C. Y. Tang

Spectra from multiphoton electron detachment of H

New data are reported on the multiphoton detachment process in a fast beam of H− ions. The angle-tuned relativistic Doppler shift is used to vary the photon energy of a focused (~10 GW/cm2) 10.6-μm CO2 laser beam from ~0.05 to ~0.4 eV in the rest frame (CM frame) of the fast ions. The ions are produced at 800 MeV (β = v/c = 0.84) by the Los Alamos Meson Physics Facility linear accelerator at Los Alamos and experience ~1-psec pulses in the CM frame as they cross the laser beam focus. Peaks in the detachment signal corresponding to each order for two- to six-photon processes are observed. At modest laser intensity in the gigawatt-per-square-centimeter range, observed shifts of the apparent two-photon threshold are found to be not more than 30–50% of the expected maximum shift, based on the value of the ponderomotive potential. Experimental uncertainties are due mainly to imprecise knowledge of the maximum laser intensity. The data analysis and modeling of the expected threshold shape experiments are continuing.

Measurement and reduction of momentum spread in the LAMPF linac beam

The use of laser spectroscopy for the determination of momentum spread and energy of a relativistic H− beam is discussed, and the results are presented of a successful attempt to reduce the momentum spread δpp by a factor of 5 from its initial value of approximately 5 × 10−4.

Photodetachment of the H− ion

Abstract Detachment of electrons from the H − ion is investigated with an experimental technique whereby an H − beam moving at a relativistic velocity (2.5 × 10 10 cm/s) is intersected with a fixed frequency laser. The Doppler effect allows systematic variation of the center-of-mass (CM) photon energy over a wide range (factor of 10) by simply adjusting the angle between the ion and laser beams. The focused output from a pulsed, linearly polarized, CO 2 TEA laser operating at 10.6 μm, with peak intensities on the order of 10 GW/cm 2 , was used to examine the multiphoton absorption process in H −2 . The fourth harmonic (266 nm) of a Nd:YAG laser was used to investigate some of the doubly-excited state resonances in H − . In the multiphoton absorption work, electron detachment was observed at photon energies where as few as 2 and as many as 8 photons are required to get above the 1-electron detachment threshold (EDT) of H − (0.754 eV). Electron yield vs photon energy plots exhibit structure that is laser intensity dependent. Electron yield vs laser pulse energy data was obtained at a few selected CM wavelengths and laser pulse energies. In the single-photon UV laser work, numerous resonances within the H − photodetachment continuum corresponding to one-photon two-electron excitation processes were observed. The doubly-excited resonances appear to be of the Feshbach type. A simple, semi-empirical recursion formula predicts the resonance energy levels. The experimental techniques described here can be used to accurately determine accelerator beam and ion source parameters such as beam energy, energy-spread, and ion density spatial distribution.

Observations of excited H° atoms produced by relativistic H− ions in carbon foils

Studies of the production of H° its various states by the passage of relativistic H− ions through thin foils are under way at the linear accelerator at Los Alamos. Laser excitation and selective field ionization are used to isolate specific states. We present evidence that the excitation of Rydberg states by a foil at energies up to 800 meV proceeds in a stepwise manner.

Laser Spectroscopy of Relativistic Beams of H- and H: Observation of e-Detachment from H- by Multiphoton Absorption

Publisher Summary This chapter discusses laser spectroscopy of relativistic beams of H- and H by multiphoton absorption. Multiphoton detachment (MPD) takes place with lower energy photons than typical multiphoton ionization (MPI) with neutrals. The lack of intermediate-states simplifies the calculations as does the fact that the final-state interaction is of short range rather than Coulomb: H- may provide a particularly clean test of MPI/MPD theory. Large changes in the MPD rate can be expected when a large static electric field is applied in addition to a laser field. In the experiment described in the chapter, detached electrons were detected with high efficiency and low background by the electron spectrometer; fast η atoms were detected by a downstream scintillator. Observed MPD rates with the smoothing tube were somewhat smaller than without it, as expected, but at the highest intensities, partial depletion saturation was observed of at least the 3-photon threshold detachment on the ∼1 psec scale of the H- transit time through the laser focus.