110 nm) by frequency tripling of tunable UV from dye laser. -Experiments on multiphoton excitation and ionization of atomic hydrogen with tunable VUV light at Lyman-α, have been carried out with the following objectives: (1) sensitive H and D atom detection by two-photon ionization i. e. 1s ω Lyα → 2 p ω uv → ion , (2) two-photon ionization of H with high resolution at Lyman-α to resolve the hyperfine groundstate doublet, aiming at the production of polarized protons, (3) state selective production of high Rydberg states ( 1 s ω Lyα → 2p ω uv → ns, nd ) , and their ionization, (4) Doppler line shape spectroscopy at Lyman-α applied to recoil energy and momentum distribution measurements of H and D atoms in dissociation processes. -Depending on the objective, different experimental conditions and systems have been employed, for instance atomic beam configuration, mass-spectroscopic ion detection, field ionization in cryogenetic environment, and for Doppler measurements laser induced fluorescence.
Two investigations are reported on the application of resonantly enhanced multiphoton ionization (RMPI) to gaseous mixtures of an atomic vapor and a few Torr of argon. Photon fluxes large enough to ionize atoms by RMPI were produced by focussing down the light from a tunable pulsed dye laser. The irradiances generated were of the order of 500MWcm⁻². Ionization was detected by a voltage biased wire electrode that simply collected the photoelectrons either directly or after some gas multiplication. One investigation was the measurement of the dependence of RMPI processes in mercury on the polarization of the incident light for comparisons with theoretical calculations. The processes were four-photon resonant absorptions to either a ¹S₀ or ¹’³D₂ level followed by single photon ionization. Complete photoionization of all atoms excited to the resonant levels is established and the measured polarization dependences are found to agree with the calculated polarization dependence for the resonant excitation step of the RMPI process. Unexplained observations of the distortions in the polarization dependence of the ion yield and the absorption linewidth of the 6d ¹D₂ resonance are discussed. The second investigation was a study of the density dependent electric dipole forbidden two-photon resonant transition ²S → ²P in three-photon RMPI spectra of the Rydberg states of potassium. Stark interactions are shown to be unlikely and too weak. From the characteristics of the spectra, the excitation process is identified as a laser-assisted collision interaction.
Two synchronised pulsed single-mode tunable laser systems are used to excite atomic hydrogen in an atomic beam. A vacuum ultraviolet (VUV) laser populates first the 2p 2 P 3/2 sublevel from the ground 1s 2 S 1/2 state. A second-step UV laser selects substates of the n=33 manifold in the presence of a weak electric field (23.6 V cm -1 ). Stark patterns of this manifold are recorded for different polarisation configurations of the exciting light beams. The n=44 substates are detected by the standard field ionisation technique. The measured relative intensities of the Stark components agree with the theoretical excitation probabilities in the two-step process, calculated to the first order of perturbation theory.
Tunable coherent ultraviolet radiation has been used to excite selectively the n=3n=3n=3 level in hydrogen and deuterium atoms via two-photon absorption from the ground state. The resulting Balmer-$\ensuremath{\alpha}$ fluorescence at 656 nm was observed as well as resonant three-photon ionization. This work demonstrates several advantages over other techniques for selective detection of neutral H and D atoms, including three-dimensional spatial resolution and a remote monitoring capability afforded by the use of laser-induced-fluorescence detection.
Tunable coherent ultraviolet radiation has been used to excite selectively the n=3n=3n=3 level in hydrogen and deuterium atoms via two-photon absorption from the ground state. The resulting Balmer-$\ensuremath{\alpha}$ fluorescence at 656 nm was observed as well as resonant three-photon ionization. This work demonstrates several advantages over other techniques for selective detection of neutral H and D atoms, including three-dimensional spatial resolution and a remote monitoring capability afforded by the use of laser-induced-fluorescence detection.